Jann Horn <jann@xxxxxxxxx> writes: > On Tue, Oct 18, 2016 at 10:35:23AM -0500, Eric W. Biederman wrote: >> 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. > > Oh, interesting. > > >> > 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. > > If you just check the owner of the mount namespace, someone could still > use a user namespace to chroot() the process. That should also be > sufficient to get the evil linker in. I think it really needs to be the > user namespace of the executing process that's checked, not the user > namespace associated with some mount namespace. Something. I will just note that this is hard to analyze and theoretically possible for now, since I don't intend to pursue that solution. >> 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. > > Hm. Yeah, I guess mode 04111 probably isn't sooo common. > From a security perspective, I think that should work. Well there is at least one common distro that installs sudo that way so I would not say uncommon. But we already ignore the suid and sgid bit when executing such executables as without having the uid or gid mapping into a user namespace suid and sgid can not be supported. So the only case that could cause a real regression/loss of functionality is if there are unreadable executables without the suid or sgid bit set. I can't find any of those. Patch for this second bug in a moment. Eric -- 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
