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SubjectRe: keys: GPF in request_key
On Wed, Feb 1, 2017 at 2:11 PM, David Howells <dhowells@redhat.com> wrote:
> Dmitry Vyukov <dvyukov@google.com> wrote:
>
>> Code: 41 54 49 89 f4 53 49 89 d7 48 89 fb 48 83 ec 08 e8 d1 50 67 ff
>> 49 8d 7c 24 10 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80>
>> 3c 02 00 0f 85 35 02 00 00 49 83 7c 24 10 00 0f 84 bb 01 00
>
> This disassembles to:
>
> 0: 41 54 push %r12
> 2: 49 89 f4 mov %rsi,%r12
> 5: 53 push %rbx
> 6: 49 89 d7 mov %rdx,%r15
> 9: 48 89 fb mov %rdi,%rbx
> c: 48 83 ec 08 sub $0x8,%rsp
> 10: e8 d1 50 67 ff callq 0xffffffffff6750e6
> 15: 49 8d 7c 24 10 lea 0x10(%r12),%rdi
> 1a: 48 b8 00 00 00 00 00 movabs $0xdffffc0000000000,%rax
> 21: fc ff df
> 24: 48 89 fa mov %rdi,%rdx
> 27: 48 c1 ea 03 shr $0x3,%rdx
> 2b:* 80 3c 02 00 cmpb $0x0,(%rdx,%rax,1) <-- trapping instruction
> 2f: 0f 85 35 02 00 00 jne 0x26a
> 35: 49 83 7c 24 10 00 cmpq $0x0,0x10(%r12)
> 3b: 0f .byte 0xf
> 3c: 84 .byte 0x84
> 3d: bb .byte 0xbb
> 3e: 01 00 add %eax,(%rax)
>
> I can see that RAX got loaded from the instruction at 0x1a, but the code
> doesn't look very much like what I get out of the compiler (your compiled
> function is also at least double the size of what I get, presumably due to
> kasan?).
>
> Can you disassemble __key_link_begin() for me and send me your config?
>
> In particular, 0xdffffc0000000000 looks very weird. Is this code validating
> the pointer in R12?


Here is the disasm:
https://gist.githubusercontent.com/dvyukov/c06f742adba0ee162dfb1ece75b5d8ab/raw/14aba22492090e09ea36bddd5db344fdcfc25065/gistfile1.txt

Yes, this is with KASAN+KCOV so size increase is expected.

Before each memory access to *p KASAN inserts:

if (*(char*)(0xdffffc0000000000 + p/8)) __kasan_report();
... *p .... // original memory accesses

You can see it here:

ffffffff8203ea84: 48 b8 00 00 00 00 00 movabs $0xdffffc0000000000,%rax
ffffffff8203ea8b: fc ff df
ffffffff8203ea8e: 48 89 fa mov %rdi,%rdx
ffffffff8203ea91: 48 c1 ea 03 shr $0x3,%rdx
ffffffff8203ea95: 80 3c 02 00 cmpb $0x0,(%rdx,%rax,1) //
<<< ACCESS that crashes

For valid kernel addresses this never crashes.
In this case the original addresses that code tried to accesses is in
%RDI. And they look bogus.

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