linux-kernel - Re: io_uring: BUG: kernel NULL pointer dereference

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Date:   Mon, 25 May 2020 13:25:22 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Stefano Garzarella <sgarzare@...hat.com>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: io_uring: BUG: kernel NULL pointer dereference

On 5/25/20 1:21 PM, Peter Zijlstra wrote:
> On Mon, May 25, 2020 at 08:10:27AM -0600, Jens Axboe wrote:
>> I think the odd part here is that task_tick_numa() checks for a
>> valid mm, and queues work if the task has it. But for the sqpoll
>> kthread, the mm can come and go. By the time the task work is run,
>> the mm is gone and we oops on current->mm == NULL.
>>
>> I think the below should fix it:
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 538ba5d94e99..24a8557f001f 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -2908,7 +2908,8 @@ static void task_tick_numa(struct rq *rq, struct task_struct *curr)
>>  	/*
>>  	 * We don't care about NUMA placement if we don't have memory.
>>  	 */
>> -	if (!curr->mm || (curr->flags & PF_EXITING) || work->next != work)
>> +	if (!curr->mm || (curr->flags & (PF_EXITING | PF_KTHREAD)) ||
>> +	    work->next != work)
>>  		return;
> 
> Ah, I think that's one more instance of '!p->mm' != is_kthread(). A
> while ago someone went and cleaned a bunch of them up. Clearly this one
> was missed.
> 
> I'm thinking just:
> 
> 	if ((curr->flags & (PF_EXITING | PF_KTHREAD)) || work->next != work)
> 
> should be enough.

Yeah it should, no point in checking both ->mm == NULL and PF_KTHREAD.

-- 
Jens Axboe