On Mon, Dec 8, 2014 at 2:44 PM, Eric W. Biederman [off-list ref] wrote:

Andy Lutomirski [off-list ref] writes:

quoted

On Mon, Dec 8, 2014 at 2:11 PM, Eric W. Biederman [off-list ref] wrote:

quoted

- Expose the knob to user space through a proc file /proc/<pid>/setgroups

  A value of 0 means the setgroups system call is disabled in the

"deny"

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  current processes user namespace and can not be enabled in the
  future in this user namespace.

  A value of 1 means the segtoups system call is enabled.

"allow"

quoted

- Descedent user namespaces inherit the value of setgroups from

s/Descedent/Descendent/

Bah.  I updated everything but the changelog comment.

quoted

quoted

--- a/kernel/groups.c
+++ b/kernel/groups.c

@@ -222,6 +222,7 @@ bool may_setgroups(void)
         * the user namespace has been established.
         */
        return userns_gid_mappings_established(user_ns) &&
+               userns_setgroups_allowed(user_ns) &&
                ns_capable(user_ns, CAP_SETGID);
 }

Can you add a comment explaining the ordering?  For example:

I need to think on what I can say to make it clear.
Perhaps: /* Careful the order of these checks is important. */

quoted

We need to check for a gid mapping before checking setgroups_allowed
because an unprivileged user can create a userns with setgroups
allowed, then disallow setgroups and add a mapping.  If we check in
the opposite order, then we have a race: we could see that setgroups
is allowed before the user clears the bit and then see that there is a
gid mapping after the other thread is done.

This text was actually my suggested comment text.

If you put smp_rmb() in this function with a comment like that, then I
think it will all make sense and be obviously correct (even with most
of the other barriers removed).

--Andy

Since these are independent atomic variables yes that ordering issue
seems to be the case.

For me it was the natural ordering of the checks so I didn't even bother
to think about what happens when you reorder them.

Eric

-- 
Andy Lutomirski
AMA Capital Management, LLC