Re: [PATCH] ARM: spectre-v2: fix smp_processor_id() warning

From: Russell King (Oracle)
Date: Wed Jun 22 2022 - 07:23:02 EST


On Wed, Jun 22, 2022 at 03:49:21PM +0900, 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@xxxxxxxxxxxxxxxxxxxxxxxxx>
> Fixes: f5fe12b1eaee220c ("ARM: spectre-v2: harden user aborts in kernel space")
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>

This may "fix" the warning, but...

> ---
> 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();

The idea is to get the function for the specific CPU, and then to run it
_on_ that CPU, and in theory the CPU that took the fault. However, I
seem to remember there are issues trying to achieve that, and I don't
have a solution for it.

--
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