linux-kernel - Re: [PATCH v3 0/3] namei: implement various lookup restriction AT

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Message-ID: <20181017152332.madyld45jvhtoog3@ryuk>
Date:   Thu, 18 Oct 2018 02:23:32 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Al Viro <viro@...iv.linux.org.uk>,
        Eric Biederman <ebiederm@...ssion.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        David Howells <dhowells@...hat.com>,
        Jann Horn <jannh@...gle.com>,
        Christian Brauner <christian@...uner.io>,
        David Drysdale <drysdale@...gle.com>,
        containers@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>, Tycho Andersen <tycho@...ho.ws>,
        dev@...ncontainers.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: [PATCH v3 0/3] namei: implement various lookup restriction AT_*
 flags

On 2018-10-09, Aleksa Sarai <cyphar@...har.com> wrote:
> The need for some sort of control over VFS's path resolution (to avoid
> malicious paths resulting in inadvertent breakouts) has been a very
> long-standing desire of many userspace applications. This patchset is a
> revival of Al Viro's old AT_NO_JUMPS[1,2] patchset (which was a variant
> of David Drysdale's O_BENEATH patchset[3] which was a spin-off of the
> Capsicum project[4]) with a few additions and changes made based on the
> previous discussion within [5] as well as others I felt were useful.

I've been thinking about this problem a little more (from the UX side of
things) and I have a feeling that adding 5 different O_* flags related
to resolution -- rather than properties related to opening the file --
might be less than ideal (even though, as discussed in previous threads,
there is a need for these flags and for them to be separated).

There is *some* precedence for this with O_PATH[**] changing fairly
large semantics of openat(2) but there are some things about O_PATH
which I think could be improved.

What if we had a resolveat(2) which acted like openat(..., O_PATH) *but*
it allowed us to have new flags and to separate the scoping flags from
the (fairly limited) space of O_* flags. Then O_PATH could effectively
just be a legacy way of doing resolveat(2) -- with only O_CLOEXEC,
O_DIRECTORY, and O_NOFOLLOW support.

And the main things we could add would be:

  * These resolution flags, with only support available from
	resolveat(2) for the moment. The idea would be that AT_EMPTY_PATH
	would be the recommended way to make use of this.

  * Support for RESOLVE_{NOPERM,RDONLY,WRONLY,RDWR} (which after some
	discussions with Eric last year might be necessary in order to make
	/proc/$pid/fd/$fd re-opening of O_PATH descriptors safer -- which is
	something that we use in both runc and LXC).

Is this idea palatable, or was this something considered during the
development of O_PATH and someone had an argument why augmenting O_PATH
is better than a new syscall?

[**] And while writing this paragraph I noticed that I didn't update the
     O_PATH "flag whitelist" to allow the scoping flags to affect it. I
	 will include a fix for this in v4 (I must've lost it in an early
	 rebase before I sent v1).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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