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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>
f7c8a4120e