binfmt_elf: Use ELF_ET_DYN_BASE only for PIE

RFC PATCH Superseded Kees Cook 3 participants 8 messages Jun 21, 2017

Subsystems (3)

x86 VFS exec and binfmt

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Kees Cook

author

2017-06-21 05:58 UTC #0 lore raw

RFC PATCH 1/1 binfmt_elf: Use ELF_ET_DYN_BASE only for PIE

The ELF_ET_DYN_BASE position was originally intended to keep loaders away from ET_EXEC binaries. (For example, running "/lib/ld-linux.so.2 /bin/cat" might cause the subsequent load of /bin/cat into where the loader had been loaded.) With the advent of PIE (ET_DYN binaries with an INTERP Program Header), ELF_ET_DYN_BASE continued to be used since the kernel was only looking at ET_DYN. However, since ELF_ET_DYN_BASE is traditionally set at the top 1/3rd of the TASK_SIZE, a substantial portion of the address space is unused.

For 32-bit tasks when RLIMIT_STACK is set to RLIM_INFINITY, programs are loaded below the mmap region. This means they can be made to collide (CVE-2017-1000370) or nearly collide (CVE-2017-1000371) with pathological stack regions. Lowering ELF_ET_DYN_BASE solves both by moving programs above the mmap region in all cases, and will now additionally avoid programs falling back to the mmap region by enforcing MAP_FIXED for program loads (i.e. if it would have collided with the stack, now it will fail to load instead of falling back to the mmap region).

To allow for a lower ELF_ET_DYN_BASE, loaders (ET_DYN without INTERP) are loaded into the mmap region, leaving space available for either an ET_EXEC binary with a fixed location or PIE being loaded into mmap by the loader. Only PIE programs are loaded offset from ELF_ET_DYN_BASE, which means architectures can now safely lower their values without risk of loaders colliding with their subsequently loaded programs.

Thanks go to PaX for inspiration on how to approach this solution.

Fixes d1fd836dcf00 ("mm: split ET_DYN ASLR from mmap ASLR")

Signed-off-by Kees Cook <[email protected]>

---
 arch/x86/include/asm/elf.h |  8 ++------
 fs/binfmt_elf.c            | 38 +++++++++++++++++++++++++++++++-------
 2 files changed, 33 insertions(+), 13 deletions(-)

arch/x86/include/asm/elf.h +2 -6

index e8ab9a46bc68..46549973ea98 100644
@@ -245,12 +245,8 @@ extern int force_personality32;
 #define CORE_DUMP_USE_REGSET
 #define ELF_EXEC_PAGESIZE	4096
 
-/* This is the location that an ET_DYN program is loaded if exec'ed.  Typical
-   use of this is to invoke "./ld.so someprog" to test out a new version of
-   the loader.  We need to make sure that it is out of the way of the program
-   that it will "exec", and that there is sufficient room for the brk.  */
-
-#define ELF_ET_DYN_BASE		(TASK_SIZE / 3 * 2)
+/* This is the base location for PIE (ET_DYN with INTERP) loads. */
+#define ELF_ET_DYN_BASE		0x400000UL
 
 /* This yields a mask that user programs can use to figure out what
    instruction set this CPU supports.  This could be done in user space,

fs/binfmt_elf.c +31 -7

index 5075fd5c62c8..a998c7251d1c 100644
@@ -930,13 +930,37 @@ static int load_elf_binary(struct linux_binprm *bprm)
 		if (loc->elf_ex.e_type == ET_EXEC || load_addr_set) {
 			elf_flags |= MAP_FIXED;
 		} else if (loc->elf_ex.e_type == ET_DYN) {
-			/* Try and get dynamic programs out of the way of the
-			 * default mmap base, as well as whatever program they
-			 * might try to exec.  This is because the brk will
-			 * follow the loader, and is not movable.  */
-			load_bias = ELF_ET_DYN_BASE - vaddr;
-			if (current->flags & PF_RANDOMIZE)
-				load_bias += arch_mmap_rnd();
+			/*
+			 * There are effectively two types of ET_DYN
+			 * binaries: programs (i.e. PIE: ET_DYN with INTERP)
+			 * and loaders (ET_DYN without INTERP, since they
+			 * _are_ the ELF interpreter). The loaders must
+			 * be loaded away from programs since the program
+			 * may otherwise collide with the loader (especially
+			 * for ET_EXEC which does not have a randomized
+			 * position). For example to handle invocations of
+			 * "./ld.so someprog" to test out a new version of
+			 * the loader, the subsequent program that the
+			 * loader loads must avoid the loader itself, so
+			 * they cannot share the same load range. Sufficient
+			 * room for the brk must be allocated with the
+			 * loader as well, since brk must be available with
+			 * the loader.
+			 *
+			 * Therefore, programs are loaded offset from
+			 * ELF_ET_DYN_BASE and loaders are loaded into the
+			 * independently randomized mmap region (0 load_bias
+			 * without MAP_FIXED).
+			 */
+			if (elf_interpreter) {
+				load_bias = ELF_ET_DYN_BASE;
+				if (current->flags & PF_RANDOMIZE)
+					load_bias += arch_mmap_rnd();
+				elf_flags |= MAP_FIXED;
+			} else
+				load_bias = 0;
+
+			load_bias -= vaddr;
 			load_bias = ELF_PAGESTART(load_bias);
 			total_size = total_mapping_size(elf_phdata,
 							loc->elf_ex.e_phnum);

Signature · 6 lines

2.7.4


-- 
Kees Cook
Pixel Security

Rik van Riel

2017-06-21 12:07 UTC #1 lore raw

Quoted 57 lines · Kees Cook

The ELF_ET_DYN_BASE position was originally intended to keep loaders away from ET_EXEC binaries. (For example, running "/lib/ld-linux.so.2 /bin/cat" might cause the subsequent load of /bin/cat into where the loader had been loaded.) With the advent of PIE (ET_DYN binaries with an INTERP Program Header), ELF_ET_DYN_BASE continued to be used since the kernel was only looking at ET_DYN. However, since ELF_ET_DYN_BASE is traditionally set at the top 1/3rd of the TASK_SIZE, a substantial portion of the address space is unused.
For 32-bit tasks when RLIMIT_STACK is set to RLIM_INFINITY, programs are loaded below the mmap region. This means they can be made to collide
(CVE-2017-1000370) or nearly collide (CVE-2017-1000371) with pathological
stack regions. Lowering ELF_ET_DYN_BASE solves both by moving programs
above the mmap region in all cases, and will now additionally avoid programs falling back to the mmap region by enforcing MAP_FIXED for program loads (i.e. if it would have collided with the stack, now it will fail to load instead of falling back to the mmap region).
To allow for a lower ELF_ET_DYN_BASE, loaders (ET_DYN without INTERP) are loaded into the mmap region, leaving space available for either an
ET_EXEC binary with a fixed location or PIE being loaded into mmap by the
loader. Only PIE programs are loaded offset from ELF_ET_DYN_BASE, which
means architectures can now safely lower their values without risk of loaders colliding with their subsequently loaded programs.
Thanks go to PaX for inspiration on how to approach this solution.
Fixes d1fd836dcf00 ("mm: split ET_DYN ASLR from mmap ASLR")
Signed-off-by Kees Cook <[email protected]>
---
arch/x86/include/asm/elf.h | 8 ++------
fs/binfmt_elf.c | 38 +++++++++++++++++++++++++++++++----- --
 2 files changed, 33 insertions(+), 13 deletions(-)
arch/x86/include/asm/elf.h +0 -0
index e8ab9a46bc68..46549973ea98 100644
@@ -245,12 +245,8 @@ extern int force_personality32;
  #define CORE_DUMP_USE_REGSET
  #define ELF_EXEC_PAGESIZE	4096
-/* This is the location that an ET_DYN program is loaded if exec'ed.  Typical
-   use of this is to invoke "./ld.so someprog" to test out a new version of
-   the loader.  We need to make sure that it is out of the way of the program
-   that it will "exec", and that there is sufficient room for the brk.  */

arch/x86/include/asm/elf.h extern int force_personality32; · line 249

Quoted 4 lines

arch/x86/include/asm/elf.h +2 -2

-
-#define ELF_ET_DYN_BASE		(TASK_SIZE / 3 * 2)
+/* This is the base location for PIE (ET_DYN with INTERP) loads. */
+#define ELF_ET_DYN_BASE		0x400000UL

This value is good for 32 bit binaries, but for 64 bit binaries you probably want to put ELF_ET_DYN_BASE at 4GB or higher.

The latter is necessary because Android uses the lower 4GB of address space for its JVM runtime, with 32 bit pointers inside that part of the otherwise 64 bit address space.

In other words:

#define ELF_ET_DYN_BASE (mmap_is_ia32() ? 0x400000UL : 0x100000000UL)

Quoted 15 lines

+++ b/fs/binfmt_elf.c
arch/x86/include/asm/elf.h +1 -0
+			 * Therefore, programs are loaded offset
from
arch/x86/include/asm/elf.h +1 -0
+			 * ELF_ET_DYN_BASE and loaders are loaded
into the
arch/x86/include/asm/elf.h +1 -0
+			 * independently randomized mmap region (0
load_bias
arch/x86/include/asm/elf.h +6 -0
+			 * without MAP_FIXED).
+			 */
+			if (elf_interpreter) {
+				load_bias = ELF_ET_DYN_BASE;
+				if (current->flags & PF_RANDOMIZE)
+					load_bias +=
arch_mmap_rnd();

arch/x86/include/asm/elf.h

Quoted 5 lines

arch/x86/include/asm/elf.h +5 -0

+				elf_flags |= MAP_FIXED;
+			} else
+				load_bias = 0;
+
+			load_bias -= vaddr;

I like this, and the big comment telling people how it works :)

Kees Cook

author

2017-06-21 17:23 UTC #2 lore raw

Quoted 8 lines · Rik van Riel

arch/x86/include/asm/elf.h extern int force_personality32; · line 249
Quoted 2 lines · Kees Cook
arch/x86/include/asm/elf.h +2 -0
+/* This is the base location for PIE (ET_DYN with INTERP) loads. */
+#define ELF_ET_DYN_BASE              0x400000UL
This value is good for 32 bit binaries, but for 64 bit binaries you probably want to put ELF_ET_DYN_BASE at 4GB or higher.

Quoted 8 lines

The latter is necessary because Android uses the lower 4GB of address space for its JVM runtime, with 32 bit pointers inside that part of the otherwise 64 bit address space.
In other words:
#define ELF_ET_DYN_BASE (mmap_is_ia32() ? 0x400000UL : 0x100000000UL)

Ah, interesting. Okay, that should be fine. I'll adjust it.

Quoted 15 lines

Quoted 15 lines
+++ b/fs/binfmt_elf.c
+ * Therefore, programs are loaded offset
from
+ * ELF_ET_DYN_BASE and loaders are loaded
into the
+ * independently randomized mmap region (0
load_bias
fs/binfmt_elf.c +6 -0
+                      * without MAP_FIXED).
+                      */
+                     if (elf_interpreter) {
+                             load_bias = ELF_ET_DYN_BASE;
+                             if (current->flags & PF_RANDOMIZE)
+                                     load_bias +=
arch_mmap_rnd();

Quoted 8 lines

fs/binfmt_elf.c static int load_elf_binary(struct linux_binprm *bprm) · line 963
Quoted 5 lines
fs/binfmt_elf.c +5 -0
+                             elf_flags |= MAP_FIXED;
+                     } else
+                             load_bias = 0;
+
+                     load_bias -= vaddr;
I like this, and the big comment telling people how it works :)

Thanks! It looks like your patch for commenting load_bias never got picked up, so I've added some more comments for that and some other things too. (Mostly for all the stuff I have to read a few times when I look at this code.)

-Kees

Signature · 2 lines

Kees Cook
Pixel Security

Daniel Micay

2017-06-21 17:27 UTC #3 lore raw

Quoted 46 lines · Kees Cook

Quoted 14 lines · Rik van Riel
Quoted 3 lines · Kees Cook
+/* This is the base location for PIE (ET_DYN with INTERP) loads. */
+#define ELF_ET_DYN_BASE 0x400000UL
This value is good for 32 bit binaries, but for 64 bit binaries you probably want to put ELF_ET_DYN_BASE at 4GB or higher.
The latter is necessary because Android uses the lower 4GB of address space for its JVM runtime, with 32 bit pointers inside that part of the otherwise 64 bit address space.
Quoted 4 lines
In other words:
#define ELF_ET_DYN_BASE (mmap_is_ia32() ? 0x400000UL :
0x100000000UL)
Ah, interesting. Okay, that should be fine. I'll adjust it.
Quoted 15 lines
Quoted 15 lines
+++ b/fs/binfmt_elf.c
+ * Therefore, programs are loaded offset
from
+ * ELF_ET_DYN_BASE and loaders are loaded
into the
+ * independently randomized mmap region (0
load_bias
fs/binfmt_elf.c +6 -0
+                      * without MAP_FIXED).
+                      */
+                     if (elf_interpreter) {
+                             load_bias = ELF_ET_DYN_BASE;
+                             if (current->flags & PF_RANDOMIZE)
+                                     load_bias +=
arch_mmap_rnd();
Quoted 8 lines
fs/binfmt_elf.c static int load_elf_binary(struct linux_binprm *bprm) · line 963
Quoted 5 lines
fs/binfmt_elf.c +5 -0
+                             elf_flags |= MAP_FIXED;
+                     } else
+                             load_bias = 0;
+
+                     load_bias -= vaddr;
I like this, and the big comment telling people how it works :)

Quoted 7 lines

Thanks! It looks like your patch for commenting load_bias never got picked up, so I've added some more comments for that and some other things too. (Mostly for all the stuff I have to read a few times when I look at this code.)
-Kees

The stack rlimit calculation fix for space potentially lost to ASLR is probably still needed too, right?

Kees Cook

author

2017-06-21 17:28 UTC #4 lore raw

Quoted 52 lines · Daniel Micay

Quoted 51 lines · Kees Cook
Quoted 14 lines · Rik van Riel
Quoted 3 lines · Kees Cook
+/* This is the base location for PIE (ET_DYN with INTERP) loads. */
+#define ELF_ET_DYN_BASE 0x400000UL
This value is good for 32 bit binaries, but for 64 bit binaries you probably want to put ELF_ET_DYN_BASE at 4GB or higher.
The latter is necessary because Android uses the lower 4GB of address space for its JVM runtime, with 32 bit pointers inside that part of the otherwise 64 bit address space.
Quoted 4 lines
In other words:
#define ELF_ET_DYN_BASE (mmap_is_ia32() ? 0x400000UL :
0x100000000UL)
Ah, interesting. Okay, that should be fine. I'll adjust it.
Quoted 15 lines
Quoted 15 lines
+++ b/fs/binfmt_elf.c
+ * Therefore, programs are loaded offset
from
+ * ELF_ET_DYN_BASE and loaders are loaded
into the
+ * independently randomized mmap region (0
load_bias
fs/binfmt_elf.c +6 -0
+                      * without MAP_FIXED).
+                      */
+                     if (elf_interpreter) {
+                             load_bias = ELF_ET_DYN_BASE;
+                             if (current->flags & PF_RANDOMIZE)
+                                     load_bias +=
arch_mmap_rnd();
Quoted 8 lines
fs/binfmt_elf.c static int load_elf_binary(struct linux_binprm *bprm) · line 963
Quoted 5 lines
fs/binfmt_elf.c +5 -0
+                             elf_flags |= MAP_FIXED;
+                     } else
+                             load_bias = 0;
+
+                     load_bias -= vaddr;
I like this, and the big comment telling people how it works :)
Thanks! It looks like your patch for commenting load_bias never got picked up, so I've added some more comments for that and some other things too. (Mostly for all the stuff I have to read a few times when I look at this code.)

Quoted 5 lines

Quoted 2 lines
-Kees
The stack rlimit calculation fix for space potentially lost to ASLR is probably still needed too, right?

Yes. Was that picked up by akpm already?

-Kees

Signature · 2 lines

Kees Cook
Pixel Security

Daniel Micay

2017-06-21 17:32 UTC #5 lore raw

Quoted 65 lines · Kees Cook

Quoted 63 lines · Daniel Micay
Quoted 61 lines · Kees Cook
Quoted 15 lines · Rik van Riel
Quoted 4 lines · Kees Cook
+/* This is the base location for PIE (ET_DYN with INTERP) loads.
*/
+#define ELF_ET_DYN_BASE 0x400000UL
This value is good for 32 bit binaries, but for 64 bit binaries you probably want to put ELF_ET_DYN_BASE at 4GB or higher.
The latter is necessary because Android uses the lower 4GB of address space for its JVM runtime, with 32 bit pointers inside that part of the otherwise 64 bit address space.
Quoted 4 lines
In other words:
#define ELF_ET_DYN_BASE (mmap_is_ia32() ? 0x400000UL :
0x100000000UL)
Ah, interesting. Okay, that should be fine. I'll adjust it.
Quoted 18 lines
Quoted 18 lines
+++ b/fs/binfmt_elf.c
+ * Therefore, programs are loaded offset
from
+ * ELF_ET_DYN_BASE and loaders are
loaded
into the
+ * independently randomized mmap region
(0
load_bias
fs/binfmt_elf.c +5 -0
+                      * without MAP_FIXED).
+                      */
+                     if (elf_interpreter) {
+                             load_bias = ELF_ET_DYN_BASE;
+                             if (current->flags &
PF_RANDOMIZE)
+ load_bias +=
arch_mmap_rnd();
Quoted 8 lines
fs/binfmt_elf.c static int load_elf_binary(struct linux_binprm *bprm) · line 963
Quoted 5 lines
fs/binfmt_elf.c +5 -0
+                             elf_flags |= MAP_FIXED;
+                     } else
+                             load_bias = 0;
+
+                     load_bias -= vaddr;
I like this, and the big comment telling people how it works :)
Thanks! It looks like your patch for commenting load_bias never got
picked up, so I've added some more comments for that and some other
things too. (Mostly for all the stuff I have to read a few times when
I look at this code.)
-Kees

Quoted 7 lines

Quoted 3 lines
The stack rlimit calculation fix for space potentially lost to ASLR is
probably still needed too, right?
Yes. Was that picked up by akpm already?
-Kees

I think it was dropped when the ET_DYN changes were dropped.

Kees Cook

author

2017-06-21 17:33 UTC #6 lore raw

Quoted 70 lines · Daniel Micay

Quoted 69 lines · Kees Cook
Quoted 66 lines · Daniel Micay
Quoted 59 lines · Kees Cook
Quoted 15 lines · Rik van Riel
Quoted 4 lines · Kees Cook
+/* This is the base location for PIE (ET_DYN with INTERP) loads.
*/
+#define ELF_ET_DYN_BASE 0x400000UL
This value is good for 32 bit binaries, but for 64 bit binaries you probably want to put ELF_ET_DYN_BASE at 4GB or higher.
The latter is necessary because Android uses the lower 4GB of address space for its JVM runtime, with 32 bit pointers inside that part of the otherwise 64 bit address space.
Quoted 4 lines
In other words:
#define ELF_ET_DYN_BASE (mmap_is_ia32() ? 0x400000UL :
0x100000000UL)
Ah, interesting. Okay, that should be fine. I'll adjust it.
Quoted 18 lines
Quoted 18 lines
+++ b/fs/binfmt_elf.c
+ * Therefore, programs are loaded offset
from
+ * ELF_ET_DYN_BASE and loaders are
loaded
into the
+ * independently randomized mmap region
(0
load_bias
fs/binfmt_elf.c +5 -0
+                      * without MAP_FIXED).
+                      */
+                     if (elf_interpreter) {
+                             load_bias = ELF_ET_DYN_BASE;
+                             if (current->flags &
PF_RANDOMIZE)
+ load_bias +=
arch_mmap_rnd();
Quoted 8 lines
fs/binfmt_elf.c static int load_elf_binary(struct linux_binprm *bprm) · line 963
Quoted 5 lines
fs/binfmt_elf.c +5 -0
+                             elf_flags |= MAP_FIXED;
+                     } else
+                             load_bias = 0;
+
+                     load_bias -= vaddr;
I like this, and the big comment telling people how it works :)
Thanks! It looks like your patch for commenting load_bias never got
picked up, so I've added some more comments for that and some other
things too. (Mostly for all the stuff I have to read a few times when
I look at this code.)
Quoted 2 lines
-Kees
The stack rlimit calculation fix for space potentially lost to ASLR is
probably still needed too, right?

Quoted 5 lines

Quoted 3 lines
Yes. Was that picked up by akpm already?
-Kees
I think it was dropped when the ET_DYN changes were dropped.

Ah! Rik, can you resend just the stack calculation fixes? I can Ack those.

-Kees

Signature · 2 lines

Kees Cook
Pixel Security

Rik van Riel

2017-06-21 18:47 UTC #7 lore raw

In reply to Daniel Micay

Quoted 11 lines · Daniel Micay

Quoted 8 lines · Kees Cook
Quoted 4 lines
The stack rlimit calculation fix for space potentially lost to ASLR
is
probably still needed too, right?
Yes. Was that picked up by akpm already?
-Kees
I think it was dropped when the ET_DYN changes were dropped.

I can resend those.