Re: general protection fault in perf_misc_flags

[Nick Desaulniers]

On Mon, Sep 21, 2020 at 1:09 AM 'Dmitry Vyukov' via Clang Built Linux
<clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
>
> On Mon, Sep 21, 2020 at 7:54 AM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> >
> > On Sat, Sep 19, 2020 at 1:08 PM Borislav Petkov <bp@xxxxxxxxx> wrote:
> > >
> > > 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?

Right, the two sequences above look almost the same, except those 4
bytes of zeros (the disassembler gets confused about the rest, but
it's the same byte sequence otherwise).  Are the two disassemblies a
comparison of the code at runtime vs. compile-time?  If so, how did
you disassemble the runtime code?  If runtime and compile time differ,
I suspect some kind of runtime patching.  I wonder if we calculated
the address of a static_key wrong (asm goto).  What function am I
looking at the disassembly of?  perf_misc_flags() in
arch/x86/events/core.c? With this config?
https://syzkaller.appspot.com/x/.config?x=cd992d74d6c7e62 (though I
don't see _any_ asm goto in the IR for this file built with this
config).  If this is deterministically reproducible, I suppose we
could set a watchpoint on the address being overwritten?

(Un-interestingly, I do get a panic trying to boot that config in
qemu, unless I bump the VMs RAM up.)

> >
> > Hi Boris,
> >
> > Memory corruption is definitely possible. There are hundreds of known
> > bugs that can potentially lead to silent memory corruptions, and some
> > observed to lead to silent memory corruptions.
> >
> > However, these tend to produce crash signatures with 1-2 crashes.
> > While this has 6 and they look similar and all happened on the only
> > instance that uses clang. So my bet would be on
> > something-clang-related rather than a silent memory corruption.
> > +clang-built-linux
>
>
> general protection fault in pvclock_gtod_notify (2) looks somewhat similar:
>  - only clang
>  - gpf in systems code
>  - happened few times
>
> https://syzkaller.appspot.com/bug?extid=1dccfcb049726389379c
> https://groups.google.com/g/syzkaller-bugs/c/0eUUkjFKrBg/m/nGfTjIfCBAAJ

Dmitry,
Is there an easy way for me to get from
https://syzkaller.appspot.com/upstream to <list of clang specific
failures>?  ctrl+f, `clang`, returns nothing on that first link; it
seems the compiler version is only included in the email?
-- 
Thanks,
~Nick Desaulniers