[SRU][Zesty][PATCH 1/2] fix unbalanced page refcounting in bio_map_user_iov

[Kleber Sacilotto de Souza]

From: Vitaly Mayatskikh <v.mayatskih at gmail.com>

bio_map_user_iov and bio_unmap_user do unbalanced pages refcounting if
IO vector has small consecutive buffers belonging to the same page.
bio_add_pc_page merges them into one, but the page reference is never
dropped.

Cc: stable at vger.kernel.org
Signed-off-by: Vitaly Mayatskikh <v.mayatskih at gmail.com>
Signed-off-by: Al Viro <viro at zeniv.linux.org.uk>

CVE-2017-12190
(cherry picked from commit 95d78c28b5a85bacbc29b8dba7c04babb9b0d467)
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza at canonical.com>
---
 block/bio.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/block/bio.c b/block/bio.c
index 17ece5b40a2f..13185834b253 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1372,6 +1372,7 @@ struct bio *bio_map_user_iov(struct request_queue *q,
 		offset = offset_in_page(uaddr);
 		for (j = cur_page; j < page_limit; j++) {
 			unsigned int bytes = PAGE_SIZE - offset;
+			unsigned short prev_bi_vcnt = bio->bi_vcnt;
 
 			if (len <= 0)
 				break;
@@ -1386,6 +1387,13 @@ struct bio *bio_map_user_iov(struct request_queue *q,
 					    bytes)
 				break;
 
+			/*
+			 * check if vector was merged with previous
+			 * drop page reference if needed
+			 */
+			if (bio->bi_vcnt == prev_bi_vcnt)
+				put_page(pages[j]);
+
 			len -= bytes;
 			offset = 0;
 		}
-- 
2.14.1