xSplice design

From the top (or left in the above diagram) the structures are:

Since RFC v2 [http://lists.xen.org/archives/html/xen-devel/2015-05/msg02142.html]
- Ingested every review comment in.

For those who prefer an diff of what changed between v2 and this I am attaching an diff to help easy reviewing.

Since RFC v2 [http://lists.xen.org/archives/html/xen-devel/2015-05/msg02142.html]
- Ingested every review comment in.

For those who prefer an diff of what changed between v2 and this I am attaching an diff to help easy reviewing.

Please see inline the RFC v3 which in general:
- Ditches the attempt at defining an ELF payload using semi-Elf language and just concentrates on structures.
- Expands on the preemption of the hypercalls
- Expands the implementation details with various topics that emerged during v2 review
- Adds ASCII art (if you can call it that), and an example.
- state diagram the command hypercall.

# xSplice Design v1 (EXTERNAL RFC v3)

## Rationale

A mechanism is required to binarily patch the running hypervisor with new opcodes that have come about due to primarily security updates.

This document describes the design of the API that would allow us to upload to the hypervisor binary patches.

The document is split in four sections:
- Detailed descriptions of the problem statement.
- Design of the data structures.
- Design of the hypercalls.
- Implementation notes that should be taken into consideration.

## Glossary

* splice - patch in the binary code with new opcodes
* trampoline - a jump to a new instruction.
* payload - telemetries of the old code along with binary blob of the new function (if needed).
* reloc - telemetries contained in the payload to construct proper trampoline.

## Multiple ways to patch

The mechanism needs to be flexible to patch the hypervisor in multiple ways and be as simple as possible. The compiled code is contiguous in memory with no gaps - so we have no luxury of 'moving' existing code and must either insert a trampoline to the new code to be executed - or only modify in-place the code if there is sufficient space. The placement of new code has to be done by hypervisor and the virtual address for the new code is allocated dynamically.

This implies that the hypervisor must compute the new offsets when splicing in the new trampoline code. Where the trampoline is added (inside the function we are patching or just the callers?) is also important.

To lessen the amount of code in hypervisor, the consumer of the API is responsible for identifying which mechanism to employ

and how many locations
to patch. Combinations of modifying in-place code, adding trampoline, etc has to be supported. The API should allow read/write any memory within the hypervisor virtual address space.

We must also have a mechanism to query what has been applied and a mechanism to revert it if needed.

We must also have a mechanism to: provide an copy of the old code - so that the hypervisor can verify it against the code in memory; the new code; the symbol name of the function to be patched; or offset from the symbol; or virtual address.

The complications that this design will encounter are explained later in this document.

## Patching code

The first mechanism to patch that comes in mind is in-place replacement. That is replace the affected code with new code. Unfortunately the x86 ISA is variable size which places limits on how much space we have available to replace the instructions.

The second mechanism is by replacing the call or jump to the old function with the address of the new function.

A third mechanism is to add a jump to the new function at the start of the old function.

### Example of trampoline and in-place splicing

As example we will assume the hypervisor does not have XSA-132 (see *domctl/sysctl: don't leak hypervisor stack to toolstacks* 4ff3449f0e9d175ceb9551d3f2aecb59273f639d) and we would like to binary patch the hypervisor with it. The original code looks as so:

<pre>

   48 89 e0                  mov    %rsp,%rax  
   48 25 00 80 ff ff         and    $0xffffffffffff8000,%rax  

</pre>

while the new patched hypervisor would be:

<pre>

   48 c7 45 b8 00 00 00 00   movq   $0x0,-0x48(%rbp)  
   48 c7 45 c0 00 00 00 00   movq   $0x0,-0x40(%rbp)  
   48 c7 45 c8 00 00 00 00   movq   $0x0,-0x38(%rbp)  
   48 89 e0                  mov    %rsp,%rax  
   48 25 00 80 ff ff         and    $0xffffffffffff8000,%rax  

</pre>

This is inside the arch_do_domctl. This new change adds 21 extra bytes of code which alters all the offsets inside the function. To alter these offsets and add the extra 21 bytes of code we might not have enough space in .text to squeze this in.

As such we could simplify this problem by only patching the site which calls arch_do_domctl:

<pre>
<do_domctl>:
e8 4b b1 05 00 callq ffff82d08015fbb9 <arch_do_domctl>
</pre>

with a new address for where the new arch_do_domctl would be (this area would be allocated dynamically).

Astute readers will wonder what we need to do if we were to patch do_domctl
- which is not called directly by hypervisor but on behalf of the guests via the compat_hypercall_table and hypercall_table.
Patching the offset in hypercall_table for `do_domctl:
(ffff82d080103079 <do_domctl>:)
<pre>

ffff82d08024d490: 79 30
ffff82d08024d492: 10 80 d0 82 ff ff

</pre>
with the new address where the new do_domctl is possible. The other place where it is used is in hvm_hypercall64_table which would need to be patched in a similar way. This would require an in-place splicing of the new virtual address of arch_do_domctl.

In summary this example patched the callee of the affected function by
* allocating memory for the new code to live in,
* changing the virtual address of all the functions which called the old

code (computing the new offset, patching the callq with a new callq).
* changing the function pointer tables with the new virtual address of the function (splicing in the new virtual address). Since this table resides in the .rodata section we would need to temporarily change the page table permissions during this part.

However it has severe drawbacks - the safety checks which have to make sure the function is not on the stack - must also check every caller. For some patches this could if there were an sufficient large amount of callers

that we would never be able to apply the update.

### Example of different trampoline patching.

An alternative mechanism exists where we can insert an trampoline in the existing function to be patched to jump directly to the new code. This lessens the locations to be patched to one but it puts pressure on the CPU branching logic (I-cache, but it is just one unconditional jump).

For this example we will assume that the hypervisor has not been compiled with fe2e079f642effb3d24a6e1a7096ef26e691d93e (XSA-125: *pre-fill structures for certain HYPERVISOR_xen_version sub-ops*) which mem-sets an structure in xen_version hypercall. This function is not called **anywhere** in the hypervisor (it is called by the guest) but referenced in the compat_hypercall_table and hypercall_table (and indirectly called from that). Patching the offset in hypercall_table for the old do_xen_version (ffff82d080112f9e <do_xen_version>)

</pre>
ffff82d08024b270 <hypercall_table>
...
ffff82d08024b2f8: 9e 2f 11 80 d0 82 ff ff

</pre>
with the new address where the new do_xen_version is possible. The other place where it is used is in hvm_hypercall64_table which would need to be patched in a similar way. This would require an in-place splicing of the new virtual address of do_xen_version.

An alternative solution would be to patch insert an trampoline in the

old do_xen_version' function to directly jump to the new do_xen_version`.

<pre>
ffff82d080112f9e <do_xen_version>:

 ffff82d080112f9e:       48 c7 c0 da ff ff ff    mov    $0xffffffffffffffda,%rax  
 ffff82d080112fa5:       83 ff 09                cmp    $0x9,%edi  
 ffff82d080112fa8:       0f 87 24 05 00 00       ja     ffff82d0801134d2 <do_xen_version+0x534>  

</pre>

with:

<pre>
ffff82d080112f9e <do_xen_version>:
ffff82d080112f9e: e9 XX YY ZZ QQ jmpq [new do_xen_version]
</pre>

which would lessen the amount of patching to just one location.

In summary this example patched the affected function to jump to the new replacement function which required:
* allocating memory for the new code to live in,
* inserting trampoline with new offset in the old function to point to the new function.
* Optionally we can insert in the old function an trampoline jump to an function

providing an BUG_ON to catch errant code.

The disadvantage of this are that the unconditional jump will consume a small I-cache penalty. However the simplicity of the patching of safety checks make this a worthwhile option.

### Security

With this method we can re-write the hypervisor - and as such we **MUST** be diligent in only allowing certain guests to perform this operation.

Furthermore with SecureBoot or tboot, we **MUST** also verify the signature of the payload to be certain it came from a trusted source.

As such the hypercall **MUST** support an XSM policy to limit the what guest is allowed.

If the system is booted with signature checking the signature checking will be enforced.

## Design of payload format

The payload **MUST** contain enough data to allow us to apply the update and also safely reverse it. As such we **MUST** know:

* What the old code is expected to be. We **MUST** be able verify it against the runtime code.

* The locations in memory to be patched. This can be determined dynamically via symbols or via virtual addresses.
* The new code (or data) to will be patched in.

* Signature to verify the payload.

This binary format can be constructed using an custom binary format but there are severe disadvantages of it:

* The format might need to be change and we need an mechanism to accommodate

that.
* It has to be platform agnostic.
* Easily constructed using existing tools.

As such having the payload in an ELF file is the sensible way. We would be carrying the various set of structures (and data) in the ELF sections under

different names and with definitions. The prefix for the ELF section name would always be: *.xsplice* to match up to the names of the structures.

Note that every structure has padding. This is added so that the hypervisor can re-use those fields as it sees fit.

Earlier design attempted to ineptly explain the relations of the ELF sections to each other without using proper ELF mechanism (sh_info, sh_link, data structures using Elf_* types, etc). This design will explain in detail the structures and how they are used together and not dig in the ELF format - except mention that the section names should match the structure names.

### ASCII art of structures.

The diagram below is omitting some entries to easy the relationship explanation.

<pre>

                                                                          /---------------------\  
                                                                       +->| xsplice_reloc_howto |  
                                                                      /   \---------------------/  
                                                /---------------\ 1:1/  
                                             +->| xsplice_reloc |   /  
                                            /   | - howto       +--/  1:1 /----------------\  
                                           /    | - symbol      +-------->| xsplice_symbol |  
                                     1:N  /     \---------------/       / \----------------/  
/----------\        /--------------\     /                             /  
| xsplice  |  1:1   | xsplice_code |    /                          1:1/  
| - new    +------->|  - relocs    +---/  1:N   /-----------------\  /  
| - old    +------->|  - sections  +----------->| xsplice_section | /  
\----------/        |  - patches   +--\         | - symbol        +/ 1:1   /----------------\  
                    \--------------/   \        | - addr          +------->| .text or .data |  
                                        \       \----------------/         \----------------/  
                                         \  
                                      1:N \  
                                           \    /----------------\  
                                            +-->| xsplice_patch  |  1:1  /----------------\  
                                                | - content      +------>| binary code or |  
                                                \----------------/       | data           |  
                                                                         \----------------/  

</pre>

### xsplice structures

From the top (or left in the above diagram) the structures are:

* xsplice. The top most structure - contains the the name of the update, the id to match against the hypervisor, the pointer to the metadata for the new code and optionally the metadata for the old code.

* xsplice_code. The structure that ties all of this together and defines the payload. Contains arrays of xsplice_reloc, xsplice_section, and xsplice_patch.

* xsplice_reloc contains telemtry used for patching - which describes the

targets to be patched and how to do it.

* xsplice_section - the safety data for the code. Contains pointer to the symbol (xsplice_symbols) and pointer to the code (.text) or data (.data), which are to be used during safety and dependency checking.

* xsplice_patch: the description of the new function to be patched in along with the binary code or data.

* xsplice_reloc_howto: the howto properly construct trampolines for an patch. We may have multiple locations for which we need to insert an trampoline for a payload and each location might require a different way of handling it.

* xsplice_symbols . The symbol that will be patched.

In short the *.xsplice* sections (with xsplice being the top) represent various structures to define the new code and safety checks for the old code (optional). The ELF provides the mechanism to glue it all together when loaded in memory.

Note that a lot of these ideas are borrowed from kSplice which is available at: https://github.com/jirislaby/ksplice

### struct xsplice

The top most structure is quite simple. It defines the name, the id of the hypervisor, pointer to the new code and an pointer to the old code (optional).

The new code uses all of the xsplice_* structures while the old code does not use the xsplice_reloc structures.

The sections defining the structures will explicitly state when they are not used.

<pre>
struct xsplice {
const char *name; /* A sensible name for the patch. Up to 40 characters. */

const char *id; /* ID of the hypervisor this binary was built against. */

struct xsplice_code *new; /* Pointer to the new code to be patched. */ struct xsplice_code *old; /* Pointer to the old code to be checked against. */ uint8_t pad[32]; /* Must be zero. */
};
</pre>

The size of this structure should be 64 bytes.

### xsplice_code

The structure embedded within this section ties the other structures together. It has the pointers with an start and end address for each set of structures. This means that an update can be split in multiple changes - for example to accomodate an update that contains both code and data and will need patching in both .text and .data sections.

<pre>
struct xsplice_code {
struct xsplice_reloc *relocs, *relocs_end; /* How to patch it. */ struct xsplice_section *sections, *sections_end; /* Safety data. */ struct xsplice_patch *patches, *patches_end; /* Patch code and data */ uint8_t pad[16]; /* Must be zero. */
};
</pre>

The size of this structure is 64 bytes.

There can be at most two of those structures in the payload. One for the new code and another for the old code (optional).

If it is for the old code the relocs, and relocs_end values will be ignored.

### xsplice_reloc

The xsplice_code defines an array of these structures. As such an singular structure defines an singular point where to patch the hypervisor.

The structure contains the address of the hypervisor (if known), the symbol associated with this address, how the patching is to be done, and platform specific details.

The isns_added is an value to be used to compute the new offset due to the quirks of the operands of the instruction. For example to patch in an jump operation to the new code - the offset is relative to the program counter of the next instruction - hence the offset value has to be subtracted by four bytes - hence this would contain -4 .

The isns_target is the offset against the symbol.

The relation of this structure with xsplice_patch is 1:1, even for inline patches. See the section detailing the structure xsplice_reloc_howto.

The relation of this structure with xsplice_section is 1:1.

This structure is as follow:

<pre>
struct xsplice_reloc {
uint64_t addr; /* The address of the relocation (if known). */
struct xsplice_symbol *symbol; /* Symbol for this relocation. */
int64_t isns_target; /* rest of the ELF addend. This is equal to the offset against the symbol that the relocation refers to. */
struct xsplice_reloc_howto *howto; /* Pointer to the above structure. */
int64_t isns_added; /* ELF addend resulting from quirks of instruction one of whose operands is the relocation. For example, this is -4 on x86 pc-relative jumps. */ uint8_t pad[24]; /* Must be zero. */
};

</pre>

The size of this structure is 64 bytes.

### xsplice_section

The structure defined in this section is used during pre-patching and during patching. Pre-patching it is used to verify that it is safe to update with the new changes - and contains safety data on the old code and what kind of matching we are to expect.

That is whether the address (either provided or resolved when payload is loaded by referencing the symbols) is:

* in memory,
* correct size,
* in it's proper ELF section,
* has been already patched (or not),
* is expected not to be the CPU stack - (or it is OK for it be on the CPU stack).

with what we expect it to be.

Some of the checks can be relaxed, as such the flag values can be or-ed together.

<pre>

#define XSPLICE_SECTION_TEXT 0x00000001 /* Section is in .text */
#define XSPLICE_SECTION_RODATA 0x00000002 /* Section is in .rodata */
#define XSPLICE_SECTION_DATA 0x00000004 /* Section is in .data */
#define XSPLICE_SECTION_STRING 0x00000008 /* Section is in .str */

#define XSPLICE_SECTION_TEXT_INLINE 0x00000200 /* Change is to be inline. */ #define XSPLICE_SECTION_MATCH_EXACT 0x00000400 /* Must match exactly. */ #define XSPLICE_SECTION_NO_STACKCHECK 0x00000800 /* Do not check the stack. */

struct xsplice_section {
struct xsplice_symbol *symbol; /* The symbol associated with this change. */ uint64_t address; /* The address of the section (if known). */ uint32_t size; /* The size of the section. */
uint32_t flags; /* Various XSPLICE_SECTION_* flags. */
uint8_t pad[12]; /* To be zero. */
};

</pre>

The size of this structure is 32 bytes.

### xsplice_patch

This structure has the binary code (or data) to be patched. Depending on the type it can either an inline patch (data or text) or require an relocation change (which requires an trampoline). Naturally it also points to a blob of the binary data to patch in, and the size of the patch.

The addr is used when the patch is for inline change. If it is an relocation (requiring an trampoline), the addr should be zero.

There must be an corresponding struct xsplice_reloc and struct xsplice_section describing this patch.

<pre>

#define XSPLICE_PATCH_INLINE_TEXT   0x1
#define XSPLICE_PATCH_INLINE_DATA   0x2
#define XSPLICE_PATCH_RELOC_TEXT    0x3

struct xsplice_patch {  
    uint32_t type; /* XSPLICE_PATCH_* .*/  
    uint32_t size; /* Size of patch. */  
    uint64_t addr; /* The address of the inline new code (or data). */  
    void *content; /* The bytes to be installed. */  
    uint8_t pad[40]; /* Must be zero. */  
};

</pre>

The size of this structure is 64 bytes.

### xsplice_symbols

The structure contains an pointer to the name of the ELF symbol to be patched and as well an unique name for the symbol.

The label is used for diagnostic purposes - such as including the name and the offset.

The structure is as follow:

<pre>
struct xsplice_symbol {
const char *name; /* The ELF name of the symbol. */ const char *label; /* A unique xSplice name for the symbol. */ uint8_t pad[16]; /* Must be zero. */
};
</pre>

The size of this structure is 32 bytes.

### xsplice_reloc_howto

The howto defines in the detail the change. It contains the type, whether the relocation is relative, the size of the relocation, bitmask for which parts of the instruction or data are to be replaced, amount the final relocation is shifted by (to drop unwanted data), and whether the replacement should be interpreted as signed value.

The structure is as follow:

<pre>
#define XSPLICE_HOWTO_RELOC_INLINE 0x1 /* It is an inline replacement. */
#define XSPLICE_HOWTO_RELOC_PATCH 0x2 /* Add an trampoline. */

#define XSPLICE_HOWTO_FLAG_PC_REL 0x1 /* Is PC relative. */
#define XSPLICE_HOWOT_FLAG_SIGN 0x2 /* Should the new value be treated as signed value. */

struct xsplice_reloc_howto {  
    uint32_t    type; /* XSPLICE_HOWTO_* */  

    uint32_t    flag; /* XSPLICE_HOWTO_FLAG_* */  
    uint32_t    size; /* Size, in bytes, of the item to be relocated. */  
    uint32_t    r_shift; /* The value the final relocation is shifted right by; used to drop unwanted data from the relocation. */  
    uint64_t    mask; /* Bitmask for which parts of the instruction or data are replaced with the relocated value. */  
    uint8_t     pad[8]; /* Must be zero. */  

};

</pre>

The size of this structure is 32 bytes.

### Example

There is a wealth of information that the payload must have to define a simple patch. For this example we will assume that the hypervisor has not been compiled with fe2e079f642effb3d24a6e1a7096ef26e691d93e (XSA-125: *pre-fill structures for certain HYPERVISOR_xen_version sub-ops*) which mem-sets an structure in xen_version hypercall. This function is not called **anywhere** in the hypervisor (it is called by the guest) but referenced in the compat_hypercall_table and hypercall_table (and indirectly called from that). There are two ways to patch this:
inline patch hvm_hypercall64_table and hvm_hypercall with a new address for the new do_xen_version , or insert
trampoline in do_xen_version code. The example will focus on the later.

The do_xen_version code is located at virtual address ffff82d080112f9e.

<pre>
struct xsplice_code xsplice_xsa125;
struct xsplice_reloc relocs[1];
struct xsplice_section sections[1];
struct xsplice_patch patches[1];
struct xsplice_symbol do_xen_version_symbol;
struct xsplice_reloc_howto do_xen_version_howto;
char do_xen_version_new_code[1728];

#ifndef HYPERVISOR_ID
#define HYPERVISOR_ID "92dd05a61556c554155b1508c9cf67d993336d28" #endif

struct xsplice xsa125 = {
.name = "xsa125",
.id = HYPERVISOR_ID,
.old = NULL,
.new = &xsplice_xsa125,
};

struct xsplice_code xsplice_xsa125 = {  
    .relocs = &relocs[0],  
    .relocs_end = &relocs[0],  

    .sections = &sections[0],  
    .sections_end = &sections[0],  
    .patches = &patches[0],  
    .patches_end = &patches[0],   
};

struct xsplice_reloc relocs[1] = {  
    {  
        .addr = 0xffff82d080112f9e,  
        .symbol = &do_xen_version_symbol,  
        .isns_target = 0,  
        .howto = &do_xen_version_howto,  
        .isns_added = -4,  
    },  
};  

struct xsplice_symbol do_xen_version_symbol = {  
    .name = "do_xen_version",  
    .label = "do_xen_version+<0x0>",  
};  

struct xsplice_reloc_howto do_xen_version_howto = {
.type = XSPLICE_HOWTO_RELOC_PATCH, .flag = XSPLICE_HOWTO_FLAG_PC_REL, .r_shift = 0,
.mask = (-1ULL),
};

struct xsplice_section sections[1] = {  
    {  
        .symbol = &do_xen_version_symbol,  
        .address = 0xffff82d080112f9e,  
        .size = 1728,  
        .flags = XSPLICE_SECTION_TEXT,  
    },  
};  

struct xsplice_patch patches[1] = {  
    {  
        .type = XSPLICE_PATCH_RELOC_TEXT,  
        .size = 1728,  
        .addr = 0,  
        .content = &do_xen_version_new_code,  
    },  
};  

char do_xen_version_new_code[1728] = { 0x83, 0xff, 0x09, /* And more code. */}; </pre>

## Signature checking requirements.

The signature checking requires that the layout of the data in memory **MUST** be same for signature to be verified. This means that the payload data layout in ELF format **MUST** match what the hypervisor would be expecting such that it can properly do signature verification.

The signature is based on the all of the payloads continuously laid out in memory. The signature is to be appended at the end of the ELF payload prefixed with the string '~Module signature appended~\n", followed by

an signature header then followed by the signature, key identifier, and signers name.

Specifically the signature header would be:

<pre>
#define PKEY_ALGO_DSA 0
#define PKEY_ALGO_RSA 1

#define PKEY_ID_PGP 0 /* OpenPGP generated key ID */
#define PKEY_ID_X509 1 /* X.509 arbitrary subjectKeyIdentifier */

#define HASH_ALGO_MD4          0  
#define HASH_ALGO_MD5          1  
#define HASH_ALGO_SHA1         2  
#define HASH_ALGO_RIPE_MD_160  3  
#define HASH_ALGO_SHA256       4  
#define HASH_ALGO_SHA384       5  
#define HASH_ALGO_SHA512       6  
#define HASH_ALGO_SHA224       7  
#define HASH_ALGO_RIPE_MD_128  8  
#define HASH_ALGO_RIPE_MD_256  9  
#define HASH_ALGO_RIPE_MD_320 10  
#define HASH_ALGO_WP_256      11  
#define HASH_ALGO_WP_384      12  
#define HASH_ALGO_WP_512      13  
#define HASH_ALGO_TGR_128     14  
#define HASH_ALGO_TGR_160     15  
#define HASH_ALGO_TGR_192     16  

struct elf_payload_signature {  
	u8	algo;		/* Public-key crypto algorithm PKEY_ALGO_*. */  
	u8	hash;		/* Digest algorithm: HASH_ALGO_*. */  
	u8	id_type;	/* Key identifier type PKEY_ID*. */  
	u8	signer_len;	/* Length of signer's name */  
	u8	key_id_len;	/* Length of key identifier */  
	u8	__pad[3];  
	__be32	sig_len;	/* Length of signature data */  
};

</pre>
(Note that this has been borrowed from Linux module signature code.).

## Hypercalls

We will employ the sub operations of the system management hypercall (sysctl). There are to be four sub-operations:

* upload the payloads.
* listing of payloads summary uploaded and their state.
* getting an particular payload summary and its state.
* command to apply, delete, or revert the payload.

The actions are asynchronous therefore the caller is responsible

to verify that it has been applied properly by retrieving the summary of it and verifying that there are no error codes associated with the payload.

We **MUST** make it asynchronous due to the nature of patching: it requires every physical CPU to be lock-step with each other. The patching mechanism while an implementation detail, is not an short operation and as such the design **MUST** assume it will be an long-running operation.

The sub-operations will spell out how preemption is to be handled (if at all).

Furthermore it is possible to have multiple different payloads for the same function. As such an unique id has to be visible to allow proper manipulation.

The hypercall is part of the xen_sysctl. The top level structure contains one uint32_t to determine the sub-operations:

<pre>

struct xen_sysctl_xsplice_op {  
    uint32_t cmd;  
	union {  
          ... see below ...  
        } u;  
};  

</pre>
while the rest of hypercall specific structures are part of the this structure.

### XEN_SYSCTL_XSPLICE_UPLOAD (0)

Upload a payload to the hypervisor. The payload is verified and if there are any issues the proper return code will be returned. The payload is not applied at this time - that is controlled by *XEN_SYSCTL_XSPLICE_ACTION*.

The caller provides:

* id unique id.

* payload the virtual address of where the ELF payload is.

The return value is zero if the payload was succesfully uploaded and the signature was verified. Otherwise an EXX return value is provided.

Duplicate id are not supported.

The payload is the ELF payload as mentioned in the Payload format section.

This operation can be preempted by the hypercall returning EAGAIN.

This is due to the nature of signature verification - which may require SecureBoot firmware calls which are unbounded.

The structure is as follow:

<pre>

struct xen_sysctl_xsplice_upload {  
    char id[40];  /* IN, name of the patch. */

uint64_t size; /* IN, size of the ELF file. */ XEN_GUEST_HANDLE_64(uint8) payload; /* ELF file. */
};
</pre>

### XEN_SYSCTL_XSPLICE_GET (1)

Retrieve an summary of an specific payload. This caller provides:

* id the unique id.
* status *MUST* be set to zero.

The summary structure contains an summary of payload which includes:

* id the unique id.
* status - whether it has been:
1. *XSPLICE_STATUS_LOADED* (0) has been loaded.
2. *XSPLICE_STATUS_PROGRESS* (1) acting on the **XEN_SYSCTL_XSPLICE_ACTION** command.
3. *XSPLICE_STATUS_CHECKED* (2) the ELF payload safety checks passed.
4. *XSPLICE_STATUS_APPLIED* (3) loaded, checked, and applied.
5. *XSPLICE_STATUS_REVERTED* (4) loaded, checked, applied and then also reverted.
6. Negative values is an error. The error would be of EXX format.

The return value is zero on success and EXX on failure. This operation is synchronous and does not require preemption.

The structure is as follow:

<pre>

#define XSPLICE_STATUS_LOADED    0  
#define XSPLICE_STATUS_PROGRESS  1  
#define XSPLICE_STATUS_CHECKED   2  
#define XSPLICE_STATUS_APPLIED   3  
#define XSPLICE_STATUS_REVERTED  4  

struct xen_sysctl_xsplice_summary {  
    char id[40];  /* IN/OUT, name of the patch. */  

    int32_t status;   /* OUT */  
}; 

</pre>

### XEN_SYSCTL_XSPLICE_LIST (2)

Retrieve an array of abbreviated summary of payloads that are loaded in the hypervisor.

The caller provides:

* version. Initially it *MUST* be zero.
* idx index iterator. Initially it *MUST* be zero.
* count the max number of entries to populate.
* summary virtual address of where to write payload summaries.

The hypercall returns zero on success and updates the idx (index) iterator with the number of payloads returned, count to the number of remaining payloads, and summary with an number of payload summaries. The version is updated on every hypercall

- if it varies from one hypercall to another
the data is stale and further calls could fail.

If the hypercall returns E2BIG the count is too big and should be lowered.

Note that due to the asynchronous nature of hypercalls the domain might have added or removed the number of payloads making this information stale. It is the responsibility of the toolstack to use the version field to check between each invocation.

This operation is synchronous and does not require preemption.

The summary structure contains an summary of payload which includes:

* version version of the data.
* id unique id.

* status - whether it has been:
1. *XSPLICE_STATUS_LOADED* (0) has been loaded.
2. *XSPLICE_STATUS_PROGRESS* (1) acting on the **XEN_SYSCTL_XSPLICE_ACTION** command.
3. *XSPLICE_STATUS_CHECKED* (2) the ELF payload safety checks passed.
4. *XSPLICE_STATUS_APPLIED* (3) loaded, checked, and applied.
5. *XSPLICE_STATUS_REVERTED* (4) loaded, checked, applied and then also reverted.
6. Any negative values means there has been error. The value is in EXX format.

The structure is as follow:

<pre>

struct xen_sysctl_xsplice_list {  
    uint32_t version; /* OUT */
    uint32_t idx;  /* IN/OUT */  
    uint32_t count;  /* IN/OUT */
    XEN_GUEST_HANDLE_64(xen_sysctl_xsplice_summary) summary;  /* OUT */  
};  

struct xen_sysctl_xsplice_summary {  
    char id[40];  /* OUT, name of the patch. */  

    int32_t status;  /* OUT */  
};  

</pre>
### XEN_SYSCTL_XSPLICE_ACTION (3)

Perform an operation on the payload structure referenced by the id field. The operation request is asynchronous and the status should be retrieved by using either **XEN_SYSCTL_XSPLICE_GET** or **XEN_SYSCTL_XSPLICE_LIST** hypercall.

There are two ways about doing preemption. Either via returning back EBUSY or the mechanism outlined here.

Doing it in userland would remove any tracking of states in the hypervisor - except the simple commands apply, unload, and revert.

However we would not be able to patch all the code that is invoked while this hypercall is in progress. That is - the do_domctl, the spinlocks, anything put on the stack, etc.

The disadvantage of the mechanism outlined here is that the hypervisor code has to keep the state atomic and have an upper bound of time on actions. If within the time the operation does not succeed the operation would go in error state.

* id the unique id.
* time the upper bound of time the cmd should take. Zero means infinite.
* cmd the command requested:
1. *XSPLICE_ACTION_CHECK* (1) check that the payload will apply properly.
2. *XSPLICE_ACTION_UNLOAD* (2) unload the payload.
Any further hypercalls against the id will result in failure unless **XEN_SYSCTL_XSPLICE_UPLOAD** hypercall is perfomed with same id.
3. *XSPLICE_ACTION_REVERT* (3) revert the payload. If the operation takes more time than the upper bound of time the status will EBUSY.
4. *XSPLICE_ACTION_APPLY* (4) apply the payload. If the operation takes more time than the upper bound of time the status will be EBUSY.
5. *XSPLICE_ACTION_LOADED* is an initial state and cannot be requested.

The return value will be zero unless the provided fields are incorrect.

The structure is as follow:

<pre>
#define XSPLICE_ACTION_LOADED 0
#define XSPLICE_ACTION_CHECK 1
#define XSPLICE_ACTION_UNLOAD 2
#define XSPLICE_ACTION_REVERT 3
#define XSPLICE_ACTION_APPLY 4

struct xen_sysctl_xsplice_action {  
    char id[40];  /* IN, name of the patch. */  
    uint64_t time; /* IN, upper bound of time (ms) for the operation to take. */  
    uint32_t cmd; /* IN */  
};  

</pre>

## State diagrams of XSPLICE_ACTION values.

There is a strict ordering state of what the commands can be. The XSPLICE_ACTION prefix has been dropped to easy reading:

<pre>

                        /->\  
                        \  /  
             /-------< CHECK <--------\  
             |          |             |  
             |           +            /  
             |    +--->UNLOAD<--\    /  
             |   /               \  /  
             | /                  \/  
      /-> APPLY -----------> REVERT --\  
      |                               |  
      \-------------------------------/  

</pre>
Or an state transition table of valid states:
<pre>

+-------+-------+--------+--------+---------+-------+------------------+  
| CHECK | APPLY | REVERT | UNLOAD | Current | Next  | Result           |  
+-------+-------+--------+--------+---------+-------+------------------+  
|   x   |       |        |        | LOADED  | CHECK | Check payload.   |  
+-------+-------+--------+--------+---------+-------+------------------+  
|   x   |       |        |        | CHECK   | CHECK | Check payload.   |  
+-------+-------+--------+--------+---------+-------+------------------+  
|       |   x   |        |        | CHECK   | APPLY | Apply payload.   |  
+-------+-------+--------+--------+---------+-------+------------------+  
|       |       |        |   x    | CHECK   | UNLOAD| Unload payload.  |  
+-------+-------+--------+--------+---------+-------+------------------+  
|       |       |   x    |        | APPLY   | REVERT| Revert payload.  |  
+-------+-------+--------+--------+---------+-------+------------------+  
|       |       |        |   x    | APPLY   | UNLOAD| unload payload.  |  
+-------+-------+--------+--------+---------+-------+------------------+  
|       |   x   |        |        | REVERT  | APPLY | Apply payload.   |  
+-------+-------+--------+--------+---------+-------+------------------+  

</pre>
All the other states are invalid.

## Sequence of events.

The normal sequence of events is to:

1. *XEN_SYSCTL_XSPLICE_UPLOAD* to upload the payload. If there are errors *STOP* here.
2. *XEN_SYSCTL_XSPLICE_GET* to check the ->status. If in *XSPLICE_STATUS_PROGRESS* spin. If in *XSPLICE_STATUS_LOADED* go to next step.
3. *XEN_SYSCTL_XSPLICE_ACTION* with *XSPLICE_ACTION_CHECK* command to verify that the payload can be succesfully applied.
4. *XEN_SYSCTL_XSPLICE_GET* to check the ->status. If in *XSPLICE_STATUS_PROGRESS* spin. If in *XSPLICE_STATUS_CHECKED* go to next step.
5. *XEN_SYSCTL_XSPLICE_ACTION* with *XSPLICE_ACTION_APPLY* to apply the patch.
6. *XEN_SYSCTL_XSPLICE_GET* to check the ->status. If in *XSPLICE_STATUS_PROGRESS* spin. If in *XSPLICE_STATUS_APPLIED* exit with success.

## Addendum

Implementation quirks should not be discussed in a design document.

However these observations can provide aid when developing against this document.

### Alternative assembler

Alternative assembler is a mechanism to use different instructions depending on what the CPU supports. This is done by providing multiple streams of code that can be patched in - or if the CPU does not support it - padded with nop operations. The alternative assembler macros cause the compiler to expand the code to place a most generic code in place - emit a special ELF .section header to tag this location. During run-time the hypervisor can leave the areas alone or patch them with an better suited opcodes.

However these sections are part of .init. and as such can't reasonably be subject to patching.

### .rodata sections

The patching might require strings to be updated as well. As such we must be also able to patch the strings as needed. This sounds simple - but the compiler has a habit of coalescing strings that are the same - which means if we in-place alter the strings - other users will be inadvertently affected as well.

This is also where pointers to functions live - and we may need to patch this as well.

To guard against that we must be prepared to do patching similar to trampoline patching or in-line depending on the flavour. If we can do in-line patching we would need to:

* alter .rodata to be writeable.
* inline patch.
* alter .rodata to be read-only.

If are doing trampoline patching we would need to:

* allocate a new memory location for the string.
* all locations which use this string will have to be updated to use the offset to the string.
* mark the region RO when we are done.

### .bss and .data sections.

Patching writable data is not suitable as it is unclear what should be done depending on the current state of data. As such it should not be attempted.

### Patching code which is in the stack.

We should not patch the code which is on the stack. That can lead to corruption.

### Trampoline (e9 opcode)

The e9 opcode used for jmpq uses a 32-bit signed displacement. That means we are limited to up to 2GB of virtual address to place the new code from the old code. That should not be a problem since Xen hypervisor has a very small footprint.
However if we need - we can always add two trampolines. One at the 2GB limit that calls the next trampoline.

### When to patch

During the discussion on the design two candidates bubbled where the call stack for each CPU would be deterministic. This would minimize the chance of the patch not being applied due to safety checks failing.

#### Rendezvous code instead of stop_machine for patching

The hypervisor's time rendezvous code runs synchronously across all CPUs every second. Using the stop_machine to patch can stall the time rendezvous code and result in NMI. As such having the patching be done at the tail of rendezvous code should avoid this problem.

#### Before entering the guest code.

Before we call VMXResume we check whether any soft IRQs need to be executed. This is a good spot because all Xen stacks are effectively empty at that point.

To randezvous all the CPUs an barrier with an maximum timeout (which could be adjusted), combined with forcing all other CPUs through the hypervisor with IPIs, can be utilized to have all the CPUs be lockstep.

The approach is similar in concept to stop_machine and the time rendezvous but is time-bound.

### Compiling the hypervisor code

Hotpatch generation often requires support for compiling the target with -ffunction-sections / -fdata-sections. Changes would have to be done to the linker scripts to support this.

### Generation of xSplice ELF payloads

The design of that is not discussed in this design.

The author of this design envisions objdump and objcopy along with special GCC parameters (see above) to create .o.xsplice files which can be used to splice an ELF with the new payload.

### Exception tables and symbol tables growth

We may need support for adapting or augmenting exception tables if patching such code. Hotpatches may need to bring their own small exception tables (similar to how Linux modules support this).

If supporting hotpatches that introduce additional exception-locations is not important, one could also change the exception table in-place and reorder it afterwards.

### xSplice interdependencies

xSplice patches interdependencies are tricky.

There are the ways this can be addressed:
* A single large patch that subsumes and replaces all previous ones. Over the life-time of patching the hypervisor this large patch grows to accumulate all the code changes.
* Hotpatch stack - where an mechanism exists that loads the hotpatches in the same order they were built in. We would need an build-id of the hypevisor to make sure the hot-patches are build against the correct build.
* Payload containing the old code to check against that. That allows the hotpatches to be loaded indepedently (if they don't overlap) - or if the old code also containst previously patched code - even if they overlap.

The disadvantage of the first large patch is that it can grow over time and not provide an bisection mechanism to identify faulty patches.

The hot-patch stack puts stricts requirements on the order of the patches being loaded and requires an hypervisor build-id to match against.

The old code allows much more flexibility and an additional guard, but is more complex to implement.

### Hypervisor ID (buid-id)

The build-id can help with:

* Prevent loading of wrong hotpatches (intended for other builds)

* Allow to identify suitable hotpatches on disk and help with runtime tooling (if laid out using build ID)

The build-id (aka hypervisor id) can be easily obtained by utilizing the ld --build-id operatin which (copied from ld):

<pre>
--build-id
--build-id=style
Request creation of ".note.gnu.build-id" ELF note section. The contents of the note are unique bits identifying this linked file. style can be "uuid" to use 128 random bits, "sha1" to use a 160-bit SHA1 hash on the normative parts of the output contents, "md5" to use a 128-bit MD5 hash on the normative parts of the output contents, or "0xhexstring" to use a chosen bit string specified as an even number of hexadecimal digits ("-" and ":" characters between digit pairs are ignored). If style is omitted, "sha1" is used.

The "md5" and "sha1" styles produces an identifier that is always the same in an identical output file, but will be unique among all nonidentical output files. It is not intended to be compared as a checksum for the file's contents. A linked file may be changed later by other tools, but the build ID bit string identifying the original linked file does not change.

Passing "none" for style disables the setting from any "--build-id" options earlier on the command line.

</pre>

### Symbol names

Xen as it is now, has a couple of non-unique symbol names which will make runtime symbol identification hard. Sometimes, static symbols simply have the same name in C files, sometimes such symbols get included via header files, and some C files are also compiled multiple times and linked under different names (guest_walk.c).

As such we need to modify the linker to make sure that the symbol table qualifies also symbols by their source file name.

find . -name \*.c -print0 | xargs -0 -n1 basename | sort | uniq -c | sort -n | tail -n10

For the awkward situations in which C-files are compiled multiple times patches we would need to some modification in the Xen code.

### Security

Only the privileged domain should be allowed to do this operation.

Hi,

I have a couple of high-level points here:

* Do you think it would be reasonable to have a minimal design without in-place patching, just using trampolines? The examples discussed below suggest so.

* Regarding tboot integration: Do you plan to report hotpatching state to guests or do you want to keep that unchanged?

* If reporting: Should we hash / sign the hotpatches in load order or should we work on an otherwise ordered set?

In other words, do the sequences
1: load + activate A, load + activate B
2: load + activate B, load + activate A

Result in the same system state and hash or not?

* Should we consider pairs of activation / deactivation events as no-ops?

In other words, should hypervisor X and hypervisor X after activating and deactivating module A report the same system state and hash?

* What about auditing? Currently the design discusses a method to query about the current state of affairs with regard to hotpatch modules. Do we need something like a audit log for hotpatch operations? We should at least report high-level operations that could impact the integrity on the console with a low threshold.

* There is a general (and mostly obscure) limitation on unloading hotpatches:

In contrast to normal kernel modules where the module code adheres to specific conventions around resource allocation and locking, hotpatches typically contain code from any context. That code is usually not aware that it can be unloaded.

That code could leave behind in Xen references to itself, e.g., by installing a function pointer in a global data structure, without incrementing something like a usage count. While most hotpatch code will probably be very simple and small, a similar effect could even be achieved by code called from the hotpatch in Xen, e.g., some code patch could dynamically generate a backtrace and later decide to inspect individual elements from the collected trace, later being a time, where the hotpatch has been unloaded again.

One could approach that proplem from multiple angles: code inspection of generated hotpatches, testing, and by making unloading a very special and exceptional operation.

... and more inline comments below.

Regards,
Martin Pohlack

Quoted 53 lines · Konrad Rzeszutek Wilk

Since RFC v2 [http://lists.xen.org/archives/html/xen-devel/2015-05/msg02142.html]
- Ingested every review comment in.

For those who prefer an diff of what changed between v2 and this I am attaching an diff to help easy reviewing.

Please see inline the RFC v3 which in general:
- Ditches the attempt at defining an ELF payload using semi-Elf language and just concentrates on structures.
- Expands on the preemption of the hypercalls
- Expands the implementation details with various topics that emerged during v2 review
- Adds ASCII art (if you can call it that), and an example.
- state diagram the command hypercall.

# xSplice Design v1 (EXTERNAL RFC v3)

## Rationale

A mechanism is required to binarily patch the running hypervisor with new opcodes that have come about due to primarily security updates.

This document describes the design of the API that would allow us to upload to the hypervisor binary patches.

The document is split in four sections:
- Detailed descriptions of the problem statement.
- Design of the data structures.
- Design of the hypercalls.
- Implementation notes that should be taken into consideration.

## Glossary

* splice - patch in the binary code with new opcodes
* trampoline - a jump to a new instruction.
* payload - telemetries of the old code along with binary blob of the new function (if needed).
* reloc - telemetries contained in the payload to construct proper trampoline.

## Multiple ways to patch

The mechanism needs to be flexible to patch the hypervisor in multiple ways and be as simple as possible. The compiled code is contiguous in memory with no gaps - so we have no luxury of 'moving' existing code and must either insert a trampoline to the new code to be executed - or only modify in-place the code if there is sufficient space. The placement of new code has to be done by hypervisor and the virtual address for the new code is allocated dynamically.

This implies that the hypervisor must compute the new offsets when splicing in the new trampoline code. Where the trampoline is added (inside the function we are patching or just the callers?) is also important.

Quoted 2 lines

To lessen the amount of code in hypervisor, the consumer of the API is responsible for identifying which mechanism to employ

The hypervisor at least needs to make sure that in-place patches fit in the old place.

Quoted 6 lines

and how many locations
to patch. Combinations of modifying in-place code, adding trampoline, etc has to be supported. The API should allow read/write any memory within the hypervisor virtual address space.

Quoted 2 lines

We must also have a mechanism to query what has been applied and a mechanism to revert it if needed.

In-place modifications are more restrictive in when they can be applied and need undo buffers in the size of the replacement code. Trampolines only need small fixed-size undo buffers (e.g., 5 bytes on x86).

If you have multiple levels of patching for a function, mixing the different types can become tricky. We need to think about where in-place patches should go, always to the original function or also potentially into replacement functions spliced via trampolines?

I feel that a pure trampoline mechanism could be enough and would result in the smallest hypervisor code footprint.

In which scenarios would you prefer an in-place solution?

Quoted 5 lines

We must also have a mechanism to: provide an copy of the old code - so that the hypervisor can verify it against the code in memory; the new code; the symbol name of the function to be patched; or offset from the symbol; or virtual address.

I am a bit confused by the "must" above:

If you consider my previous build ID suggestion as a hash over all hypervisor code and you embed a designated build ID in each hotpatch module together with a list of required previous modules, comparing the two IDs and checking for required modules gives you a *safety* guarantee of similar strength with a simpler implementation, right?

Providing a copy of the old code provides you with additional flexibility as you can potentially load a specific hotpatch into more Xen builds or you have more freedom regarding module load order.

We should think a bit about expected workflows of higher-level tools that manage hotpatch stacks on production machines:

* The first obvious task is loading all available / suggested hotpatches around system start.
* Whenever new hotpatches are installed, they should be loaded too.
* One wants to query which modules have been loaded at runtime.
* If unloading is deemed safe (see comment far below), one may want to support a workflow where a specific hotpatch is marked as bad and unloaded.
* If we do no restrict module activation order and want to report tboot state on sequences, we might have a complexity explosion problem, in what system hashes should be considered acceptable.

Quoted 10 lines

The complications that this design will encounter are explained later in this document.

## Patching code

The first mechanism to patch that comes in mind is in-place replacement. That is replace the affected code with new code. Unfortunately the x86 ISA is variable size which places limits on how much space we have available to replace the instructions.

I see no special limitation here for variable instruction size ISAs except that you may have to fill the end of a smaller in-place patch with nops. You will have similar space trouble with all ISAs when the replacement code is longer.

Quoted 29 lines

The second mechanism is by replacing the call or jump to the old function with the address of the new function.

A third mechanism is to add a jump to the new function at the start of the old function.

### Example of trampoline and in-place splicing

As example we will assume the hypervisor does not have XSA-132 (see *domctl/sysctl: don't leak hypervisor stack to toolstacks* 4ff3449f0e9d175ceb9551d3f2aecb59273f639d) and we would like to binary patch the hypervisor with it. The original code looks as so:

<pre>

   48 89 e0                  mov    %rsp,%rax  
   48 25 00 80 ff ff         and    $0xffffffffffff8000,%rax  

</pre>

while the new patched hypervisor would be:

<pre>

   48 c7 45 b8 00 00 00 00   movq   $0x0,-0x48(%rbp)  
   48 c7 45 c0 00 00 00 00   movq   $0x0,-0x40(%rbp)  
   48 c7 45 c8 00 00 00 00   movq   $0x0,-0x38(%rbp)  
   48 89 e0                  mov    %rsp,%rax  
   48 25 00 80 ff ff         and    $0xffffffffffff8000,%rax  

</pre>

Quoted 4 lines

This is inside the arch_do_domctl. This new change adds 21 extra bytes of code which alters all the offsets inside the function. To alter these offsets and add the extra 21 bytes of code we might not have enough space in .text to squeze this in.

squeze -> squeeze

Quoted 28 lines

As such we could simplify this problem by only patching the site which calls arch_do_domctl:

<pre>
<do_domctl>:
e8 4b b1 05 00 callq ffff82d08015fbb9 <arch_do_domctl>
</pre>

with a new address for where the new arch_do_domctl would be (this area would be allocated dynamically).

Astute readers will wonder what we need to do if we were to patch do_domctl
- which is not called directly by hypervisor but on behalf of the guests via the compat_hypercall_table and hypercall_table.
Patching the offset in hypercall_table for `do_domctl:
(ffff82d080103079 <do_domctl>:)
<pre>

ffff82d08024d490: 79 30
ffff82d08024d492: 10 80 d0 82 ff ff

</pre>
with the new address where the new do_domctl is possible. The other place where it is used is in hvm_hypercall64_table which would need to be patched in a similar way. This would require an in-place splicing of the new virtual address of arch_do_domctl.

Quoted 3 lines

In summary this example patched the callee of the affected function by
* allocating memory for the new code to live in,
* changing the virtual address of all the functions which called the old

"of all" -> "in all" ?

Quoted 8 lines

code (computing the new offset, patching the callq with a new callq).
* changing the function pointer tables with the new virtual address of the function (splicing in the new virtual address). Since this table resides in the .rodata section we would need to temporarily change the page table permissions during this part.

Quoted 3 lines

However it has severe drawbacks - the safety checks which have to make sure the function is not on the stack - must also check every caller. For some patches this could if there were an sufficient large amount of callers

"could" -> "could mean"

Quoted 30 lines

that we would never be able to apply the update.

### Example of different trampoline patching.

An alternative mechanism exists where we can insert an trampoline in the existing function to be patched to jump directly to the new code. This lessens the locations to be patched to one but it puts pressure on the CPU branching logic (I-cache, but it is just one unconditional jump).

For this example we will assume that the hypervisor has not been compiled with fe2e079f642effb3d24a6e1a7096ef26e691d93e (XSA-125: *pre-fill structures for certain HYPERVISOR_xen_version sub-ops*) which mem-sets an structure in xen_version hypercall. This function is not called **anywhere** in the hypervisor (it is called by the guest) but referenced in the compat_hypercall_table and hypercall_table (and indirectly called from that). Patching the offset in hypercall_table for the old do_xen_version (ffff82d080112f9e <do_xen_version>)

</pre>
ffff82d08024b270 <hypercall_table>
...
ffff82d08024b2f8: 9e 2f 11 80 d0 82 ff ff

</pre>
with the new address where the new do_xen_version is possible. The other place where it is used is in hvm_hypercall64_table which would need to be patched in a similar way. This would require an in-place splicing of the new virtual address of do_xen_version.

Quoted 1 line

An alternative solution would be to patch insert an trampoline in the

"an trampoline" -> "a trampoline"

Quoted 17 lines

old do_xen_version' function to directly jump to the new do_xen_version`.

<pre>
ffff82d080112f9e <do_xen_version>:

 ffff82d080112f9e:       48 c7 c0 da ff ff ff    mov    $0xffffffffffffffda,%rax  
 ffff82d080112fa5:       83 ff 09                cmp    $0x9,%edi  
 ffff82d080112fa8:       0f 87 24 05 00 00       ja     ffff82d0801134d2 <do_xen_version+0x534>  

</pre>

with:

<pre>
ffff82d080112f9e <do_xen_version>:
ffff82d080112f9e: e9 XX YY ZZ QQ jmpq [new do_xen_version]
</pre>

Quoted 8 lines

which would lessen the amount of patching to just one location.

In summary this example patched the affected function to jump to the new replacement function which required:
* allocating memory for the new code to live in,
* inserting trampoline with new offset in the old function to point to the new function.
* Optionally we can insert in the old function an trampoline jump to an function

"an trampoline" -> "a trampoline"

Quoted 1 line

providing an BUG_ON to catch errant code.

Where exactly in the old function would you put that second trampoline?

Quoted 4 lines

The disadvantage of this are that the unconditional jump will consume a small I-cache penalty. However the simplicity of the patching of safety checks make this a worthwhile option.

I don't understand the "of safety checks".

Quoted 8 lines

### Security

With this method we can re-write the hypervisor - and as such we **MUST** be diligent in only allowing certain guests to perform this operation.

Furthermore with SecureBoot or tboot, we **MUST** also verify the signature of the payload to be certain it came from a trusted source.

+ "... and integrity was intact."

Quoted 3 lines

As such the hypercall **MUST** support an XSM policy to limit the what guest is allowed.

"to limit the what guest is allowed." -> "to limit what guest is allowed to invoke it." ?

Quoted 3 lines

If the system is booted with signature checking the signature checking will be enforced.

Are there any plans to extend signature reporting on hotpatch loading or do you plan to hide that completely from guests?

What about a hotpatch loading and unloading sequence? Do you consider the hypervirsor to be in the same state with regard to its signature as before the loading sequence and should it report the same signature after such a pair of operations?

Do you plan for load order to be reflected in such updated signatures, that is, do you plan the hash the set or sequence of hotpatches loaded?

Quoted 8 lines

## Design of payload format

The payload **MUST** contain enough data to allow us to apply the update and also safely reverse it. As such we **MUST** know:

* What the old code is expected to be. We **MUST** be able verify it against the runtime code.

See comment above about build IDs.

Quoted 4 lines

* The locations in memory to be patched. This can be determined dynamically via symbols or via virtual addresses.
* The new code (or data) to will be patched in.

to -> that

Quoted 7 lines

* Signature to verify the payload.

This binary format can be constructed using an custom binary format but there are severe disadvantages of it:

* The format might need to be change and we need an mechanism to accommodate

change -> changed

Quoted 6 lines

that.
* It has to be platform agnostic.
* Easily constructed using existing tools.

As such having the payload in an ELF file is the sensible way. We would be carrying the various set of structures (and data) in the ELF sections under

set -> sets

Quoted 13 lines

different names and with definitions. The prefix for the ELF section name would always be: *.xsplice* to match up to the names of the structures.

Note that every structure has padding. This is added so that the hypervisor can re-use those fields as it sees fit.

Earlier design attempted to ineptly explain the relations of the ELF sections to each other without using proper ELF mechanism (sh_info, sh_link, data structures using Elf_* types, etc). This design will explain in detail the structures and how they are used together and not dig in the ELF format - except mention that the section names should match the structure names.

Quoted 3 lines

### ASCII art of structures.

The diagram below is omitting some entries to easy the relationship explanation.

ommitting -> omitting

Quoted 36 lines

<pre>

                                                                          /---------------------\  
                                                                       +->| xsplice_reloc_howto |  
                                                                      /   \---------------------/  
                                                /---------------\ 1:1/  
                                             +->| xsplice_reloc |   /  
                                            /   | - howto       +--/  1:1 /----------------\  
                                           /    | - symbol      +-------->| xsplice_symbol |  
                                     1:N  /     \---------------/       / \----------------/  
/----------\        /--------------\     /                             /  
| xsplice  |  1:1   | xsplice_code |    /                          1:1/  
| - new    +------->|  - relocs    +---/  1:N   /-----------------\  /  
| - old    +------->|  - sections  +----------->| xsplice_section | /  
\----------/        |  - patches   +--\         | - symbol        +/ 1:1   /----------------\  
                    \--------------/   \        | - addr          +------->| .text or .data |  
                                        \       \----------------/         \----------------/  
                                         \  
                                      1:N \  
                                           \    /----------------\  
                                            +-->| xsplice_patch  |  1:1  /----------------\  
                                                | - content      +------>| binary code or |  
                                                \----------------/       | data           |  
                                                                         \----------------/  

</pre>

### xsplice structures

From the top (or left in the above diagram) the structures are:

* xsplice. The top most structure - contains the the name of the update, the id to match against the hypervisor, the pointer to the metadata for the new code and optionally the metadata for the old code.

Quoted 5 lines

* xsplice_code. The structure that ties all of this together and defines the payload. Contains arrays of xsplice_reloc, xsplice_section, and xsplice_patch.

* xsplice_reloc contains telemtry used for patching - which describes the

telemtry -> telemetry

Quoted 14 lines

targets to be patched and how to do it.

* xsplice_section - the safety data for the code. Contains pointer to the symbol (xsplice_symbols) and pointer to the code (.text) or data (.data), which are to be used during safety and dependency checking.

* xsplice_patch: the description of the new function to be patched in along with the binary code or data.

* xsplice_reloc_howto: the howto properly construct trampolines for an patch. We may have multiple locations for which we need to insert an trampoline for a payload and each location might require a different way of handling it.

Quoted 6 lines

* xsplice_symbols . The symbol that will be patched.

In short the *.xsplice* sections (with xsplice being the top) represent various structures to define the new code and safety checks for the old code (optional). The ELF provides the mechanism to glue it all together when loaded in memory.

I am wondering if it would be useful to provide special call-once hooks for activation and deactivation for scenarios where once-off data transformations should happen when activating or deactivating a patch, for example, for small changes to data structures. Or for debug-type modules that allow to dump or change some runtime state (debug level).

Quoted 16 lines

Note that a lot of these ideas are borrowed from kSplice which is available at: https://github.com/jirislaby/ksplice

### struct xsplice

The top most structure is quite simple. It defines the name, the id of the hypervisor, pointer to the new code and an pointer to the old code (optional).

The new code uses all of the xsplice_* structures while the old code does not use the xsplice_reloc structures.

The sections defining the structures will explicitly state when they are not used.

Quoted 3 lines

<pre>
struct xsplice {
const char *name; /* A sensible name for the patch. Up to 40 characters. */

A version for the module might come in handy, depending on the tooling around it. This information could also be encoded in the name.

Quoted 2 lines

const char *id; /* ID of the hypervisor this binary was built against. */

Should we define an ID type here with a fixed length? Otherwise you would need to rely on 0 termination and store the ID as hex string instead of binary.

Quoted 27 lines

struct xsplice_code *new; /* Pointer to the new code to be patched. */ struct xsplice_code *old; /* Pointer to the old code to be checked against. */ uint8_t pad[32]; /* Must be zero. */
};
</pre>

The size of this structure should be 64 bytes.

### xsplice_code

The structure embedded within this section ties the other structures together. It has the pointers with an start and end address for each set of structures. This means that an update can be split in multiple changes - for example to accomodate an update that contains both code and data and will need patching in both .text and .data sections.

<pre>
struct xsplice_code {
struct xsplice_reloc *relocs, *relocs_end; /* How to patch it. */ struct xsplice_section *sections, *sections_end; /* Safety data. */ struct xsplice_patch *patches, *patches_end; /* Patch code and data */ uint8_t pad[16]; /* Must be zero. */
};
</pre>

Quoted 4 lines

The size of this structure is 64 bytes.

There can be at most two of those structures in the payload. One for the new code and another for the old code (optional).

You mentioned several times that you also want to support patching data sections but restrict yourself here to code. Would there be a use-case for having the old-data that you expect to be there before applying changes?

Quoted 54 lines

If it is for the old code the relocs, and relocs_end values will be ignored.

### xsplice_reloc

The xsplice_code defines an array of these structures. As such an singular structure defines an singular point where to patch the hypervisor.

The structure contains the address of the hypervisor (if known), the symbol associated with this address, how the patching is to be done, and platform specific details.

The isns_added is an value to be used to compute the new offset due to the quirks of the operands of the instruction. For example to patch in an jump operation to the new code - the offset is relative to the program counter of the next instruction - hence the offset value has to be subtracted by four bytes - hence this would contain -4 .

The isns_target is the offset against the symbol.

The relation of this structure with xsplice_patch is 1:1, even for inline patches. See the section detailing the structure xsplice_reloc_howto.

The relation of this structure with xsplice_section is 1:1.

This structure is as follow:

<pre>
struct xsplice_reloc {
uint64_t addr; /* The address of the relocation (if known). */
struct xsplice_symbol *symbol; /* Symbol for this relocation. */
int64_t isns_target; /* rest of the ELF addend. This is equal to the offset against the symbol that the relocation refers to. */
struct xsplice_reloc_howto *howto; /* Pointer to the above structure. */
int64_t isns_added; /* ELF addend resulting from quirks of instruction one of whose operands is the relocation. For example, this is -4 on x86 pc-relative jumps. */ uint8_t pad[24]; /* Must be zero. */
};

</pre>

The size of this structure is 64 bytes.

### xsplice_section

The structure defined in this section is used during pre-patching and during patching. Pre-patching it is used to verify that it is safe to update with the new changes - and contains safety data on the old code and what kind of matching we are to expect.

That is whether the address (either provided or resolved when payload is loaded by referencing the symbols) is:

Quoted 5 lines

* in memory,
* correct size,
* in it's proper ELF section,
* has been already patched (or not),
* is expected not to be the CPU stack - (or it is OK for it be on the CPU stack).

"the CPU stack" -> "on any CPU stack"?

Quoted 12 lines

with what we expect it to be.

Some of the checks can be relaxed, as such the flag values can be or-ed together.

<pre>

#define XSPLICE_SECTION_TEXT 0x00000001 /* Section is in .text */
#define XSPLICE_SECTION_RODATA 0x00000002 /* Section is in .rodata */
#define XSPLICE_SECTION_DATA 0x00000004 /* Section is in .data */
#define XSPLICE_SECTION_STRING 0x00000008 /* Section is in .str */

Quoted 3 lines

#define XSPLICE_SECTION_TEXT_INLINE 0x00000200 /* Change is to be inline. */ #define XSPLICE_SECTION_MATCH_EXACT 0x00000400 /* Must match exactly. */ #define XSPLICE_SECTION_NO_STACKCHECK 0x00000800 /* Do not check the stack. */

Depending on the time when patching is done, stack checking might not be required.

For stack checking, do you need to compile with framepointers?

Quoted 20 lines

struct xsplice_section {
struct xsplice_symbol *symbol; /* The symbol associated with this change. */ uint64_t address; /* The address of the section (if known). */ uint32_t size; /* The size of the section. */
uint32_t flags; /* Various XSPLICE_SECTION_* flags. */
uint8_t pad[12]; /* To be zero. */
};

</pre>

The size of this structure is 32 bytes.

### xsplice_patch

This structure has the binary code (or data) to be patched. Depending on the type it can either an inline patch (data or text) or require an relocation change (which requires an trampoline). Naturally it also points to a blob of the binary data to patch in, and the size of the patch.

Quoted 2 lines

The addr is used when the patch is for inline change. If it is an relocation (requiring an trampoline), the addr should be zero.

Why? Do you expect inline patches not to cover whole functions?

Could you not still address the area of the inline relative to a symbol?

Quoted 50 lines

There must be an corresponding struct xsplice_reloc and struct xsplice_section describing this patch.

<pre>

#define XSPLICE_PATCH_INLINE_TEXT   0x1
#define XSPLICE_PATCH_INLINE_DATA   0x2
#define XSPLICE_PATCH_RELOC_TEXT    0x3

struct xsplice_patch {  
    uint32_t type; /* XSPLICE_PATCH_* .*/  
    uint32_t size; /* Size of patch. */  
    uint64_t addr; /* The address of the inline new code (or data). */  
    void *content; /* The bytes to be installed. */  
    uint8_t pad[40]; /* Must be zero. */  
};

</pre>

The size of this structure is 64 bytes.

### xsplice_symbols

The structure contains an pointer to the name of the ELF symbol to be patched and as well an unique name for the symbol.

The label is used for diagnostic purposes - such as including the name and the offset.

The structure is as follow:

<pre>
struct xsplice_symbol {
const char *name; /* The ELF name of the symbol. */ const char *label; /* A unique xSplice name for the symbol. */ uint8_t pad[16]; /* Must be zero. */
};
</pre>

The size of this structure is 32 bytes.

### xsplice_reloc_howto

The howto defines in the detail the change. It contains the type, whether the relocation is relative, the size of the relocation, bitmask for which parts of the instruction or data are to be replaced, amount the final relocation is shifted by (to drop unwanted data), and whether the replacement should be interpreted as signed value.

Quoted 5 lines

The structure is as follow:

<pre>
#define XSPLICE_HOWTO_RELOC_INLINE 0x1 /* It is an inline replacement. */
#define XSPLICE_HOWTO_RELOC_PATCH 0x2 /* Add an trampoline. */

_RELOC_ ...

Quoted 6 lines

#define XSPLICE_HOWTO_FLAG_PC_REL 0x1 /* Is PC relative. */
#define XSPLICE_HOWOT_FLAG_SIGN 0x2 /* Should the new value be treated as signed value. */

struct xsplice_reloc_howto {  
    uint32_t    type; /* XSPLICE_HOWTO_* */  

... refers to type? Rename to relationship clear?

Quoted 48 lines

    uint32_t    flag; /* XSPLICE_HOWTO_FLAG_* */  
    uint32_t    size; /* Size, in bytes, of the item to be relocated. */  
    uint32_t    r_shift; /* The value the final relocation is shifted right by; used to drop unwanted data from the relocation. */  
    uint64_t    mask; /* Bitmask for which parts of the instruction or data are replaced with the relocated value. */  
    uint8_t     pad[8]; /* Must be zero. */  

};

</pre>

The size of this structure is 32 bytes.

### Example

There is a wealth of information that the payload must have to define a simple patch. For this example we will assume that the hypervisor has not been compiled with fe2e079f642effb3d24a6e1a7096ef26e691d93e (XSA-125: *pre-fill structures for certain HYPERVISOR_xen_version sub-ops*) which mem-sets an structure in xen_version hypercall. This function is not called **anywhere** in the hypervisor (it is called by the guest) but referenced in the compat_hypercall_table and hypercall_table (and indirectly called from that). There are two ways to patch this:
inline patch hvm_hypercall64_table and hvm_hypercall with a new address for the new do_xen_version , or insert
trampoline in do_xen_version code. The example will focus on the later.

The do_xen_version code is located at virtual address ffff82d080112f9e.

<pre>
struct xsplice_code xsplice_xsa125;
struct xsplice_reloc relocs[1];
struct xsplice_section sections[1];
struct xsplice_patch patches[1];
struct xsplice_symbol do_xen_version_symbol;
struct xsplice_reloc_howto do_xen_version_howto;
char do_xen_version_new_code[1728];

#ifndef HYPERVISOR_ID
#define HYPERVISOR_ID "92dd05a61556c554155b1508c9cf67d993336d28" #endif

struct xsplice xsa125 = {
.name = "xsa125",
.id = HYPERVISOR_ID,
.old = NULL,
.new = &xsplice_xsa125,
};

Quoted 3 lines

struct xsplice_code xsplice_xsa125 = {  
    .relocs = &relocs[0],  
    .relocs_end = &relocs[0],  

Odd to have end point to the beginning of the last element. An "int n_relocs = 1" feels more readable and does not have to be relocated.

Similar for all _end pointers below.

Quoted 48 lines

    .sections = &sections[0],  
    .sections_end = &sections[0],  
    .patches = &patches[0],  
    .patches_end = &patches[0],   
};

struct xsplice_reloc relocs[1] = {  
    {  
        .addr = 0xffff82d080112f9e,  
        .symbol = &do_xen_version_symbol,  
        .isns_target = 0,  
        .howto = &do_xen_version_howto,  
        .isns_added = -4,  
    },  
};  

struct xsplice_symbol do_xen_version_symbol = {  
    .name = "do_xen_version",  
    .label = "do_xen_version+<0x0>",  
};  

struct xsplice_reloc_howto do_xen_version_howto = {
.type = XSPLICE_HOWTO_RELOC_PATCH, .flag = XSPLICE_HOWTO_FLAG_PC_REL, .r_shift = 0,
.mask = (-1ULL),
};

struct xsplice_section sections[1] = {  
    {  
        .symbol = &do_xen_version_symbol,  
        .address = 0xffff82d080112f9e,  
        .size = 1728,  
        .flags = XSPLICE_SECTION_TEXT,  
    },  
};  

struct xsplice_patch patches[1] = {  
    {  
        .type = XSPLICE_PATCH_RELOC_TEXT,  
        .size = 1728,  
        .addr = 0,  
        .content = &do_xen_version_new_code,  
    },  
};  

Quoted 2 lines

char do_xen_version_new_code[1728] = { 0x83, 0xff, 0x09, /* And more code. */}; </pre>

Nice example.

Quoted 8 lines

## Signature checking requirements.

The signature checking requires that the layout of the data in memory **MUST** be same for signature to be verified. This means that the payload data layout in ELF format **MUST** match what the hypervisor would be expecting such that it can properly do signature verification.

Quoted 3 lines

The signature is based on the all of the payloads continuously laid out in memory. The signature is to be appended at the end of the ELF payload prefixed with the string '~Module signature appended~\n", followed by

Unbalanced quotes break my syntax highlighting :-/

Quoted 51 lines

an signature header then followed by the signature, key identifier, and signers name.

Specifically the signature header would be:

<pre>
#define PKEY_ALGO_DSA 0
#define PKEY_ALGO_RSA 1

#define PKEY_ID_PGP 0 /* OpenPGP generated key ID */
#define PKEY_ID_X509 1 /* X.509 arbitrary subjectKeyIdentifier */

#define HASH_ALGO_MD4          0  
#define HASH_ALGO_MD5          1  
#define HASH_ALGO_SHA1         2  
#define HASH_ALGO_RIPE_MD_160  3  
#define HASH_ALGO_SHA256       4  
#define HASH_ALGO_SHA384       5  
#define HASH_ALGO_SHA512       6  
#define HASH_ALGO_SHA224       7  
#define HASH_ALGO_RIPE_MD_128  8  
#define HASH_ALGO_RIPE_MD_256  9  
#define HASH_ALGO_RIPE_MD_320 10  
#define HASH_ALGO_WP_256      11  
#define HASH_ALGO_WP_384      12  
#define HASH_ALGO_WP_512      13  
#define HASH_ALGO_TGR_128     14  
#define HASH_ALGO_TGR_160     15  
#define HASH_ALGO_TGR_192     16  

struct elf_payload_signature {  
	u8	algo;		/* Public-key crypto algorithm PKEY_ALGO_*. */  
	u8	hash;		/* Digest algorithm: HASH_ALGO_*. */  
	u8	id_type;	/* Key identifier type PKEY_ID*. */  
	u8	signer_len;	/* Length of signer's name */  
	u8	key_id_len;	/* Length of key identifier */  
	u8	__pad[3];  
	__be32	sig_len;	/* Length of signature data */  
};

</pre>
(Note that this has been borrowed from Linux module signature code.).

## Hypercalls

We will employ the sub operations of the system management hypercall (sysctl). There are to be four sub-operations:

Quoted 4 lines

* upload the payloads.
* listing of payloads summary uploaded and their state.
* getting an particular payload summary and its state.
* command to apply, delete, or revert the payload.

Runtime querying of the hypervisor ID of the currently active hypervisor could also be part of this design.

Quoted 2 lines

The actions are asynchronous therefore the caller is responsible

"listing" and "getting payload summary" are probably not asynchronous.

Quoted 8 lines

to verify that it has been applied properly by retrieving the summary of it and verifying that there are no error codes associated with the payload.

We **MUST** make it asynchronous due to the nature of patching: it requires every physical CPU to be lock-step with each other. The patching mechanism while an implementation detail, is not an short operation and as such the design **MUST** assume it will be an long-running operation.

I disagree with the strong wording here, but we had some discussions about this before. The specific quiescencing mechanism plays a role here.

Quoted 5 lines

The sub-operations will spell out how preemption is to be handled (if at all).

Furthermore it is possible to have multiple different payloads for the same function. As such an unique id has to be visible to allow proper manipulation.

Unique ID per hotpatch?

Quoted 17 lines

The hypercall is part of the xen_sysctl. The top level structure contains one uint32_t to determine the sub-operations:

<pre>

struct xen_sysctl_xsplice_op {  
    uint32_t cmd;  
	union {  
          ... see below ...  
        } u;  
};  

</pre>
while the rest of hypercall specific structures are part of the this structure.

### XEN_SYSCTL_XSPLICE_UPLOAD (0)

Quoted 7 lines

Upload a payload to the hypervisor. The payload is verified and if there are any issues the proper return code will be returned. The payload is not applied at this time - that is controlled by *XEN_SYSCTL_XSPLICE_ACTION*.

The caller provides:

* id unique id.

The hypervisor could return a unique ID as return value of the operation for future reference. How can the caller generate a unique ID?

After some thinking about this:

* You probably have something like a UUID in mind that stays fixed forever for a given hotpatch module. Maybe a hash-like build ID of the module itself. It could be embedded into the module at creation time.

It could also be used for specifying hotpatch module interdependencies. That would be nice.

* I had a transient ID in mind, that the hypervisor could simply create by counting up a 64-bit value and that would only be used for addressing specific payloads at runtime, but not necessarily uniquely over time and accross different machines.

Quoted 5 lines

* payload the virtual address of where the ELF payload is.

The return value is zero if the payload was succesfully uploaded and the signature was verified. Otherwise an EXX return value is provided.

That sounds synchronous.

Quoted 6 lines

Duplicate id are not supported.

The payload is the ELF payload as mentioned in the Payload format section.

This operation can be preempted by the hypercall returning EAGAIN.

Would the client be expected to submit the request again or should he spin a bit and query for the results?

Quoted 9 lines

This is due to the nature of signature verification - which may require SecureBoot firmware calls which are unbounded.

The structure is as follow:

<pre>

struct xen_sysctl_xsplice_upload {  
    char id[40];  /* IN, name of the patch. */

Does this have to be unique? Earlier there was a distinction between the patch name and a request ID.

I was a bit confused about the different IDs at play here, I suggest to introduce a term (e.g., payload ID) for the IDs referring to a specific payload. It seems you expect the ID for a given payload to remain constant, ideally over time?

Where does the size 40 come from?

Inspecting the common ld --buil-id options, sha1 with 160 bits is the largest and can be represented with 40 characters in ASCII representation or 20 bytes binary.

sha-256 would also fit in binary representation with 32 bytes.

Is there more behind this?

Quoted 36 lines

uint64_t size; /* IN, size of the ELF file. */ XEN_GUEST_HANDLE_64(uint8) payload; /* ELF file. */
};
</pre>

### XEN_SYSCTL_XSPLICE_GET (1)

Retrieve an summary of an specific payload. This caller provides:

* id the unique id.
* status *MUST* be set to zero.

The summary structure contains an summary of payload which includes:

* id the unique id.
* status - whether it has been:
1. *XSPLICE_STATUS_LOADED* (0) has been loaded.
2. *XSPLICE_STATUS_PROGRESS* (1) acting on the **XEN_SYSCTL_XSPLICE_ACTION** command.
3. *XSPLICE_STATUS_CHECKED* (2) the ELF payload safety checks passed.
4. *XSPLICE_STATUS_APPLIED* (3) loaded, checked, and applied.
5. *XSPLICE_STATUS_REVERTED* (4) loaded, checked, applied and then also reverted.
6. Negative values is an error. The error would be of EXX format.

The return value is zero on success and EXX on failure. This operation is synchronous and does not require preemption.

The structure is as follow:

<pre>

#define XSPLICE_STATUS_LOADED    0  
#define XSPLICE_STATUS_PROGRESS  1  
#define XSPLICE_STATUS_CHECKED   2  
#define XSPLICE_STATUS_APPLIED   3  
#define XSPLICE_STATUS_REVERTED  4  

Quoted 2 lines

struct xen_sysctl_xsplice_summary {  
    char id[40];  /* IN/OUT, name of the patch. */  

ID or name (comment)?

Quoted 17 lines

    int32_t status;   /* OUT */  
}; 

</pre>

### XEN_SYSCTL_XSPLICE_LIST (2)

Retrieve an array of abbreviated summary of payloads that are loaded in the hypervisor.

The caller provides:

* version. Initially it *MUST* be zero.
* idx index iterator. Initially it *MUST* be zero.
* count the max number of entries to populate.
* summary virtual address of where to write payload summaries.

Quoted 4 lines

The hypercall returns zero on success and updates the idx (index) iterator with the number of payloads returned, count to the number of remaining payloads, and summary with an number of payload summaries. The version is updated on every hypercall

It remains a bit unclear what is updated when. I am guessing:

Each operation in Xen that mutates relevant payload state updates the single global payload-version field in Xen.

Each invocation of XEN_SYSCTL_XSPLICE_LIST will also return a current copy thereoff. An acquired snapshot in userland is only consistent if all individual operations returned the same version.

Quoted 3 lines

- if it varies from one hypercall to another
the data is stale and further calls could fail.

... or result in an inconsistent snapshot even if returning succesfully?

Quoted 9 lines

If the hypercall returns E2BIG the count is too big and should be lowered.

Note that due to the asynchronous nature of hypercalls the domain might have added or removed the number of payloads making this information stale. It is the responsibility of the toolstack to use the version field to check between each invocation.

Quoted 6 lines

This operation is synchronous and does not require preemption.

The summary structure contains an summary of payload which includes:

* version version of the data.
* id unique id.

Per payload?

Quoted 19 lines

* status - whether it has been:
1. *XSPLICE_STATUS_LOADED* (0) has been loaded.
2. *XSPLICE_STATUS_PROGRESS* (1) acting on the **XEN_SYSCTL_XSPLICE_ACTION** command.
3. *XSPLICE_STATUS_CHECKED* (2) the ELF payload safety checks passed.
4. *XSPLICE_STATUS_APPLIED* (3) loaded, checked, and applied.
5. *XSPLICE_STATUS_REVERTED* (4) loaded, checked, applied and then also reverted.
6. Any negative values means there has been error. The value is in EXX format.

The structure is as follow:

<pre>

struct xen_sysctl_xsplice_list {  
    uint32_t version; /* OUT */
    uint32_t idx;  /* IN/OUT */  
    uint32_t count;  /* IN/OUT */
    XEN_GUEST_HANDLE_64(xen_sysctl_xsplice_summary) summary;  /* OUT */  
};  

Quoted 2 lines

struct xen_sysctl_xsplice_summary {  
    char id[40];  /* OUT, name of the patch. */  

Again, name or module ID?

Quoted 7 lines

    int32_t status;  /* OUT */  
};  

</pre>
### XEN_SYSCTL_XSPLICE_ACTION (3)

Quoted 6 lines

Perform an operation on the payload structure referenced by the id field. The operation request is asynchronous and the status should be retrieved by using either **XEN_SYSCTL_XSPLICE_GET** or **XEN_SYSCTL_XSPLICE_LIST** hypercall.

There are two ways about doing preemption. Either via returning back EBUSY or the mechanism outlined here.

What do you mean by preemption? Are you referring to quiescing Xen?

Quoted 3 lines

Doing it in userland would remove any tracking of states in the hypervisor - except the simple commands apply, unload, and revert.

?

Quoted 80 lines

However we would not be able to patch all the code that is invoked while this hypercall is in progress. That is - the do_domctl, the spinlocks, anything put on the stack, etc.

The disadvantage of the mechanism outlined here is that the hypervisor code has to keep the state atomic and have an upper bound of time on actions. If within the time the operation does not succeed the operation would go in error state.

* id the unique id.
* time the upper bound of time the cmd should take. Zero means infinite.
* cmd the command requested:
1. *XSPLICE_ACTION_CHECK* (1) check that the payload will apply properly.
2. *XSPLICE_ACTION_UNLOAD* (2) unload the payload.
Any further hypercalls against the id will result in failure unless **XEN_SYSCTL_XSPLICE_UPLOAD** hypercall is perfomed with same id.
3. *XSPLICE_ACTION_REVERT* (3) revert the payload. If the operation takes more time than the upper bound of time the status will EBUSY.
4. *XSPLICE_ACTION_APPLY* (4) apply the payload. If the operation takes more time than the upper bound of time the status will be EBUSY.
5. *XSPLICE_ACTION_LOADED* is an initial state and cannot be requested.

The return value will be zero unless the provided fields are incorrect.

The structure is as follow:

<pre>
#define XSPLICE_ACTION_LOADED 0
#define XSPLICE_ACTION_CHECK 1
#define XSPLICE_ACTION_UNLOAD 2
#define XSPLICE_ACTION_REVERT 3
#define XSPLICE_ACTION_APPLY 4

struct xen_sysctl_xsplice_action {  
    char id[40];  /* IN, name of the patch. */  
    uint64_t time; /* IN, upper bound of time (ms) for the operation to take. */  
    uint32_t cmd; /* IN */  
};  

</pre>

## State diagrams of XSPLICE_ACTION values.

There is a strict ordering state of what the commands can be. The XSPLICE_ACTION prefix has been dropped to easy reading:

<pre>

                        /->\  
                        \  /  
             /-------< CHECK <--------\  
             |          |             |  
             |           +            /  
             |    +--->UNLOAD<--\    /  
             |   /               \  /  
             | /                  \/  
      /-> APPLY -----------> REVERT --\  
      |                               |  
      \-------------------------------/  

</pre>
Or an state transition table of valid states:
<pre>

+-------+-------+--------+--------+---------+-------+------------------+  
| CHECK | APPLY | REVERT | UNLOAD | Current | Next  | Result           |  
+-------+-------+--------+--------+---------+-------+------------------+  
|   x   |       |        |        | LOADED  | CHECK | Check payload.   |  
+-------+-------+--------+--------+---------+-------+------------------+  
|   x   |       |        |        | CHECK   | CHECK | Check payload.   |  
+-------+-------+--------+--------+---------+-------+------------------+  
|       |   x   |        |        | CHECK   | APPLY | Apply payload.   |  
+-------+-------+--------+--------+---------+-------+------------------+  
|       |       |        |   x    | CHECK   | UNLOAD| Unload payload.  |  
+-------+-------+--------+--------+---------+-------+------------------+  
|       |       |   x    |        | APPLY   | REVERT| Revert payload.  |  
+-------+-------+--------+--------+---------+-------+------------------+  
|       |       |        |   x    | APPLY   | UNLOAD| unload payload.  |  
+-------+-------+--------+--------+---------+-------+------------------+  
|       |   x   |        |        | REVERT  | APPLY | Apply payload.   |  
+-------+-------+--------+--------+---------+-------+------------------+  

Quoted 2 lines

</pre>
All the other states are invalid.

"states" -> "state transistions" ?

Quoted 40 lines

## Sequence of events.

The normal sequence of events is to:

1. *XEN_SYSCTL_XSPLICE_UPLOAD* to upload the payload. If there are errors *STOP* here.
2. *XEN_SYSCTL_XSPLICE_GET* to check the ->status. If in *XSPLICE_STATUS_PROGRESS* spin. If in *XSPLICE_STATUS_LOADED* go to next step.
3. *XEN_SYSCTL_XSPLICE_ACTION* with *XSPLICE_ACTION_CHECK* command to verify that the payload can be succesfully applied.
4. *XEN_SYSCTL_XSPLICE_GET* to check the ->status. If in *XSPLICE_STATUS_PROGRESS* spin. If in *XSPLICE_STATUS_CHECKED* go to next step.
5. *XEN_SYSCTL_XSPLICE_ACTION* with *XSPLICE_ACTION_APPLY* to apply the patch.
6. *XEN_SYSCTL_XSPLICE_GET* to check the ->status. If in *XSPLICE_STATUS_PROGRESS* spin. If in *XSPLICE_STATUS_APPLIED* exit with success.

## Addendum

Implementation quirks should not be discussed in a design document.

However these observations can provide aid when developing against this document.

### Alternative assembler

Alternative assembler is a mechanism to use different instructions depending on what the CPU supports. This is done by providing multiple streams of code that can be patched in - or if the CPU does not support it - padded with nop operations. The alternative assembler macros cause the compiler to expand the code to place a most generic code in place - emit a special ELF .section header to tag this location. During run-time the hypervisor can leave the areas alone or patch them with an better suited opcodes.

However these sections are part of .init. and as such can't reasonably be subject to patching.

### .rodata sections

The patching might require strings to be updated as well. As such we must be also able to patch the strings as needed. This sounds simple - but the compiler has a habit of coalescing strings that are the same - which means if we in-place alter the strings - other users will be inadvertently affected as well.

Quoted 2 lines

This is also where pointers to functions live - and we may need to patch this as well.

switch-style jump tables sometimes too.

Quoted 24 lines

To guard against that we must be prepared to do patching similar to trampoline patching or in-line depending on the flavour. If we can do in-line patching we would need to:

* alter .rodata to be writeable.
* inline patch.
* alter .rodata to be read-only.

If are doing trampoline patching we would need to:

* allocate a new memory location for the string.
* all locations which use this string will have to be updated to use the offset to the string.
* mark the region RO when we are done.

### .bss and .data sections.

Patching writable data is not suitable as it is unclear what should be done depending on the current state of data. As such it should not be attempted.

### Patching code which is in the stack.

Quoted 2 lines

We should not patch the code which is on the stack. That can lead to corruption.

To clarify:
* Are you referreing to code that is generated on actual stacks (where is that done in Xen?), or
* Code which is target to return addresses that are live in specific CPU stacks?

Quoted 9 lines

### Trampoline (e9 opcode)

The e9 opcode used for jmpq uses a 32-bit signed displacement. That means we are limited to up to 2GB of virtual address to place the new code from the old code. That should not be a problem since Xen hypervisor has a very small footprint.
However if we need - we can always add two trampolines. One at the 2GB limit that calls the next trampoline.

There is a small limitation for trampolines in function entries: The target function (+ trailing padding) must be able to accomodate the trampoline. On x86 with +-2 GB relative jumps, this means 5 bytes are required.

Depending on compiler settings, there are several functions in Xen that are smaller (without inter-function padding).

readelf -sW xen-syms | grep " FUNC " | \
    awk '{ if ($3 < 5) print $3, $4, $5, $8 }'

...
3 FUNC LOCAL wbinvd_ipi
3 FUNC LOCAL shadow_l1_index
...

A compile-time check for, e.g., a minimum alignment of functions or a runtime check that verifies symbol size (+ padding to next symbols) for that in the hypervisor seems advised.

Quoted 8 lines

### When to patch

During the discussion on the design two candidates bubbled where the call stack for each CPU would be deterministic. This would minimize the chance of the patch not being applied due to safety checks failing.

Quoted 6 lines

#### Rendezvous code instead of stop_machine for patching

The hypervisor's time rendezvous code runs synchronously across all CPUs every second. Using the stop_machine to patch can stall the time rendezvous code and result in NMI. As such having the patching be done at the tail of rendezvous code should avoid this problem.

Could you indicate what functions you would expect to be live at this point in each CPU? Can the rendezvous code interrupt other hypervisor operations or is that entered via the softirq mechanism close to return from hypervisor code?

Quoted 23 lines

#### Before entering the guest code.

Before we call VMXResume we check whether any soft IRQs need to be executed. This is a good spot because all Xen stacks are effectively empty at that point.

To randezvous all the CPUs an barrier with an maximum timeout (which could be adjusted), combined with forcing all other CPUs through the hypervisor with IPIs, can be utilized to have all the CPUs be lockstep.

The approach is similar in concept to stop_machine and the time rendezvous but is time-bound.

### Compiling the hypervisor code

Hotpatch generation often requires support for compiling the target with -ffunction-sections / -fdata-sections. Changes would have to be done to the linker scripts to support this.

### Generation of xSplice ELF payloads

Quoted 5 lines

The design of that is not discussed in this design.

The author of this design envisions objdump and objcopy along with special GCC parameters (see above) to create .o.xsplice files which can be used to splice an ELF with the new payload.

I would expect ksplice userland code to be a good source of inspiration at least.

Quoted 71 lines

### Exception tables and symbol tables growth

We may need support for adapting or augmenting exception tables if patching such code. Hotpatches may need to bring their own small exception tables (similar to how Linux modules support this).

If supporting hotpatches that introduce additional exception-locations is not important, one could also change the exception table in-place and reorder it afterwards.

### xSplice interdependencies

xSplice patches interdependencies are tricky.

There are the ways this can be addressed:
* A single large patch that subsumes and replaces all previous ones. Over the life-time of patching the hypervisor this large patch grows to accumulate all the code changes.
* Hotpatch stack - where an mechanism exists that loads the hotpatches in the same order they were built in. We would need an build-id of the hypevisor to make sure the hot-patches are build against the correct build.
* Payload containing the old code to check against that. That allows the hotpatches to be loaded indepedently (if they don't overlap) - or if the old code also containst previously patched code - even if they overlap.

The disadvantage of the first large patch is that it can grow over time and not provide an bisection mechanism to identify faulty patches.

The hot-patch stack puts stricts requirements on the order of the patches being loaded and requires an hypervisor build-id to match against.

The old code allows much more flexibility and an additional guard, but is more complex to implement.

### Hypervisor ID (buid-id)

The build-id can help with:

* Prevent loading of wrong hotpatches (intended for other builds)

* Allow to identify suitable hotpatches on disk and help with runtime tooling (if laid out using build ID)

The build-id (aka hypervisor id) can be easily obtained by utilizing the ld --build-id operatin which (copied from ld):

<pre>
--build-id
--build-id=style
Request creation of ".note.gnu.build-id" ELF note section. The contents of the note are unique bits identifying this linked file. style can be "uuid" to use 128 random bits, "sha1" to use a 160-bit SHA1 hash on the normative parts of the output contents, "md5" to use a 128-bit MD5 hash on the normative parts of the output contents, or "0xhexstring" to use a chosen bit string specified as an even number of hexadecimal digits ("-" and ":" characters between digit pairs are ignored). If style is omitted, "sha1" is used.

The "md5" and "sha1" styles produces an identifier that is always the same in an identical output file, but will be unique among all nonidentical output files. It is not intended to be compared as a checksum for the file's contents. A linked file may be changed later by other tools, but the build ID bit string identifying the original linked file does not change.

Passing "none" for style disables the setting from any "--build-id" options earlier on the command line.

</pre>

### Symbol names

Quoted 8 lines

Xen as it is now, has a couple of non-unique symbol names which will make runtime symbol identification hard. Sometimes, static symbols simply have the same name in C files, sometimes such symbols get included via header files, and some C files are also compiled multiple times and linked under different names (guest_walk.c).

As such we need to modify the linker to make sure that the symbol table qualifies also symbols by their source file name.

The convention for file-type symbols (that would allow to map many symbols to their compilation unit) says that only the basename (i.e., without directories) is embedded. This creates another layer of confusion for duplicate file names in the build tree.

Quoted 1 line

find . -name \*.c -print0 | xargs -0 -n1 basename | sort | uniq -c | sort -n | tail -n10

      3 shutdown.c
      3 sysctl.c
      3 time.c
      3 xenoprof.c
      4 gdbstub.c
      4 irq.c
      5 domain.c
      5 mm.c
      5 pci.c
      5 traps.c

Quoted 7 lines

For the awkward situations in which C-files are compiled multiple times patches we would need to some modification in the Xen code.

### Security

Only the privileged domain should be allowed to do this operation.

From 3ecaf6e01e262273335e3d751c585e5d5d497d5c Mon Sep 17 00:00:00 2001