linux-kernel - Re: [PATCH 2/3] namei: implement AT_THIS

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Date:   Sat, 29 Sep 2018 10:25:17 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Jann Horn <jannh@...gle.com>
Cc:     cyphar@...har.com, "Eric W. Biederman" <ebiederm@...ssion.com>,
        jlayton@...nel.org, Bruce Fields <bfields@...ldses.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>, shuah@...nel.org,
        David Howells <dhowells@...hat.com>,
        Andy Lutomirski <luto@...nel.org>, christian@...uner.io,
        Tycho Andersen <tycho@...ho.ws>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-kselftest@...r.kernel.org, dev@...ncontainers.org,
        containers@...ts.linux-foundation.org,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH 2/3] namei: implement AT_THIS_ROOT chroot-like path resolution



> On Sep 29, 2018, at 9:35 AM, Jann Horn <jannh@...gle.com> wrote:
> 
> +cc linux-api; please keep them in CC for future versions of the patch
> 
>> On Sat, Sep 29, 2018 at 4:29 PM Aleksa Sarai <cyphar@...har.com> wrote:
>> The primary motivation for the need for this flag is container runtimes
>> which have to interact with malicious root filesystems in the host
>> namespaces. One of the first requirements for a container runtime to be
>> secure against a malicious rootfs is that they correctly scope symlinks
>> (that is, they should be scoped as though they are chroot(2)ed into the
>> container's rootfs) and ".."-style paths. The already-existing AT_XDEV
>> and AT_NO_PROCLINKS help defend against other potential attacks in a
>> malicious rootfs scenario.
> 
> So, I really like the concept for patch 1 of this series (but haven't
> read the code yet); but I dislike this patch because of its footgun
> potential.
> 

The code could do it differently: do the path walk and then, before accepting the result, walk back up and make sure the result is under the starting point.

This is *not* a full solution, though, since a walk above the root gas side effects on timing, various caches, and possibly network traffic, so it’s open to Spectre-like attacks in which a malicious container could use a runtime-initiated AT_THIS_ROOT to infer the existence of directories outside the container.

But what’s the container usecase?  Any sane container is based on pivot_root or similar, so the runtime can just do the walk in the container context. IOW I’m a bit confused as to the exact intended use of the whole series. Can you elaborate?