loop: access lo_backing_file only when the loop device is Lo_bound
Commit758a58d0bc
("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 commit7ccd0791d9
("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:7ccd0791d9
("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>
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@ -656,7 +656,7 @@ static int loop_validate_file(struct file *file, struct block_device *bdev)
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return -EBADF;
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l = f->f_mapping->host->i_bdev->bd_disk->private_data;
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if (l->lo_state == Lo_unbound) {
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if (l->lo_state != Lo_bound) {
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return -EINVAL;
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}
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f = l->lo_backing_file;
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