[PATCH] loop: drop caches if offset is changed

[Jaegeuk Kim <jaegeuk@kernel.org>]

If we don't drop caches used in old offset, we can get old data from new
offset, which gives unexpected data to user.

Martijn found a loopback bug in the below scenario.
1) LOOP_SET_FD loads first two pages on loop file
2) LOOP_SET_STATUS64 changes the offset on the loop file
3) mount is failed due to the cached pages having wrong superblock

This patch drops caches when we change lo->offset.

Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Reported-by: Martijn Coenen <maco@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
---
 drivers/block/loop.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index cb0cc8685076..f073a3f1a7cd 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1154,6 +1154,12 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
 
 	if (lo->lo_offset != info->lo_offset ||
 	    lo->lo_sizelimit != info->lo_sizelimit) {
+		struct block_device *bdev = lo->lo_device;
+
+		/* drop stale caches used in old offset */
+		sync_blockdev(bdev);
+		kill_bdev(bdev);
+
 		if (figure_loop_size(lo, info->lo_offset, info->lo_sizelimit)) {
 			err = -EFBIG;
 			goto exit;
-- 
2.19.0.605.g01d371f741-goog