io_uring: support using a registered personality for commands

For personalities previously registered via IORING_REGISTER_PERSONALITY,
allow any command to select them. This is done through setting
sqe->personality to the id returned from registration, and then flagging
sqe->flags with IOSQE_PERSONALITY.
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -4626,9 +4626,10 @@ static inline void io_queue_link_head(struct io_kiocb *req)
 static bool io_submit_sqe(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 			  struct io_submit_state *state, struct io_kiocb **link)
 {
+	const struct cred *old_creds = NULL;
 	struct io_ring_ctx *ctx = req->ctx;
 	unsigned int sqe_flags;
-	int ret;
+	int ret, id;
 
 	sqe_flags = READ_ONCE(sqe->flags);
 
@@ -4637,6 +4638,19 @@ static bool io_submit_sqe(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 		ret = -EINVAL;
 		goto err_req;
 	}
+
+	id = READ_ONCE(sqe->personality);
+	if (id) {
+		const struct cred *personality_creds;
+
+		personality_creds = idr_find(&ctx->personality_idr, id);
+		if (unlikely(!personality_creds)) {
+			ret = -EINVAL;
+			goto err_req;
+		}
+		old_creds = override_creds(personality_creds);
+	}
+
 	/* same numerical values with corresponding REQ_F_*, safe to copy */
 	req->flags |= sqe_flags & (IOSQE_IO_DRAIN|IOSQE_IO_HARDLINK|
 					IOSQE_ASYNC);
@@ -4646,6 +4660,8 @@ static bool io_submit_sqe(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 err_req:
 		io_cqring_add_event(req, ret);
 		io_double_put_req(req);
+		if (old_creds)
+			revert_creds(old_creds);
 		return false;
 	}
 
@@ -4706,6 +4722,8 @@ static bool io_submit_sqe(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 		}
 	}
 
+	if (old_creds)
+		revert_creds(old_creds);
 	return true;
 }
 

include/uapi/linux/io_uring.h

@@ -40,7 +40,12 @@ struct io_uring_sqe {
 	};
 	__u64	user_data;	/* data to be passed back at completion time */
 	union {
-		__u16	buf_index;	/* index into fixed buffers, if used */
+		struct {
+			/* index into fixed buffers, if used */
+			__u16	buf_index;
+			/* personality to use, if used */
+			__u16	personality;
+		};
 		__u64	__pad2[3];
 	};
 };