From: Vitaly Mayatskikh <v.mayatskih@gmail.com>
Date: Fri, 22 Sep 2017 01:18:39 -0400
Subject: fix unbalanced page refcounting in bio_map_user_iov
References: bsc#1062568 CVE-2017-12190
Git-commit: 95d78c28b5a85bacbc29b8dba7c04babb9b0d467 
Patch-mainline: v4.14-rc5

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@vger.kernel.org
Signed-off-by: Vitaly Mayatskikh <v.mayatskih@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Johannes Thumshirn <jthumshirn@suse.de>
---
 block/bio.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/block/bio.c b/block/bio.c
index b38e962fa83e..0d6439e89acb 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1383,6 +1383,7 @@ 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;
@@ -1397,6 +1398,13 @@ 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;
 		}
-- 
2.12.3