linux-kernel - Re: [PATCH] arm64: fix bug for reloading FPSIMD state after execve on cpu 0.

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Date:	Wed, 26 Aug 2015 12:39:22 +0100
From:	Will Deacon <will.deacon@....com>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	Chunyan Zhang <chunyan.zhang@...eadtrum.com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"jianhua.ljh@...il.com" <jianhua.ljh@...il.com>,
	"orson.zhai@...eadtrum.com" <orson.zhai@...eadtrum.com>,
	"xiongshan.an@...eadtrum.com" <xiongshan.an@...eadtrum.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm64: fix bug for reloading FPSIMD state after execve
 on cpu 0.

On Wed, Aug 26, 2015 at 12:32:03PM +0100, Ard Biesheuvel wrote:
> On 26 August 2015 at 13:12, Will Deacon <will.deacon@....com> wrote:
> > On Wed, Aug 26, 2015 at 03:40:41AM +0100, Chunyan Zhang wrote:
> >> From: Janet Liu <janet.liu@...eadtrum.com>
> >>
> >> If process A is running on CPU 0 and do execve syscall and after sched_exec,
> >> dest_cpu is 0, fpsimd_state.cpu is 0. If at the time Process A get scheduled
> >> out and after some kernel threads running on CPU 0, process A is back in CPU 0,
> >> A's fpsimd_state.cpu is current cpu id "0", and per_cpu(fpsimd_last_state)
> >> points A's fpsimd_state, TIF_FOREIGN_FPSTATE will be clear, kernel will not
> >> reload the context during it return to userspace. so set the cpu's
> >> fpsimd_last_state to NULL to avoid this.
> >
> > AFAICT, this is only a problem if one of the kernel threads uses the fpsimd
> > registers, right? However, kernel_neon_begin_partial clobbers
> > fpsimd_last_state, so I'm struggling to see the problem.
> >
> 
> I think the problem is real, but it would be better to set the
> fpsimd_state::cpu field to an invalid value like we do in
> fpsimd_flush_task_state()
> 
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index 44d6f7545505..c56956a16d3f 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -158,6 +158,7 @@ void fpsimd_thread_switch(struct task_struct *next)
>  void fpsimd_flush_thread(void)
>  {
>         memset(&current->thread.fpsimd_state, 0, sizeof(struct fpsimd_state));
> +       fpsimd_flush_task_state(current);
>         set_thread_flag(TIF_FOREIGN_FPSTATE);
>  }
> 
> (note the memset erroneously initializes that field to CPU 0)

Aha, I see. So the problem is actually that we get a view on our fpsimd
state before the exec, rather than a view on some kernel state.

> This more accurately reflects the state of the process after forking,
> i.e., that its FPSIMD state has never been loaded into any CPU.

Yup, that's much clearer.

Will
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