Quoting Serge E. Hallyn (serge@xxxxxxxxxx): > Quoting Eric W. Biederman (ebiederm@xxxxxxxxxxxx): > > Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes: > > > > > If a process gets access to a mount from a descendent or unrelated > > > user namespace, that process should not be able to take advantage of > > > setuid files or selinux entrypoints from that filesystem. > > > > > > This will make it safer to allow more complex filesystems to be > > > mounted in non-root user namespaces. > > > > > > This does not remove the need for MNT_LOCK_NOSUID. The setuid, > > > setgid, and file capability bits can no longer be abused if code in > > > a user namespace were to clear nosuid on an untrusted filesystem, > > > but this patch, by itself, is insufficient to protect the system > > > from abuse of files that, when execed, would increase MAC privilege. > > > > > > As a more concrete explanation, any task that can manipulate a > > > vfsmount associated with a given user namespace already has > > > capabilities in that namespace and all of its descendents. If they > > > can cause a malicious setuid, setgid, or file-caps executable to > > > appear in that mount, then that executable will only allow them to > > > elevate privileges in exactly the set of namespaces in which they > > > are already privileges. > > > > > > On the other hand, if they can cause a malicious executable to > > > appear with a dangerous MAC label, running it could change the > > > caller's security context in a way that should not have been > > > possible, even inside the namespace in which the task is confined. > > > > As presented this is complete and total nonsense. Mount propgation > > strongly weakens if not completely breaks the assumptions you are making > > in this code. > > > > To write any generic code that knows anything we need to capture a user > > namespace on struct super. > > > > Further I think all we really want is to filter out security labels from > > unprivileged mounts. uids/gids and the like should be completely fine > > because of the uid mappings. > > > > Having been down the route of comparing uids as userns uid tuples I am > > convinced that anything requires us to take the user namespace into > > account on a routine basis in the core will simply be broken for someone > > forgetting somewhere. This looks like a design that has that kind of > > susceptibility. > > The above paragraph is very compelling. However Andy's patch is a step > in the right direction from what we've got. I think given what you say > below and given Andy's rationale above, simply tweaking his patch to > ignore the parent-userns loop, and return false if current_user_ns() != > mount_userns, should be right? It'll prevent a child userns from > setting a selinux/apparmor entrypoint or POSIX file capabilities on a > file and having the parent userns trip over those. Ok, Andy's fn does the opposite, which will protect the parent userns, which is good. I suspect simply insisting that the user_ns's be equal is still better. It fits better with the idea that POSIX caps (and LSM entrypoints) are orthogonal to DAC. Kinda. -serge -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
