Re: general protection fault in perf_misc_flags

[Borislav Petkov]

On Sat, Sep 19, 2020 at 01:32:14AM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    92ab97ad Merge tag 'sh-for-5.9-part2' of git://git.libc.or..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1069669b900000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=cd992d74d6c7e62
> dashboard link: https://syzkaller.appspot.com/bug?extid=ce179bc99e64377c24bc
> compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+ce179bc99e64377c24bc@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> general protection fault, probably for non-canonical address 0xffff518084501e28: 0000 [#1] PREEMPT SMP KASAN
> KASAN: maybe wild-memory-access in range [0xfffaac042280f140-0xfffaac042280f147]
> CPU: 0 PID: 17449 Comm: syz-executor.5 Not tainted 5.9.0-rc5-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> RIP: 0010:perf_misc_flags+0x125/0x150 arch/x86/events/core.c:2638
> Code: e4 48 83 e6 03 41 0f 94 c4 31 ff e8 95 fa 73 00 bb 02 00 00 00 4c 29 e3 49 81 c6 90 00 00 00 4c 89 f0 48 c1 e8 00 00 00 00 38 <00> 74 08 4c 89 f7 e8 40 c0 b3 00 41 8b 06 83 e0 08 48 c1 e0 0b 48

Hmm, so converting this back to opcodes with decodecode gives:

Code: e4 48 83 e6 03 41 0f 94 c4 31 ff e8 95 fa 73 00 bb 02 00 00 00 4c 29 e3 49 81 c6 90 00 00 00 4c 89 f0 48 c1 e8 00 00 00 00 38 <00> 74 08 4c 89 f7 e8 40 c0 b3 00 41 8b 06 83 e0 08 48 c1 e0 0b 48
All code
========
   0:   e4 48                   in     $0x48,%al
   2:   83 e6 03                and    $0x3,%esi
   5:   41 0f 94 c4             sete   %r12b
   9:   31 ff                   xor    %edi,%edi
   b:   e8 95 fa 73 00          callq  0x73faa5
  10:   bb 02 00 00 00          mov    $0x2,%ebx
  15:   4c 29 e3                sub    %r12,%rbx
  18:   49 81 c6 90 00 00 00    add    $0x90,%r14
  1f:   4c 89 f0                mov    %r14,%rax
  22:   48 c1 e8 00             shr    $0x0,%rax
  26:   00 00                   add    %al,(%rax)
  28:   00 38                   add    %bh,(%rax)
  2a:*  00 74 08 4c             add    %dh,0x4c(%rax,%rcx,1)            <-- trapping instruction
  2e:   89 f7                   mov    %esi,%edi
  30:   e8 40 c0 b3 00          callq  0xb3c075
  35:   41 8b 06                mov    (%r14),%eax
  38:   83 e0 08                and    $0x8,%eax
  3b:   48 c1 e0 0b             shl    $0xb,%rax
  3f:   48                      rex.W

and those ADDs before the rIP look real strange. Just as if something
wrote 4 bytes of 0s there. And building your config with clang-10 gives
around that area:

ffffffff8101177c:       48 83 e6 03             and    $0x3,%rsi
ffffffff81011780:       41 0f 94 c4             sete   %r12b
ffffffff81011784:       31 ff                   xor    %edi,%edi
ffffffff81011786:       e8 05 c9 73 00          callq  ffffffff8174e090 <__sanitizer_cov_trace_const_cmp8>
ffffffff8101178b:       bb 02 00 00 00          mov    $0x2,%ebx
ffffffff81011790:       4c 29 e3                sub    %r12,%rbx
ffffffff81011793:       49 81 c6 90 00 00 00    add    $0x90,%r14
ffffffff8101179a:       4c 89 f0                mov    %r14,%rax
ffffffff8101179d:       48 c1 e8 03             shr    $0x3,%rax
ffffffff810117a1:       42 80 3c 38 00          cmpb   $0x0,(%rax,%r15,1)
ffffffff810117a6:       74 08                   je     ffffffff810117b0 <perf_misc_flags+0x130>
ffffffff810117a8:       4c 89 f7                mov    %r14,%rdi
ffffffff810117ab:       e8 20 75 b3 00          callq  ffffffff81b48cd0 <__asan_report_load8_noabort>
ffffffff810117b0:       41 8b 06                mov    (%r14),%eax
ffffffff810117b3:       83 e0 08                and    $0x8,%eax
ffffffff810117b6:       48 c1 e0 0b             shl    $0xb,%rax

and I can pretty much follow it instruction by instruction until I reach
that SHR. Your SHR is doing a shift by 0 bytes and that already looks
suspicious.

After it, your output has a bunch of suspicious ADDs and mine has a CMP;
JE instead. And that looks really strange too.

Could it be that something has scribbled in guest memory and corrupted
that area, leading to that strange discrepancy in the opcodes?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette