* [PATCH] objtool/x86: objtool can confuse memory and stack access
@ 2024-03-28 13:46 Alexandre Chartre
2024-03-29 1:39 ` Josh Poimboeuf
0 siblings, 1 reply; 3+ messages in thread
From: Alexandre Chartre @ 2024-03-28 13:46 UTC (permalink / raw)
To: linux-kernel, jpoimboe, peterz; +Cc: alexandre.chartre
The encoding of an x86 instruction can include a ModR/M and a SIB
(Scale-Index-Base) byte to describe the addressing mode of the
instruction.
objtool processes all addressing mode with a SIB base of 5 as having
%rbp as the base register. However, a SIB base of 5 means that the
effective address has either no base (if ModR/M mod is zero) or %rbp
as the base (if ModR/M mod is 1 or 2). This can cause objtool to confuse
an absolute address access with a stack operation.
For example, objtool will see the following instruction:
4c 8b 24 25 e0 ff ff mov 0xffffffffffffffe0,%r12
as a stack operation (i.e. similar to: mov -0x20(%rbp), %r12).
[Note that this kind of weird absolute address access is added by the
compiler when using KASAN.]
If this perceived stack operation happens to reference the location
where %r12 was pushed on the stack then the objtool validation will
think that %r12 is being restored and this can cause a stack state
mismatch.
This kind behavior was seen on xfs code, after a minor change (convert
kmem_alloc() to kmalloc()):
>> fs/xfs/xfs.o: warning: objtool: xfs_da_grow_inode_int+0x6c1: stack state mismatch: reg1[12]=-2-48 reg2[12]=-1+0
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202402220435.MGN0EV6l-lkp@intel.com/
Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
---
tools/objtool/arch/x86/decode.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index 3a1d80a7878d3..18a9140173326 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -412,6 +412,14 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
if (!rex_w)
break;
+ /*
+ * If the SIB base is 5, and ModRM mod is 0 then there
+ * is no base. But SIB decoding will set sib_base to
+ * CFI_BP (register 5).
+ */
+ if (have_SIB() && sib_base == CFI_BP && modrm_mod == 0)
+ break;
+
if (rm_is_mem(CFI_BP)) {
/* mov disp(%rbp), reg */
--
2.39.3
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] objtool/x86: objtool can confuse memory and stack access
2024-03-28 13:46 [PATCH] objtool/x86: objtool can confuse memory and stack access Alexandre Chartre
@ 2024-03-29 1:39 ` Josh Poimboeuf
2024-03-29 13:22 ` Alexandre Chartre
0 siblings, 1 reply; 3+ messages in thread
From: Josh Poimboeuf @ 2024-03-29 1:39 UTC (permalink / raw)
To: Alexandre Chartre; +Cc: linux-kernel, peterz
On Thu, Mar 28, 2024 at 02:46:34PM +0100, Alexandre Chartre wrote:
> The encoding of an x86 instruction can include a ModR/M and a SIB
> (Scale-Index-Base) byte to describe the addressing mode of the
> instruction.
>
> objtool processes all addressing mode with a SIB base of 5 as having
> %rbp as the base register. However, a SIB base of 5 means that the
> effective address has either no base (if ModR/M mod is zero) or %rbp
> as the base (if ModR/M mod is 1 or 2). This can cause objtool to confuse
> an absolute address access with a stack operation.
>
> For example, objtool will see the following instruction:
>
> 4c 8b 24 25 e0 ff ff mov 0xffffffffffffffe0,%r12
>
> as a stack operation (i.e. similar to: mov -0x20(%rbp), %r12).
>
> [Note that this kind of weird absolute address access is added by the
> compiler when using KASAN.]
>
> If this perceived stack operation happens to reference the location
> where %r12 was pushed on the stack then the objtool validation will
> think that %r12 is being restored and this can cause a stack state
> mismatch.
>
> This kind behavior was seen on xfs code, after a minor change (convert
> kmem_alloc() to kmalloc()):
>
> >> fs/xfs/xfs.o: warning: objtool: xfs_da_grow_inode_int+0x6c1: stack state mismatch: reg1[12]=-2-48 reg2[12]=-1+0
>
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202402220435.MGN0EV6l-lkp@intel.com/
> Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Nice, thanks for finding and debugging this.
Would it make sense to make the check more generic by putting it into
rm_is()?
--
Josh
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] objtool/x86: objtool can confuse memory and stack access
2024-03-29 1:39 ` Josh Poimboeuf
@ 2024-03-29 13:22 ` Alexandre Chartre
0 siblings, 0 replies; 3+ messages in thread
From: Alexandre Chartre @ 2024-03-29 13:22 UTC (permalink / raw)
To: Josh Poimboeuf; +Cc: linux-kernel, peterz
On 3/29/24 02:39, Josh Poimboeuf wrote:
> On Thu, Mar 28, 2024 at 02:46:34PM +0100, Alexandre Chartre wrote:
>> The encoding of an x86 instruction can include a ModR/M and a SIB
>> (Scale-Index-Base) byte to describe the addressing mode of the
>> instruction.
>>
>> objtool processes all addressing mode with a SIB base of 5 as having
>> %rbp as the base register. However, a SIB base of 5 means that the
>> effective address has either no base (if ModR/M mod is zero) or %rbp
>> as the base (if ModR/M mod is 1 or 2). This can cause objtool to confuse
>> an absolute address access with a stack operation.
>>
>> For example, objtool will see the following instruction:
>>
>> 4c 8b 24 25 e0 ff ff mov 0xffffffffffffffe0,%r12
>>
>> as a stack operation (i.e. similar to: mov -0x20(%rbp), %r12).
>>
>> [Note that this kind of weird absolute address access is added by the
>> compiler when using KASAN.]
>>
>> If this perceived stack operation happens to reference the location
>> where %r12 was pushed on the stack then the objtool validation will
>> think that %r12 is being restored and this can cause a stack state
>> mismatch.
>>
>> This kind behavior was seen on xfs code, after a minor change (convert
>> kmem_alloc() to kmalloc()):
>>
>>>> fs/xfs/xfs.o: warning: objtool: xfs_da_grow_inode_int+0x6c1: stack state mismatch: reg1[12]=-2-48 reg2[12]=-1+0
>>
>> Reported-by: kernel test robot <lkp@intel.com>
>> Closes: https://lore.kernel.org/oe-kbuild-all/202402220435.MGN0EV6l-lkp@intel.com/
>> Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
>
> Nice, thanks for finding and debugging this.
>
> Would it make sense to make the check more generic by putting it into
> rm_is()?
>
Yes. Making the change.
Thanks,
alex.
^ permalink raw reply [flat|nested] 3+ messages in thread
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2024-03-28 13:46 [PATCH] objtool/x86: objtool can confuse memory and stack access Alexandre Chartre
2024-03-29 1:39 ` Josh Poimboeuf
2024-03-29 13:22 ` Alexandre Chartre
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