[SRU][X][A][B][PATCH 1/1] loop: fix concurrent lo_open/lo_release

[Kleber Sacilotto de Souza]

From: Linus Torvalds <torvalds at linux-foundation.org>

CVE-2018-5344

范龙飞 reports that KASAN can report a use-after-free in __lock_acquire.
The reason is due to insufficient serialization in lo_release(), which
will continue to use the loop device even after it has decremented the
lo_refcnt to zero.

In the meantime, another process can come in, open the loop device
again as it is being shut down. Confusion ensues.

Reported-by: 范龙飞 <long7573 at 126.com>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Jens Axboe <axboe at kernel.dk>
(cherry picked from commit ae6650163c66a7eff1acd6eb8b0f752dcfa8eba5)
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza at canonical.com>
---
 drivers/block/loop.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index bc8e61506968..d5fe720cf149 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1581,9 +1581,8 @@ static int lo_open(struct block_device *bdev, fmode_t mode)
 	return err;
 }
 
-static void lo_release(struct gendisk *disk, fmode_t mode)
+static void __lo_release(struct loop_device *lo)
 {
-	struct loop_device *lo = disk->private_data;
 	int err;
 
 	if (atomic_dec_return(&lo->lo_refcnt))
@@ -1610,6 +1609,13 @@ static void lo_release(struct gendisk *disk, fmode_t mode)
 	mutex_unlock(&lo->lo_ctl_mutex);
 }
 
+static void lo_release(struct gendisk *disk, fmode_t mode)
+{
+	mutex_lock(&loop_index_mutex);
+	__lo_release(disk->private_data);
+	mutex_unlock(&loop_index_mutex);
+}
+
 static const struct block_device_operations lo_fops = {
 	.owner =	THIS_MODULE,
 	.open =		lo_open,
-- 
2.14.1