loop: fix concurrent lo_open/lo_release

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@126.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files 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 @@ out:
         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,