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

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Message-ID: <27d52fe4-3159-2f19-33cd-01ef4b3e530b@intel.com>
Date:   Wed, 26 May 2021 14:21:39 -0700
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>,
        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>
Subject: Re: [syzbot] WARNING in ex_handler_fprestore

On 5/26/2021 12:00 AM, Dmitry Vyukov wrote:
> 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?
> 

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.