linux-kernel - Re: [tip: x86/fpu] x86/fpu/xstate: Define new functions for clearing fpregs and xstates

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Message-ID: <87y2c28zir.ffs@nanos.tec.linutronix.de>
Date:   Tue, 25 May 2021 20:00:12 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Yu\, Yu-cheng" <yu-cheng.yu@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     linux-tip-commits@...r.kernel.org,
        Fenghua Yu <fenghua.yu@...el.com>,
        Borislav Petkov <bp@...e.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>, x86 <x86@...nel.org>,
        "Shankar\, Ravi V" <ravi.v.shankar@...el.com>
Subject: Re: [tip: x86/fpu] x86/fpu/xstate: Define new functions for clearing fpregs and xstates

On Tue, May 25 2021 at 10:44, Yu-cheng Yu wrote:
> On 5/24/2021 9:34 AM, Andy Lutomirski wrote:
>> So I'm guessing that syzbot may have misattributed the problem.  But
>> we definitely need to clean up the XRSTOR #GP handling before CET
>> lands.
>> 
>  From the crash dump, the system is doing syscall_exit_to_user_mode() 
> for __x64_sys_futex().  The futex syscall does not seem to modify 
> xstates,

Of course does the futex syscall not modify anything, but the task can
schedule out before returning from the syscall so it has to restore the
FPU state.

> but upon returning to user mode, XRSTORS gets a GP.  Can this 
> be some memory corruption?  fpu__clear() is merely helping to clear the 
> mess and seems to be innocent.

What kind of analysis is that?

Thanks,

        tglx