loop: fix concurrent lo_open/lo_release

commit ae6650163c66a7eff1acd6eb8b0f752dcfa8eba5 upstream.

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>
Cc: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 85de673..a2a0dce 100644 (file)
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1576,9 +1576,8 @@ 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))
@@ -1605,6 +1604,13 @@ 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,