linux-kernel - Re: new ...at() flag: AT_NO

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Date:   Fri, 5 May 2017 01:30:30 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Jann Horn <jannh@...gle.com>
Cc:     Linux API <linux-api@...r.kernel.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: new ...at() flag: AT_NO_JUMPS

On Mon, May 01, 2017 at 07:36:52PM +0200, Jann Horn wrote:

> Oh, nice!
> 
> It looks like this is somewhat similar to the old O_BENEATH proposal,
> but because the intentions behind the proposals are different
> (application sandboxing versus permitting an application to restrict its
> own filesystem accesses), the semantics differ: AT_NO_JUMPS
> doesn't prevent starting the path with "/", but does prevent mountpoint
> traversal. Is that correct?

It prevents both, actually - I missed that in description, but this
        if (unlikely(nd->flags & LOOKUP_NO_JUMPS))
                return -ELOOP;
in nd_jump_root() affects absolute pathnames same way as it affects
absolute symlinks.

It's not quite O_BENEATH, and IMO it's saner that way - a/b/c/../d is
bloody well allowed, and so are relative symlinks that do not lead out of
the subtree.  If somebody has a good argument in favour of flat-out
ban on .. (_other_ than "other guys do it that way, and it doesn't need
to make sense 'cuz security!!1!!!", please), I'd be glad to hear it.

As for mountpoint crossing...  it might make sense to split those.
O_BENEATH allowed it, and if we want AT_BENEATH to match that - let's
do it.  Then this one would become AT_BENEATH | AT_XDEV (the latter named
after find(1) option, obviously).

So how about this:

AT_BENEATH:
	* no absolute pathnames
	* no absolute symlinks
	* no procfs-style symlinks
	* no traversal of .. when we are at the same place where we'd started
(dir/../file is allowed, dir/../.. isn't)

AT_XDEV:
	* no mountpoint crossing allowed

For the latter I would prefer -EXDEV, for obvious reasons.  For the former...
not sure.  I'm not too happy about -ELOOP, but -EPERM (as with O_BENEATH)
is an atrocity - it's even more overloaded.

Suggestions?