Re: [syzbot] WARNING in ex_handler_fprestore
From: Andy Lutomirski
Date: Wed May 26 2021 - 18:37:13 EST
On 5/26/21 2:21 PM, Yu, Yu-cheng wrote:
> 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:
>>>> syzbot found the following issue on:
>>>> HEAD commit: 45af60e7 Merge tag 'for-5.13-rc2-tag' of
>>>> git tree: upstream
>>>> console output:
>>>> kernel config:
>>>> dashboard link:
>>>> compiler: Debian clang version 11.0.1-2
>>>> syz repro:
>>> 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.
It's a regression caused by your fpu__clear_user() patch. tglx and I
are working on it.
The syzbot report is confusing but correct.