io_uring: fix drain stalls by invalid SQE

cq_extra is protected by ->completion_lock, which io_get_sqe() misses.
The bug is harmless as it doesn't happen in real life, requires invalid
SQ index array and racing with submission, and only messes up the
userspace, i.e. stall requests execution but will be cleaned up on
ring destruction.

Fixes: 15641e4 ("io_uring: don't cache number of dropped SQEs")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/66096d54651b1a60534bb2023f2947f09f50ef73.1691538547.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/io_uring.c

@@ -2374,7 +2374,9 @@ static bool io_get_sqe(struct io_ring_ctx *ctx, const struct io_uring_sqe **sqe)
 	}
 
 	/* drop invalid entries */
+	spin_lock(&ctx->completion_lock);
 	ctx->cq_extra--;
+	spin_unlock(&ctx->completion_lock);
 	WRITE_ONCE(ctx->rings->sq_dropped,
 		   READ_ONCE(ctx->rings->sq_dropped) + 1);
 	return false;