On Thu 18-07-19 16:15:42, Kai-Heng Feng wrote: > Hi Jan, > > at 21:34, Jens Axboe <axboe@xxxxxxxxx> wrote: > > > On 5/27/19 6:29 AM, Jan Kara wrote: > > > On Thu 16-05-19 14:44:07, Jens Axboe wrote: > > > > 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@xxxxxxxxxxxxxxxxxxxxxxxxx > > > > > Signed-off-by: Jan Kara <jack@xxxxxxx> > > > > > --- > > > > > 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. > > > > > > Will you pick up the patch please? I cannot find it in your tree... > > > Thanks! > > > > Done! > > This patch introduced a regression [1]. > A reproducer can be found at [2]. > > [1] https://bugs.launchpad.net/bugs/1836914 > [2] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1836914/comments/4 Thanks for the notice and the references. What's your version of util-linux? What your test script does is indeed racy. You have there: echo Running: for i in {a..z}{a..z}; do mount $i.squash /mnt/$i & done So all mount(8) commands will run in parallel and race to setup loop devices with LOOP_SET_FD and mount them. However util-linux (at least in the current version) seems to handle EBUSY from LOOP_SET_FD just fine and retries with the new loop device. So at this point I don't see why the patch makes difference... I guess I'll need to reproduce and see what's going on in detail. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
