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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 4 Sep 2019 14:09:48 -0700
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Subject: Re: [PATCH v12 10/12] namei: aggressively check for nd->root escape
 on ".." resolution
To:     Aleksa Sarai <cyphar@cyphar.com>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
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On Wed, Sep 4, 2019 at 1:23 PM Aleksa Sarai <cyphar@cyphar.com> 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.

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.

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.

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?

                 Linus