Re: [PATCH] arm: fix using smp_processor_id() in preemptible context

[Marc Zyngier <marc.zyngier@arm.com>]

On 27/05/2019 10:39, Yongliang Gao wrote:
> harden_branch_predictor() call smp_processor_id() in preemptible
> context, this would cause a bug messages.
> 
> The bug messages is as follows:
> BUG: using smp_processor_id() in preemptible [00000000] code: syz-executor0/17992
> caller is harden_branch_predictor arch/arm/include/asm/system_misc.h:27 [inline]
> caller is __do_user_fault+0x34/0x114 arch/arm/mm/fault.c:200
> CPU: 1 PID: 17992 Comm: syz-executor0 Tainted: G O 4.4.176 #1
> Hardware name: Hisilicon A9
> [<c0114ae4>] (unwind_backtrace) from [<c010e6fc>] (show_stack+0x18/0x1c)
> [<c010e6fc>] (show_stack) from [<c0379514>] (dump_stack+0xc8/0x118)
> [<c0379514>] (dump_stack) from [<c039b5a0>] (check_preemption_disabled+0xf4/0x138)
> [<c039b5a0>] (check_preemption_disabled) from [<c011abe4>] (__do_user_fault+0x34/0x114)
> [<c011abe4>] (__do_user_fault) from [<c053b0d0>] (do_page_fault+0x3b4/0x3d8)
> [<c053b0d0>] (do_page_fault) from [<c01013dc>] (do_DataAbort+0x58/0xf8)
> [<c01013dc>] (do_DataAbort) from [<c053a880>] (__dabt_usr+0x40/0x60)
> 
> Reported-by: Jingwen Qiu <qiujingwen@huawei.com>
> Signed-off-by: Yongliang Gao <gaoyongliang@huawei.com>
> Cc: <stable@vger.kernel.org>
> ---
>  arch/arm/include/asm/system_misc.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/include/asm/system_misc.h b/arch/arm/include/asm/system_misc.h
> index 66f6a3a..4a55cfb 100644
> --- a/arch/arm/include/asm/system_misc.h
> +++ b/arch/arm/include/asm/system_misc.h
> @@ -22,9 +22,10 @@
>  static inline void harden_branch_predictor(void)
>  {
>  	harden_branch_predictor_fn_t fn = per_cpu(harden_branch_predictor_fn,
> -						  smp_processor_id());
> +						  get_cpu());
>  	if (fn)
>  		fn();
> +	put_cpu();
>  }
>  #else
>  #define harden_branch_predictor() do { } while (0)
> 

This doesn't look like the right fix. If we're in a preemptible context,
then we could invalidate the branch predictor on the wrong CPU.

The right fix would be to move the call to a point where we haven't
enabled preemption yet.

Thanks,

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