linux-kernel - Re: [syzbot] WARNING in ex_handler

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Date:   Wed, 26 May 2021 09:00:35 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     syzbot <syzbot+2067e764dbcd10721e2e@...kaller.appspotmail.com>,
        Borislav Petkov <bp@...en8.de>, Borislav Petkov <bp@...e.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        fenghua.yu@...el.com, "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>, tony.luck@...el.com,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [syzbot] WARNING in ex_handler_fprestore

On Wed, May 26, 2021 at 2:33 AM Andy Lutomirski <luto@...nel.org> 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?