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    SubjectRe: io_uring: BUG: kernel NULL pointer dereference
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    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

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