Now that setgroups can be disabled and not reenabled, setting gid_map
without privielge can now be enabled when setgroups is disabled.

This restores most of the functionality that was lost when unprivilege
setting of gid_map was removed.  Applications that use this
functionality will need to check to see if they use setgroups or
init_groups, and if they don't they can be fixed by simply
disabling of setgroups before they run.

Cc: stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Signed-off-by: "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
---
 kernel/user_namespace.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 3d128f91ced3..459c7f647072 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c

@@ -828,6 +828,11 @@ static bool new_idmap_permitted(const struct file *file,
 			kuid_t uid = make_kuid(ns->parent, id);
 			if (uid_eq(uid, cred->euid))
 				return true;
+		} else if (cap_setid == CAP_SETGID) {
+			kgid_t gid = make_kgid(ns->parent, id);
+			if (!userns_setgroups_allowed(ns) &&
+			    gid_eq(gid, cred->egid))
+				return true;
 		}
 	}
 

-- 
1.9.1