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! Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
