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. -- Jens Axboe
