io_uring: used cached copies of sq->dropped and cq->overflow

We currently use the ring values directly, but that can lead to issues
if the application is malicious and changes these values on our behalf.
Created in-kernel cached versions of them, and just overwrite the user
side when we update them. This is similar to how we treat the sq/cq
ring tail/head updates.
Reported-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -197,6 +197,7 @@ struct io_ring_ctx {
 		unsigned		sq_entries;
 		unsigned		sq_mask;
 		unsigned		sq_thread_idle;
+		unsigned		cached_sq_dropped;
 		struct io_uring_sqe	*sq_sqes;
 
 		struct list_head	defer_list;
@@ -212,6 +213,7 @@ struct io_ring_ctx {
 
 	struct {
 		unsigned		cached_cq_tail;
+		atomic_t		cached_cq_overflow;
 		unsigned		cq_entries;
 		unsigned		cq_mask;
 		struct wait_queue_head	cq_wait;
@@ -420,7 +422,8 @@ static struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
 static inline bool __io_sequence_defer(struct io_ring_ctx *ctx,
 				       struct io_kiocb *req)
 {
-	return req->sequence != ctx->cached_cq_tail + ctx->rings->sq_dropped;
+	return req->sequence != ctx->cached_cq_tail + ctx->cached_sq_dropped
+					+ atomic_read(&ctx->cached_cq_overflow);
 }
 
 static inline bool io_sequence_defer(struct io_ring_ctx *ctx,
@@ -567,9 +570,8 @@ static void io_cqring_fill_event(struct io_ring_ctx *ctx, u64 ki_user_data,
 		WRITE_ONCE(cqe->res, res);
 		WRITE_ONCE(cqe->flags, 0);
 	} else {
-		unsigned overflow = READ_ONCE(ctx->rings->cq_overflow);
-
-		WRITE_ONCE(ctx->rings->cq_overflow, overflow + 1);
+		WRITE_ONCE(ctx->rings->cq_overflow,
+				atomic_inc_return(&ctx->cached_cq_overflow));
 	}
 }
 
@@ -2564,7 +2566,8 @@ static bool io_get_sqring(struct io_ring_ctx *ctx, struct sqe_submit *s)
 
 	/* drop invalid entries */
 	ctx->cached_sq_head++;
-	rings->sq_dropped++;
+	ctx->cached_sq_dropped++;
+	WRITE_ONCE(rings->sq_dropped, ctx->cached_sq_dropped);
 	return false;
 }