On 2018-09-29, Andy Lutomirski email@example.com wrote:
On Sat, Sep 29, 2018 at 4:29 PM Aleksa Sarai firstname.lastname@example.org 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.