Re: [syzbot] WARNING in ex_handler_fprestore

[Yu, Yu-cheng]

On 5/26/2021 12:00 AM, Dmitry Vyukov wrote:
> On Wed, May 26, 2021 at 2:33 AM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> > On 5/24/21 1:51 AM, syzbot wrote:
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    45af60e7 Merge tag 'for-5.13-rc2-tag' of git://git.kernel...
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=1591e9f7d00000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=18fade5827eb74f7
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=2067e764dbcd10721e2e
> > > compiler:       Debian clang version 11.0.1-2
> > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11be6bd1d00000
> >
> > Hi syz people and x86 people-
> >
> > I entirely believe that this bug is real and that syzbot bisected it
> > correctly, but I'm puzzled by the reproducer.  It says:
> >
> > ptrace$setregs(0xd, r0, 0x0, &(0x7f0000000080))
> >
> > I would really, really expect this to result from PTRACE_SETREGSET or
> > PTRACE_SETFPREGS, but this is PTRACE_SETREGS.
> >
> > Am I missing something really obvious here?
>
> Hi Andy,
>
> Sometimes syzkaller uses data format from one syscall variant, but
> actually invokes another.
> But here it does _not_ seem to be the case: 0xd is actually
> PTRACE_SETREGS. And the other ptrace calls in the reproducer are
> PTRACE_SEIZE and PTRACE_SINGLESTEP.
> So I would assume somehow it happened with PTRACE_SETREGS.
> Is there any indication from hardware as to what's wrong with fpregs?

PTRACE_SETREGS can change segment registers.  The PTRACE_SETREGS is 
using some uninitialized memory area.  One possibility would be that 
XRSTORS has a memory operand outside of segment limits.