Re: WARNING: can't access registers at asm_sysvec_call_function_single

[Dmitry Vyukov]

On Mon, Sep 14, 2020 at 6:51 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > > On Tue, Sep 08 2020 at 13:16, syzbot wrote:
> > >
> > > > Hello,
> > > >
> > > > syzbot found the following issue on:
> > > >
> > > > HEAD commit:    f4d51dff Linux 5.9-rc4
> > > > git tree:       upstream
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1455d4f9900000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=a9075b36a6ae26c9
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=bc9dbf05dcc151e9b972
> > > > compiler:       gcc (GCC) 10.1.0-syz 20200507
> > > >
> > > > 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+bc9dbf05dcc151e9b972@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > >
> > > > WARNING: can't access registers at asm_sysvec_call_function_single+0x12/0x20 arch/x86/include/asm/idtentry.h:589
> > >
> > > That's the ORC unwinder complaining, but I have no idea why...
> >
> > 0000000000000cd0 <asm_sysvec_call_function_single>:
> >      cd0:     90                      nop
> >      cd1:     90                      nop
> >      cd2:     90                      nop
> >      cd3:     6a ff                   pushq  $0xffffffffffffffff
> >      cd5:     e8 b6 03 00 00          callq  1090 <error_entry>
> >      cda:     48 89 e7                mov    %rsp,%rdi
> >      cdd:     e8 00 00 00 00          callq  ce2 <asm_sysvec_call_function_single+0x12>
> >                       cde: R_X86_64_PLT32     sysvec_call_function_single-0x4
> >      ce2:     e9 69 04 00 00          jmpq   1150 <error_return>
> >      ce7:     66 0f 1f 84 00 00 00    nopw   0x0(%rax,%rax,1)
> >      cee:     00 00
> >
> > This is a head scratcher.  The ORC data looks ok, at least when I build
> > it on my system.  It's expecting to find pt_regs at the top of the stack
> > after returning from sysvec_call_function_single().  But it can't
> > dereference the regs for some reason.
> >
> > So either it's a bug in ORC data or unwinder (not likely) or the stack
> > pointer got corrupted somehow (also seems unlikely)...
>
> The true bug may be revealed elsewhere on the stack... I probably need
> to start enabling ORC stack dumps like I did for FP.

Hi Josh,

Is there a config for this? We could enable it on syzbot. More
informative kernel diagnostics are always better.