Re: general protection fault in perf_misc_flags

From: Dmitry Vyukov
Date: Tue Sep 22 2020 - 01:15:37 EST

On Mon, Sep 21, 2020 at 10:59 PM 'Nick Desaulniers' via syzkaller-bugs
<syzkaller-bugs@xxxxxxxxxxxxxxxx> wrote:
>
> 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

Hi Nick,

Yes, it should be this config. Borislav looked at the crash in the
first email in the thread and that's the config provided there. If
syzbot provides a crash and a config, these always match.

And the exact compiler used to produce the build is this:
compiler: clang version 10.0.0
(https://github.com/llvm/llvm-project/
c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
https://storage.googleapis.com/syzkaller/clang_install_c2443155.tar.gz


> (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
>
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