Jann Horn <jann@xxxxxxxxx> writes: > On Tue, Oct 18, 2016 at 09:56:53AM -0500, Eric W. Biederman wrote: >> Michal Hocko <mhocko@xxxxxxxxxx> writes: >> >> > On Mon 17-10-16 11:39:49, Eric W. Biederman wrote: >> >> >> >> During exec dumpable is cleared if the file that is being executed is >> >> not readable by the user executing the file. A bug in >> >> ptrace_may_access allows reading the file if the executable happens to >> >> enter into a subordinate user namespace (aka clone(CLONE_NEWUSER), >> >> unshare(CLONE_NEWUSER), or setns(fd, CLONE_NEWUSER). >> >> >> >> This problem is fixed with only necessary userspace breakage by adding >> >> a user namespace owner to mm_struct, captured at the time of exec, >> >> so it is clear in which user namespace CAP_SYS_PTRACE must be present >> >> in to be able to safely give read permission to the executable. >> >> >> >> The function ptrace_may_access is modified to verify that the ptracer >> >> has CAP_SYS_ADMIN in task->mm->user_ns instead of task->cred->user_ns. >> >> This ensures that if the task changes it's cred into a subordinate >> >> user namespace it does not become ptraceable. >> > >> > I haven't studied your patch too deeply but one thing that immediately >> > raised a red flag was that mm might be shared between processes (aka >> > thread groups). What prevents those two to sit in different user >> > namespaces? >> > >> > I am primarily asking because this generated a lot of headache for the >> > memcg handling as those processes might sit in different cgroups while >> > there is only one correct memcg for them which can disagree with the >> > cgroup associated with one of the processes. >> >> That is a legitimate concern, but I do not see any of those kinds of >> issues here. >> >> Part of the memcg pain comes from the fact that control groups are >> process centric, and part of the pain comes from the fact that it is >> possible to change control groups. What I am doing is making the mm >> owned by a user namespace (at creation time), and I am not allowing >> changes to that ownership. The credentials of the tasks that use that mm >> may be in the same user namespace or descendent user namespaces. >> >> The core goal is to enforce the unreadability of an mm when an >> non-readable file is executed. This is a time of mm creation property. >> The enforcement of which fits very well with the security/permission >> checking role of the user namespace. > > How is that going to work? I thought the core goal was better security for > entering containers. The better security when entering containers came from fixing the the check for unreadable files. Because that is fundamentally what the mm dumpable settings are for. > If I want to dump a non-readable file, afaik, I can just make a new user > namespace, then run the file in there and dump its memory. > I guess you could fix that by entirely prohibiting the execution of a > non-readable file whose owner UID is not mapped. (Adding more dumping > restrictions wouldn't help much because you could still e.g. supply a > malicious dynamic linker if you control the mount namespace.) That seems to be a part of this puzzle I have incompletely addressed, thank you. It looks like I need to change either the owning user namespace or fail the exec. Malicious dynamic linkers are doubly interesting. As mount name spaces are also owned if I have privileges I can address the possibility of a malicious dynamic linker that way. AKA who cares about the link if the owner of the mount namespace has permissions to read the file. I am going to look at failing the exec if the owning user namespace of the mm would not have permissions to read the file. That should just be a couple of lines of code and easy to maintain. Plus it does not appear that non-readable executables are particularly common. Eric -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>