[PATCH] block: fix uint overflow when merging io requests

[Junxiao Bi <junxiao.bi@oracle.com>]

This uint overflow will cause req->__data_len < req->bio->bi_size,
this will confuse block layer and device driver.

I watched a panic caused by this when mkfs.ext4 a volume of a large
virtual disk on vm guest, blkdev_issue_discard() issue two bio with
a total size over UINT_MAX, but the check in ll_back_merge_fn() didn't
take affect due to the overflow and they were merged into one request.
After the request is done, in blk_end_request_all(), BUG_ON(pending)
was triggered and kernel panic. "pending" is true is because
blk_update_request() return ture when req->__data_len is less
than req->bio->bi_size.

Signed-off-by: Junxiao Bi <junxiao.bi@oracle.com>
---
 block/blk-merge.c |   40 ++++++++++++++++++++++++++++++++++------
 1 file changed, 34 insertions(+), 6 deletions(-)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index b3bf0df..340c0a7 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -325,11 +325,41 @@ no_merge:
 	return 0;
 }
 
-int ll_back_merge_fn(struct request_queue *q, struct request *req,
+static inline bool ll_allow_merge_bio(struct request *req,
 		     struct bio *bio)
 {
 	if (blk_rq_sectors(req) + bio_sectors(bio) >
-	    blk_rq_get_max_sectors(req)) {
+		blk_rq_get_max_sectors(req))
+		return false;
+
+	/* check uint overflow */
+	if (blk_rq_sectors(req) + bio_sectors(bio) < blk_rq_sectors(req)
+		|| blk_rq_sectors(req) + bio_sectors(bio) < bio_sectors(bio))
+		return false;
+
+	return true;
+}
+
+static inline bool ll_allow_merge_req(struct request *req,
+		     struct request *next)
+{
+	if (blk_rq_sectors(req) + blk_rq_sectors(next) >
+		blk_rq_get_max_sectors(req))
+		return false;
+
+	/* check uint overflow */
+	if (blk_rq_sectors(req) + blk_rq_sectors(next) < blk_rq_sectors(req)
+		|| blk_rq_sectors(req) + blk_rq_sectors(next) <
+		blk_rq_sectors(next))
+		return false;
+
+	return true;
+}
+
+int ll_back_merge_fn(struct request_queue *q, struct request *req,
+		     struct bio *bio)
+{
+	if (!ll_allow_merge_bio(req, bio)) {
 		req->cmd_flags |= REQ_NOMERGE;
 		if (req == q->last_merge)
 			q->last_merge = NULL;
@@ -346,8 +376,7 @@ int ll_back_merge_fn(struct request_queue *q, struct request *req,
 int ll_front_merge_fn(struct request_queue *q, struct request *req,
 		      struct bio *bio)
 {
-	if (blk_rq_sectors(req) + bio_sectors(bio) >
-	    blk_rq_get_max_sectors(req)) {
+	if (!ll_allow_merge_bio(req, bio)) {
 		req->cmd_flags |= REQ_NOMERGE;
 		if (req == q->last_merge)
 			q->last_merge = NULL;
@@ -389,8 +418,7 @@ static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
 	/*
 	 * Will it become too large?
 	 */
-	if ((blk_rq_sectors(req) + blk_rq_sectors(next)) >
-	    blk_rq_get_max_sectors(req))
+	if (!ll_allow_merge_req(req, next))
 		return 0;
 
 	total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
-- 
1.7.9.5