io_uring: fix current->mm NULL dereference on exit

Don't reissue requests from io_iopoll_reap_events(), the task may not
have mm, which ends up with NULL. It's better to kill everything off on
exit anyway.

[  677.734670] RIP: 0010:io_iopoll_complete+0x27e/0x630
...
[  677.734679] Call Trace:
[  677.734695]  ? __send_signal+0x1f2/0x420
[  677.734698]  ? _raw_spin_unlock_irqrestore+0x24/0x40
[  677.734699]  ? send_signal+0xf5/0x140
[  677.734700]  io_iopoll_getevents+0x12f/0x1a0
[  677.734702]  io_iopoll_reap_events.part.0+0x5e/0xa0
[  677.734703]  io_ring_ctx_wait_and_kill+0x132/0x1c0
[  677.734704]  io_uring_release+0x20/0x30
[  677.734706]  __fput+0xcd/0x230
[  677.734707]  ____fput+0xe/0x10
[  677.734709]  task_work_run+0x67/0xa0
[  677.734710]  do_exit+0x35d/0xb70
[  677.734712]  do_group_exit+0x43/0xa0
[  677.734713]  get_signal+0x140/0x900
[  677.734715]  do_signal+0x37/0x780
[  677.734717]  ? enqueue_hrtimer+0x41/0xb0
[  677.734718]  ? recalibrate_cpu_khz+0x10/0x10
[  677.734720]  ? ktime_get+0x3e/0xa0
[  677.734721]  ? lapic_next_deadline+0x26/0x30
[  677.734723]  ? tick_program_event+0x4d/0x90
[  677.734724]  ? __hrtimer_get_next_event+0x4d/0x80
[  677.734726]  __prepare_exit_to_usermode+0x126/0x1c0
[  677.734741]  prepare_exit_to_usermode+0x9/0x40
[  677.734742]  idtentry_exit_cond_rcu+0x4c/0x60
[  677.734743]  sysvec_reschedule_ipi+0x92/0x160
[  677.734744]  ? asm_sysvec_reschedule_ipi+0xa/0x20
[  677.734745]  asm_sysvec_reschedule_ipi+0x12/0x20
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -890,6 +890,7 @@ static int __io_sqe_files_update(struct io_ring_ctx *ctx,
 				 struct io_uring_files_update *ip,
 				 unsigned nr_args);
 static int io_grab_files(struct io_kiocb *req);
+static void io_complete_rw_common(struct kiocb *kiocb, long res);
 static void io_cleanup_req(struct io_kiocb *req);
 static int io_file_get(struct io_submit_state *state, struct io_kiocb *req,
 		       int fd, struct file **out_file, bool fixed);
@@ -1749,6 +1750,14 @@ static void io_iopoll_queue(struct list_head *again)
 	do {
 		req = list_first_entry(again, struct io_kiocb, list);
 		list_del(&req->list);
+
+		/* shouldn't happen unless io_uring is dying, cancel reqs */
+		if (unlikely(!current->mm)) {
+			io_complete_rw_common(&req->rw.kiocb, -EAGAIN);
+			io_put_req(req);
+			continue;
+		}
+
 		refcount_inc(&req->refs);
 		io_queue_async_work(req);
 	} while (!list_empty(again));