Re: [PATCH] loop: Don't change loop device under exclusive opener

[Jens Axboe <axboe@kernel.dk>]

On 5/16/19 8:01 AM, Jan Kara wrote:
> 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>
> ---
>  drivers/block/loop.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
> 
> Hi Jens!
> 
> What do you think about this patch? It fixes the problem but it also
> changes user visible behavior so there are chances it breaks some
> existing setup (although I have hard time coming up with a realistic
> scenario where it would matter).

I also have a hard time thinking about valid cases where this would be a
problem. I think, in the end, that fixing the issue is more important
than a potentially hypothetical use case.

> Alternatively we could change getblk() code handle changing block
> size. That would fix the particular issue syzkaller found as well but
> I'm not sure what else is broken when block device changes while fs
> driver is working with it.

I think your solution here is saner.

-- 
Jens Axboe