On Tue, Oct 18, 2016 at 09:56:53AM -0500, Eric W. Biederman wrote: > Michal Hocko <mho...@kernel.org> 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. 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.)
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