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SubjectRe: fs: proc: gpf in find_entry
From
2014-12-22 18:51 GMT+03:00 Eric W. Biederman <ebiederm@xmission.com>:
> Andrey Ryabinin <ryabinin.a.a@gmail.com> writes:
>
>> 2014-12-22 17:37 GMT+03:00 Sasha Levin <sasha.levin@oracle.com>:
>>> Hi all,
>>>
>>> While fuzzing with trinity inside a KVM tools guest running the latest -next
>>> kernel, I've stumbled on the following spew:
>>>
>>> [ 2015.960381] general protection fault: 0000 [#1] PREEMPT SMP KASAN
>>
>> Actually this is NULL-ptr dereference. Since you are using kasan with
>> inline instrumentation
>> NULL-ptr deref transforms into GPF.
>>
>>
>>> [ 2015.970534] RAX: 0000000000000000 RBX: ffff88000003a960 RCX: 0000000000000073
>>> [ 2015.970534] RDX: 1ffff10101c8f3c4 RSI: 0000000000000000 RDI: ffff88080e479e20
>>> [ 2015.970534] RBP: ffff88080e477a28 R08: 0000000000000066 R09: 0000000000000073
>>> [ 2015.970534] R10: ffffda0017d55630 R11: dfffe90000000000 R12: ffff88005fc644b8
>>> [ 2015.970534] R13: dfffe90000000000 R14: ffffffff92464884 R15: 0000000000000000
>>
>> [...]
>>
>>> All code
>>> ========
>>> 0: e8 03 42 80 3c callq 0x3c804208
>>> 5: 28 00 sub %al,(%rax)
>>> 7: 0f 85 ff 01 00 00 jne 0x20c
>>> d: 4c 8b 7b 18 mov 0x18(%rbx),%r15
>>> 11: 4d 85 ff test %r15,%r15
>>> 14: 0f 84 de 01 00 00 je 0x1f8
>>> 1a: 41 f6 c7 07 test $0x7,%r15b
>>> 1e: 0f 85 d4 01 00 00 jne 0x1f8
>>> 24: 4c 89 f8 mov %r15,%rax
>>> 27: 48 c1 e8 03 shr $0x3,%rax
>>> 2b:* 42 80 3c 28 00 cmpb $0x0,(%rax,%r13,1) <-- trapping instruction
>>
>> Three commands above are result of KASAN's instrumentation.
>> They check shadow for address in %r15:
>> if (*((%r15 >> 3) + kasan_shadow_offset)
>>
>>
>>> 30: 0f 85 b5 01 00 00 jne 0x1eb
>>> 36: 4d 8b 37 mov (%r15),%r14
>>
>> And here is memory access, that KASAN checking.
>
> Then frankly I suspect this is a KASAN bug.
>

Sure it is possible, but I don't see any evidence of kasan bug here.

> These two instructions:
>>> 11: 4d 85 ff test %r15,%r15
>>> 14: 0f 84 de 01 00 00 je 0x1f8
>
> Should prevent a NULL %r15 value from ever reaching the trapping
> instruction.

If they were executed, then yes. But I think there was jump from somewhere
to the instructions below those two.

>
> What other horrible things does KASAN do to the machine code?
>

kasan insert something like following before any memory access:

s8 *shadow_addr = (add >> 3) + shadow_offset;

if (unlikely(*shadow_addr))
if (unlikely(addr & 7 >= *shadow_addr))
report_bug(addr);


I suspect that Sasha is using kasan along with ubsan.
In that case generated code much more horrid.


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