[PATCH 4.9 07/21] nbd: fix use-after-free of rq/bio in the xmit path

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jens Axboe <axboe@fb.com>

commit 429a787be6793554ee02aacc7e1f11ebcecc4453 upstream.

For writes, we can get a completion in while we're still iterating
the request and bio chain. If that happens, we're reading freed
memory and we can crash.

Break out after the last segment and avoid having the iterator
read freed memory.

Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/block/nbd.c |   32 +++++++++++++++++++++++---------
 1 file changed, 23 insertions(+), 9 deletions(-)

--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -272,6 +272,7 @@ static int nbd_send_cmd(struct nbd_devic
 	int result, flags;
 	struct nbd_request request;
 	unsigned long size = blk_rq_bytes(req);
+	struct bio *bio;
 	u32 type;
 
 	if (req->cmd_type == REQ_TYPE_DRV_PRIV)
@@ -305,16 +306,20 @@ static int nbd_send_cmd(struct nbd_devic
 		return -EIO;
 	}
 
-	if (type == NBD_CMD_WRITE) {
-		struct req_iterator iter;
+	if (type != NBD_CMD_WRITE)
+		return 0;
+
+	flags = 0;
+	bio = req->bio;
+	while (bio) {
+		struct bio *next = bio->bi_next;
+		struct bvec_iter iter;
 		struct bio_vec bvec;
-		/*
-		 * we are really probing at internals to determine
-		 * whether to set MSG_MORE or not...
-		 */
-		rq_for_each_segment(bvec, req, iter) {
-			flags = 0;
-			if (!rq_iter_last(bvec, iter))
+
+		bio_for_each_segment(bvec, bio, iter) {
+			bool is_last = !next && bio_iter_last(bvec, iter);
+
+			if (is_last)
 				flags = MSG_MORE;
 			dev_dbg(nbd_to_dev(nbd), "request %p: sending %d bytes data\n",
 				cmd, bvec.bv_len);
@@ -325,7 +330,16 @@ static int nbd_send_cmd(struct nbd_devic
 					result);
 				return -EIO;
 			}
+			/*
+			 * The completion might already have come in,
+			 * so break for the last one instead of letting
+			 * the iterator do it. This prevents use-after-free
+			 * of the bio.
+			 */
+			if (is_last)
+				break;
 		}
+		bio = next;
 	}
 	return 0;
 }