io_uring: Check current->io_uring in io_uring_cancel_sqpoll

syzkaller identified KASAN: null-ptr-deref Write in
io_uring_cancel_sqpoll.

io_uring_cancel_sqpoll is called by io_sq_thread before calling
io_uring_alloc_task_context. This leads to current->io_uring being NULL.
io_uring_cancel_sqpoll should not have to deal with threads where
current->io_uring is NULL.

In order to cast a wider safety net, perform input sanitisation directly
in io_uring_cancel_sqpoll and return for NULL value of current->io_uring.
This is safe since if current->io_uring isn't set, then there's no way
for the task to have submitted any requests.

Reported-by: syzbot+be51ca5a4d97f017cd50@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Palash Oswal <hello@oswalpalash.com>
Link: https://lore.kernel.org/r/20210427125148.21816-1-hello@oswalpalash.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -9044,6 +9044,8 @@ static void io_uring_cancel_sqpoll(struct io_sq_data *sqd)
 	s64 inflight;
 	DEFINE_WAIT(wait);
 
+	if (!current->io_uring)
+		return;
 	WARN_ON_ONCE(!sqd || sqd->thread != current);
 
 	atomic_inc(&tctx->in_idle);