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

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

On 03/06/2019 15:44, gaoyongliang wrote:
> 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?

Since we land here from do_page_fault, it seems natural to move the
invalidation up there, probably before we re-enable interrupts.

Thanks,

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