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Date:   Wed, 22 Jun 2022 14:50:15 +0100
From:   Marc Zyngier <maz@kernel.org>
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     Russell King <linux@armlinux.org.uk>,
        Tony Lindgren <tony@atomide.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] ARM: spectre-v2: fix smp_processor_id() warning
In-Reply-To: <795c9463-452e-bf64-1cc0-c318ccecb1da@I-love.SAKURA.ne.jp>
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On 2022-06-22 07:49, Tetsuo Handa wrote:
> syzbot complains smp_processor_id() from harden_branch_predictor()
>  from page fault path [1]. Explicitly disable preemption and use
> raw_smp_processor_id().
> 
> Link: https://syzkaller.appspot.com/bug?extid=a7ee43e564223f195c84 [1]
> Reported-by: syzbot 
> <syzbot+a7ee43e564223f195c84@syzkaller.appspotmail.com>
> Fixes: f5fe12b1eaee220c ("ARM: spectre-v2: harden user aborts in kernel 
> space")
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> ---
> This patch is completely untested.
> 
>  arch/arm/include/asm/system_misc.h | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/include/asm/system_misc.h
> b/arch/arm/include/asm/system_misc.h
> index 98b37340376b..a92446769acd 100644
> --- a/arch/arm/include/asm/system_misc.h
> +++ b/arch/arm/include/asm/system_misc.h
> @@ -20,8 +20,11 @@ typedef void (*harden_branch_predictor_fn_t)(void);
>  DECLARE_PER_CPU(harden_branch_predictor_fn_t, 
> harden_branch_predictor_fn);
>  static inline void harden_branch_predictor(void)
>  {
> -	harden_branch_predictor_fn_t fn = per_cpu(harden_branch_predictor_fn,
> -						  smp_processor_id());
> +	harden_branch_predictor_fn_t fn;
> +
> +	preempt_disable_notrace();
> +	fn = per_cpu(harden_branch_predictor_fn, raw_smp_processor_id());
> +	preempt_enable_no_resched_notrace();
>  	if (fn)
>  		fn();
>  }

I don't think that's required.

harden_branch_predictor() is always called on the fault path,
from __do_user_fault(), and that's always non-preemptible.

My hunch is that we're missing some tracking that indicates
to the kernel that we're already non-preemptible by virtue
of being in an exception handler.

Russell, what do you think?

         M.
-- 
Jazz is not dead. It just smells funny...