[PATCH 4.19 38/45] loop: access lo_backing_file only when the loop device is Lo_bound

From: Greg Kroah-Hartman
Date: Tue Mar 26 2019 - 02:43:27 EST

4.19-stable review patch. If anyone has any objections, please let me know.


From: Dongli Zhang <dongli.zhang@xxxxxxxxxx>

commit f7c8a4120eedf24c36090b7542b179ff7a649219 upstream.

Commit 758a58d0bc67 ("loop: set GENHD_FL_NO_PART_SCAN after
blkdev_reread_part()") separates "lo->lo_backing_file = NULL" and
"lo->lo_state = Lo_unbound" into different critical regions protected by

However, there is below race that the NULL lo->lo_backing_file would be
accessed when the backend of a loop is another loop device, e.g., loop0's
backend is a file, while loop1's backend is loop0.

loop0's backend is file loop1's backend is loop0

lo->lo_backing_file = NULL; --> set to NULL
f = l->lo_backing_file; --> NULL
access if loop0 is not Lo_unbound
lo->lo_state = Lo_unbound;

lo->lo_backing_file should be accessed only when the loop device is

In fact, the problem has been introduced already in commit 7ccd0791d985
("loop: Push loop_ctl_mutex down into loop_clr_fd()") after which
loop_validate_file() could see devices in Lo_rundown state with which it
did not count. It was harmless at that point but still.

Fixes: 7ccd0791d985 ("loop: Push loop_ctl_mutex down into loop_clr_fd()")
Reported-by: syzbot+9bdc1adc1c55e7fe765b@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

drivers/block/loop.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -657,7 +657,7 @@ static int loop_validate_file(struct fil
return -EBADF;

l = f->f_mapping->host->i_bdev->bd_disk->private_data;
- if (l->lo_state == Lo_unbound) {
+ if (l->lo_state != Lo_bound) {
return -EINVAL;
f = l->lo_backing_file;