linux-kernel - Re: WARNING: CPU: 0 PID: 3031 at ./arch/x86/include/asm/fpu/internal.h:530 fpu_

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Message-ID: <CALCETrW3r89hbU3u2xG2u+ZDrk-WzwmicL9ZL-L1GVGZfH-sdQ@mail.gmail.com>
Date:	Mon, 15 Feb 2016 18:25:10 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>
Subject: Re: WARNING: CPU: 0 PID: 3031 at ./arch/x86/include/asm/fpu/internal.h:530
 fpu__restore+0x90/0x130()

On Feb 15, 2016 12:14 PM, "Borislav Petkov" <bp@...en8.de> wrote:
>
> ---
> From: Borislav Petkov <bp@...e.de>
> Date: Mon, 15 Feb 2016 19:50:33 +0100
> Subject: [RFC PATCH] x86/FPU: Fix double FPU regs activation
>
> On the entry_INT80_32->do_syscall_32_irqs_on path on 32-bit we run with
> interrupts enabled.

I would change this a little bit.

sys_sigreturn calls fpu__restore_sig with interrupts enabled.  When
restoring a 32-bit signal frame, it can happen that...

> And it can happen that we get preempted right after
> setting ->fpstate_active in a task's FPU.
>
> After we get preempted, we switch between tasks merrily and eventually
> are about to switch to that task above whose ->fpstate_active we
> set. We enter __switch_to() and do switch_fpu_prepare(). Our task gets
> ->fpregs_active set, we find ourselves back on the call stack below and
> especially in __fpu__restore_sig() which sets ->fpregs_active again.
>
> Leading to that whoops below.

--Andy