Re: [PATCH 2/3] namei: implement AT_THIS_ROOT chroot-like path resolution

On 2018-09-29, Andy Lutomirski [off-list ref] wrote:

quoted

quoted

On Sat, Sep 29, 2018 at 4:29 PM Aleksa Sarai [off-list ref] 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.

I think that one way to solve this problem might be to have more strict
checks on nd->root in follow_dotdot(). The problem here (as far as I can
tell) is that ".." could end up skipping past the root because of a
rename, however walking *down* into a path shouldn't be a problem (even
absolute symlinks shouldn't be a problem because they will nd_jump_root
and will land back in the root).

However, I'm not entirely sure what happens to nd->root if it gets
renamed -- can you still safely do checks against it (we'd need to do
some sort of is_descendant() check on the current path before we handle
".." in follow_dotdot).

That way, we wouldn't shouldn't have the spectre-like attack problem
(since the attack would be halted at the ".." stage -- before the path
walk can proceed into host paths). Would this be sufficient or is there
a more serious issue I'm missing?

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?

I went into this in my response to Jann.

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