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

[gaoyongliang <gaoyongliang@huawei.com>]

Hi Marc,

On 2019/6/3 18:17, Marc Zyngier wrote:
> 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.

Sorry, my bad, thanks a lot for the good catch.

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

I took a look at the code, and find out that the caller of
harden_branch_predictor(), __do_user_fault(), is called by do_page_fault()
and do_bad_area(), those two function's context are both running with
preemption enabled, so I didn't find a good place to move the call,
could you please give some suggestion for my next step?

Best Regards
Yongliang