Messages in this thread | | | From | Dmitry Vyukov <> | Date | Wed, 23 Sep 2020 11:24:48 +0200 | Subject | Re: general protection fault in perf_misc_flags |
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On Wed, Sep 23, 2020 at 11:03 AM Borislav Petkov <bp@alien8.de> wrote: > > On Tue, Sep 22, 2020 at 11:56:04AM -0700, Nick Desaulniers wrote: > > So I think there's an issue with "deterministically reproducible." > > The syzcaller report has: > > > > Unfortunately, I don't have any reproducer for this issue yet. > > Yeah, Dmitry gave two other links of similar reports, the first one > works for me: > > https://syzkaller.appspot.com/bug?extid=1dccfcb049726389379c > > and that one doesn't have a reproducer either. The bytes look familiar > though: > > Code: c1 e8 03 42 80 3c 20 00 74 05 e8 79 7a a7 00 49 8b 47 10 48 89 05 f6 d8 ef 09 49 8d 7f 08 48 89 f8 48 c1 e8 03 42 80 3c 00 00 <00> 00 e8 57 7a a7 00 49 8b 47 08 48 89 05 dc d8 ef 09 49 8d 7f 18 > All code > ======== > 0: c1 e8 03 shr $0x3,%eax > 3: 42 80 3c 20 00 cmpb $0x0,(%rax,%r12,1) > 8: 74 05 je 0xf > a: e8 79 7a a7 00 callq 0xa77a88 > f: 49 8b 47 10 mov 0x10(%r15),%rax > 13: 48 89 05 f6 d8 ef 09 mov %rax,0x9efd8f6(%rip) # 0x9efd910 > 1a: 49 8d 7f 08 lea 0x8(%r15),%rdi > 1e: 48 89 f8 mov %rdi,%rax > 21: 48 c1 e8 03 shr $0x3,%rax > 25: 42 80 3c 00 00 cmpb $0x0,(%rax,%r8,1) > 2a:* 00 00 add %al,(%rax) <-- trapping instruction > 2c: e8 57 7a a7 00 callq 0xa77a88 > 31: 49 8b 47 08 mov 0x8(%r15),%rax > 35: 48 89 05 dc d8 ef 09 mov %rax,0x9efd8dc(%rip) # 0x9efd918 > 3c: 49 8d 7f 18 lea 0x18(%r15),%rdi > > 4 zero bytes again. And that .config has kasan stuff enabled too so > could the failure be related to having kasan stuff enabled and it > messing up offsets? > > That is, provided this is the mechanism how it would happen. We still > don't know what and when wrote those zeroes in there. Not having a > reproducer is nasty but looking at those reports above and if I'm > reading this correctly, rIP points to > > RIP: 0010:update_pvclock_gtod arch/x86/kvm/x86.c:1743 [inline] > > each time and the URL says they're 9 crashes total. And each have > happened at that rIP. So all we'd need is set a watchpoint when that > address is being written and dump stuff. > > Dmitry, can the syzkaller do debugging stuff like that?
syzbot does not have direct support for such things. It uses CONFIG_DEBUG_AID_FOR_SYZBOT=y: https://github.com/google/syzkaller/blob/master/docs/syzbot.md#no-custom-patches But that's generally useful for linux-next only and the clang build is on the upstream tree...
Options I see: 1. Add stricter debug checks for code that overwrites code. Then maybe we can catch it red handed. 2. Setup clang instance on linux-next 3. Run syzkaller locally with custom patches.
> > Following my hypothesis about having a bad address calculation; the > > tricky part is I'd need to look through the relocations and try to see > > if any could resolve to the address that was accidentally modified. I > > suspect objtool could be leveraged for that; > > If you can find this at compile time... > > > maybe it could check whether each `struct jump_entry`'s `target` > > member referred to either a NOP or a CMP, and error otherwise? (Do we > > have other non-NOP or CMP targets? IDK) > > Follow jump_label_transform() - it does verify what it is going to > patch. And while I'm looking at this, I realize that the jump labels > patch 5 bytes but the above zeroes are 4 bytes. In the other opcode > bytes I decoded it is 4 bytes too. So this might not be caused by the > jump labels patching... > > > This hypothesis might also be incorrect, and thus would be chasing a > > red herring...not really sure how else to pursue debugging this. > > Yeah, this one is tricky to debug. > > -- > Regards/Gruss, > Boris. > > https://people.kernel.org/tglx/notes-about-netiquette
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