xen: detect uninitialized xenbus in xenbus_init

PATCH Superseded Stefano Stabellini 3 participants 5 messages Nov 19, 2021 – Nov 22, 2021

Subsystems (2)

Stable branch Xen hypervisor interface

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Stefano Stabellini

author maintainer

2021-11-19 20:29 UTC #0 lore raw

PATCH v2 1/1 xen: detect uninitialized xenbus in xenbus_init

From: Stefano Stabellini <[email protected]>

If the xenstore page hasn't been allocated properly, reading the value of the related hvm_param (HVM_PARAM_STORE_PFN) won't actually return error. Instead, it will succeed and return zero. Instead of attempting to xen_remap a bad guest physical address, detect this condition and return early.

Note that although a guest physical address of zero for HVM_PARAM_STORE_PFN is theoretically possible, it is not a good choice and zero has never been validly used in that capacity.

Also recognize the invalid value of INVALID_PFN which is ULLONG_MAX.

For 32-bit Linux, any pfn above ULONG_MAX would get truncated. Pfns above ULONG_MAX should never be passed by the Xen tools to HVM guests anyway, so check for this condition and return early.

Cc [email protected]

Signed-off-by Stefano Stabellini <[email protected]>

---
Changes in v2:
- add check for ULLONG_MAX (unitialized)
- add check for ULONG_MAX #if BITS_PER_LONG == 32 (actual error)
- add pr_err error message

 drivers/xen/xenbus/xenbus_probe.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

drivers/xen/xenbus/xenbus_probe.c +14 -0

index 94405bb3829e..c7472ff27a93 100644
@@ -951,6 +951,20 @@ static int __init xenbus_init(void)
 		err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
 		if (err)
 			goto out_error;
+		/* Uninitialized. */
+		if (v == 0 || v == ULLONG_MAX) {
+			err = -ENOENT;
+			goto out_error;
+		}
+		/* Avoid truncation on 32-bit. */
+#if BITS_PER_LONG == 32
+		if (v > ULONG_MAX) {
+			pr_err("%s: cannot handle HVM_PARAM_STORE_PFN=%llx > ULONG_MAX\n",
+			       __func__, v);
+			err = -EINVAL;
+			goto out_error;
+		}
+#endif
 		xen_store_gfn = (unsigned long)v;
 		xen_store_interface =
 			xen_remap(xen_store_gfn << XEN_PAGE_SHIFT,

Signature · 1 line

2.25.1

Boris Ostrovsky

Reviewed-by on v2 1/1

2021-11-19 22:32 UTC #1 lore raw

Quoted 12 lines · Stefano Stabellini

From: Stefano Stabellini <[email protected]>
If the xenstore page hasn't been allocated properly, reading the value of the related hvm_param (HVM_PARAM_STORE_PFN) won't actually return error. Instead, it will succeed and return zero. Instead of attempting to xen_remap a bad guest physical address, detect this condition and return early.
Note that although a guest physical address of zero for HVM_PARAM_STORE_PFN is theoretically possible, it is not a good choice and zero has never been validly used in that capacity.

Quoted 8 lines

Also recognize the invalid value of INVALID_PFN which is ULLONG_MAX.
For 32-bit Linux, any pfn above ULONG_MAX would get truncated. Pfns above ULONG_MAX should never be passed by the Xen tools to HVM guests anyway, so check for this condition and return early.
Cc [email protected]
Signed-off-by Stefano Stabellini <[email protected]>

Reviewed-by Boris Ostrovsky <[email protected]>

Jan Beulich

2021-11-22 07:57 UTC #2 lore raw

In reply to Stefano Stabellini

drivers/xen/xenbus/xenbus_probe.c static int __init xenbus_init(void) · line 955

Quoted 8 lines · Stefano Stabellini

drivers/xen/xenbus/xenbus_probe.c +2 -0

@@ -951,6 +951,20 @@ static int __init xenbus_init(void)
 		err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
 		if (err)
 			goto out_error;
+		/* Uninitialized. */
+		if (v == 0 || v == ULLONG_MAX) {

Didn't you have a comment in v1 here regarding the check against 0? Or was that just like now only in the description? IOW I think there ought to be a code comment justifying the theoretically wrong check ...

Also, while I realize there are various other similar assumptions elsewhere, I would generally recommend to avoid such: There's no guarantee that now and forever unsigned long long and uint64_t are the same thing. And it's easy in cases like this one:

		if (!v || !(v + 1)) {

Jan

Stefano Stabellini

author maintainer

2021-11-22 22:04 UTC #3 lore raw

Quoted 13 lines · Jan Beulich

drivers/xen/xenbus/xenbus_probe.c static int __init xenbus_init(void) · line 955
Quoted 8 lines · Stefano Stabellini
drivers/xen/xenbus/xenbus_probe.c +2 -0
@@ -951,6 +951,20 @@ static int __init xenbus_init(void)
 		err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
 		if (err)
 			goto out_error;
+		/* Uninitialized. */
+		if (v == 0 || v == ULLONG_MAX) {
Didn't you have a comment in v1 here regarding the check against 0? Or was that just like now only in the description? IOW I think there ought to be a code comment justifying the theoretically wrong check ...

Yeah, I added all the info in the commit message and shortened the in-code comment this time. I am also happy to keep the details in the in-code comment, e.g.:

/*
* If the xenstore page hasn't been allocated properly, reading the
* value of the related hvm_param (HVM_PARAM_STORE_PFN) won't actually
* return error. Instead, it will succeed and return zero. Instead of
* attempting to xen_remap a bad guest physical address, detect this
* condition and return early.
*
* Note that although a guest physical address of zero for
* HVM_PARAM_STORE_PFN is theoretically possible, it is not a good
* choice and zero has never been validly used in that capacity. *
* Also recognize the invalid value of INVALID_PFN which is ULLONG_MAX. */

Quoted 6 lines

Also, while I realize there are various other similar assumptions elsewhere, I would generally recommend to avoid such: There's no guarantee that now and forever unsigned long long and uint64_t are the same thing. And it's easy in cases like this one:
		if (!v || !(v + 1)) {

I am happy to use this.

Stefano Stabellini

author maintainer

2021-11-22 22:17 UTC #4 lore raw

Quoted 19 lines · Stefano Stabellini

Quoted 13 lines · Jan Beulich
drivers/xen/xenbus/xenbus_probe.c static int __init xenbus_init(void) · line 955
Quoted 8 lines · Stefano Stabellini
drivers/xen/xenbus/xenbus_probe.c +2 -0
@@ -951,6 +951,20 @@ static int __init xenbus_init(void)
 		err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
 		if (err)
 			goto out_error;
+		/* Uninitialized. */
+		if (v == 0 || v == ULLONG_MAX) {
Didn't you have a comment in v1 here regarding the check against 0? Or was that just like now only in the description? IOW I think there ought to be a code comment justifying the theoretically wrong check ...
Yeah, I added all the info in the commit message and shortened the in-code comment this time. I am also happy to keep the details in the in-code comment, e.g.:

Quoted 13 lines

/*
* If the xenstore page hasn't been allocated properly, reading the
* value of the related hvm_param (HVM_PARAM_STORE_PFN) won't actually
* return error. Instead, it will succeed and return zero. Instead of
* attempting to xen_remap a bad guest physical address, detect this
* condition and return early.
*
* Note that although a guest physical address of zero for
* HVM_PARAM_STORE_PFN is theoretically possible, it is not a good
* choice and zero has never been validly used in that capacity. *
* Also recognize the invalid value of INVALID_PFN which is ULLONG_MAX. */

I sent a new version of the patch with the check below and slightly more concise version of this comment.

Quoted 8 lines

Quoted 6 lines
Also, while I realize there are various other similar assumptions elsewhere, I would generally recommend to avoid such: There's no guarantee that now and forever unsigned long long and uint64_t are the same thing. And it's easy in cases like this one:
		if (!v || !(v + 1)) {
I am happy to use this.