On 2019-09-04, Linus Torvalds wrote: > On Wed, Sep 4, 2019 at 1:23 PM Aleksa Sarai wrote: > > This patch allows for LOOKUP_BENEATH and LOOKUP_IN_ROOT to safely permit > > ".." resolution (in the case of LOOKUP_BENEATH the resolution will still > > fail if ".." resolution would resolve a path outside of the root -- > > while LOOKUP_IN_ROOT will chroot(2)-style scope it). Magic-link jumps > > are still disallowed entirely because now they could result in > > inconsistent behaviour if resolution encounters a subsequent ".."[*]. > > This is the only patch in the series that makes me go "umm". > > Why is it ok to re-initialize m_seq, which is used by other things > too? I think it's because we're out of RCU lookup, but there's no > comment about it, and it looks iffy to me. I'd rather have a separate > sequence count that doesn't have two users with different lifetime > rules. Yeah, the reasoning was that it's because we're out of RCU lookup and if we didn't re-grab ->m_seq we'd hit path_is_under() on every subsequent ".." (even though we've checked that it's safe). But yes, I should've used a different field to avoid confusion (and stop it looking unnecessarily dodgy). I will fix that. > But even apart from that, I think from a "patch continuity" standpoint > it would be better to introduce the sequence counts as just an error > condition first - iow, not have the "path_is_under()" check, but just > return -EXDEV if the sequence number doesn't match. Ack, will do. > So you'd have three stages: > > 1) ".." always returns -EXDEV > > 2) ".." returns -EXDEV if there was a concurrent rename/mount > > 3) ".." returns -EXDEV if there was a concurrent rename/mount and we > reset the sequence numbers and check if you escaped. > > becasue the sequence number reset really does make me go "hmm", plus I > get this nagging little feeling in the back of my head that you can > cause nasty O(n^2) lookup cost behavior with deep paths, lots of "..", > and repeated path_is_under() calls. The reason for doing the concurrent-{rename,mount} checks was to try to avoid the O(n^2) in most cases, but you're right that if you have an attacker that is spamming renames (or you're on a box with a lot of renames and/or mounts going on *anywhere*) you will hit an O(n^2) here (more pedantically, O(m*n) but who's counting?). Unfortunately, I'm not sure what the best solution would be for this one. If -EAGAIN retries are on the table, we could limit how many times we're willing to do path_is_under() and then just return -EAGAIN. > So (1) sounds safe. (2) sounds simple. And (3) is where I think subtle > things start happening. > > Also, I'm not 100% convinced that (3) is needed at all. I think the > retry could be done in user space instead, which needs to have a > fallback anyway. Yes? No? Hinting to userspace to do a retry (with -EAGAIN as you mention in your other mail) wouldn't be a bad thing at all, though you'd almost certainly get quite a few spurious -EAGAINs -- &{mount,rename}_lock are global for the entire machine, after all. But if the only significant roadblock is that (3) seems a bit too hairy, I would be quite happy with landing (2) as a first step (with -EAGAIN). -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH