Re: [PATCH] procfs: change the owner of non-dumpable and writeable files

[Aleksa Sarai]

> > In order to protect against ptrace(2) and similar attacks on container
> > runtimes when they join namespaces, many runtimes set mm->dumpable to
> > SUID_DUMP_DISABLE. However, doing this means that attempting to set up
> > an unprivileged user namespace will fail because an unprivileged process
> > can no longer access /proc/self/{setgroups,{uid,gid}_map} for the
> > container process (which is the same uid as the runtime process).
> >
> > Fix this by changing pid_getattr to *also* change the owner of regular
> > files that have a mode of 0644 (when the process is not dumpable). This
> > ensures that the important /proc/[pid]/... files mentioned above are
> > properly accessible by a container runtime in a rootless container
> > context.
> >
> > The most blantant issue is that a non-dumpable process in a rootless
> > container context is unable to open /proc/self/setgroups, because it
> > doesn't own the file.
> >
> > int main(void)
> > {
> > 	prctl(PR_SET_DUMPABLE, 0, 0, 0, 0);
> > 	unshare(CLONE_NEWUSER);
> >
> > 	/* This will fail. */
> > 	int fd = open("/proc/self/setgroups", O_WRONLY);
> > 	if (fd < 0)
> > 		abort();
> >
> > 	return 0;
> > }
>
> I do agree that failing to open anything in /proc/self/ is more than
> unexepcted! I cannot judge the patch but my gut feeling tells me that
> the fix should be somewhere in the open handler.

Maybe that would suffice as a more specific fix (for the special case of 
/proc/self), but the fact that none of the users and groups are 
correctly set in /proc/[pid] will cause issues for runC and other 
container runtimes (because they don't go through /proc/self -- it's 
accessing /proc/[pid] from another process).

Though I get the feeling that the *correct* fix would be to remove the 
conditional and *always* change the owner -- maybe I'm missing something 
but I can't think of the security issue that this code currently fixes 
(since all of the important permission checks are *in addition* to the 
generic_permission used for /proc/self/..., which use ptrace_may_access).

--
Aleksa Sarai
Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/