[PATCH 3.2 118/199] nbd: fix use-after-free of rq/bio in the xmit path

[Ben Hutchings]

3.2.87-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Jens Axboe <axboe@xxxxxx>

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@xxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxx>
[bwh: Backported to 3.2:
 - bio_for_each_segment() uses iterator of type int
 - Open-code bio_iter_last()
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/block/nbd.c | 32 +++++++++++++++++++++++---------
 1 file changed, 23 insertions(+), 9 deletions(-)

--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -235,6 +235,7 @@ static int nbd_send_req(struct nbd_devic
 	int result, flags;
 	struct nbd_request request;
 	unsigned long size = blk_rq_bytes(req);
+	struct bio *bio;
 
 	request.magic = htonl(NBD_REQUEST_MAGIC);
 	request.type = htonl(nbd_cmd(req));
@@ -255,16 +256,20 @@ static int nbd_send_req(struct nbd_devic
 		goto error_out;
 	}
 
-	if (nbd_cmd(req) == NBD_CMD_WRITE) {
-		struct req_iterator iter;
+	if (nbd_cmd(req) != NBD_CMD_WRITE)
+		return 0;
+
+	flags = 0;
+	bio = req->bio;
+	while (bio) {
+		struct bio *next = bio->bi_next;
+		int i;
 		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(req, iter))
+
+		bio_for_each_segment(bvec, bio, i) {
+			bool is_last = !next && i == bio->bi_vcnt - 1;
+
+			if (is_last)
 				flags = MSG_MORE;
 			dprintk(DBG_TX, "%s: request %p: sending %d bytes data\n",
 					lo->disk->disk_name, req, bvec->bv_len);
@@ -275,7 +280,16 @@ static int nbd_send_req(struct nbd_devic
 					result);
 				goto error_out;
 			}
+			/*
+			 * 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;