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

On Sep 29, 2018, at 9:35 AM, Jann Horn [off-list ref] wrote:

+cc linux-api; please keep them in CC for future versions of the patch

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.

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?