4.19-stable review patch. If anyone has any objections, please let me know.
------------------
From: Dongli Zhang dongli.zhang@oracle.com
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 loop_ctl_mutex.
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
__loop_clr_fd() mutex_lock(&loop_ctl_mutex); lo->lo_backing_file = NULL; --> set to NULL mutex_unlock(&loop_ctl_mutex); loop_set_fd() mutex_lock_killable(&loop_ctl_mutex); loop_validate_file() f = l->lo_backing_file; --> NULL access if loop0 is not Lo_unbound mutex_lock(&loop_ctl_mutex); lo->lo_state = Lo_unbound; mutex_unlock(&loop_ctl_mutex);
lo->lo_backing_file should be accessed only when the loop device is Lo_bound.
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@syzkaller.appspotmail.com Signed-off-by: Dongli Zhang dongli.zhang@oracle.com Reviewed-by: Jan Kara jack@suse.cz Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- 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;