On 2019-09-04, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
On Wed, Sep 4, 2019 at 1:23 PM Aleksa Sarai <cyphar@xxxxxxxxxx> 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
<https://www.cyphar.com/>
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