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loop: Don't change loop device under exclusive opener

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Loop module allows calling LOOP_SET_FD while there are other openers of
the loop device. Even exclusive ones. This can lead to weird
consequences such as kernel deadlocks like:

mount_bdev()				lo_ioctl()
  udf_fill_super()
    udf_load_vrs()
      sb_set_blocksize() - sets desired block size B
      udf_tread()
        sb_bread()
          __bread_gfp(bdev, block, B)
					  loop_set_fd()
					    set_blocksize()
            - now __getblk_slow() indefinitely loops because B != bdev
              block size

Fix the problem by disallowing LOOP_SET_FD ioctl when there are
exclusive openers of a loop device.

[Deliberately chosen not to CC stable as a user with priviledges to
trigger this race has other means of taking the system down and this
has a potential of breaking some weird userspace setup]

Reported-and-tested-by: syzbot+10007d66ca02b08f0e60@syzkaller.appspotmail.com
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Jan Kara 2019-05-16 16:01:27 +02:00 committed by Jens Axboe
parent a278682dad
commit 33ec3e53e7
1 changed files with 17 additions and 1 deletions
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18
drivers/block/loop.c
View File

@ -945,9 +945,20 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
if (!file)
goto out;
/*
* If we don't hold exclusive handle for the device, upgrade to it
* here to avoid changing device under exclusive owner.
*/
if (!(mode & FMODE_EXCL)) {
bdgrab(bdev);
error = blkdev_get(bdev, mode | FMODE_EXCL, loop_set_fd);
if (error)
goto out_putf;
}
error = mutex_lock_killable(&loop_ctl_mutex);
if (error)
goto out_putf;
goto out_bdev;
error = -EBUSY;
if (lo->lo_state != Lo_unbound)
@ -1012,10 +1023,15 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
mutex_unlock(&loop_ctl_mutex);
if (partscan)
loop_reread_partitions(lo, bdev);
if (!(mode & FMODE_EXCL))
blkdev_put(bdev, mode | FMODE_EXCL);
return 0;
out_unlock:
mutex_unlock(&loop_ctl_mutex);
out_bdev:
if (!(mode & FMODE_EXCL))
blkdev_put(bdev, mode | FMODE_EXCL);
out_putf:
fput(file);
out: