[CFT][PATCH 8/8] userns: Allow setting gid_maps without privilege when setgroups is disabled

[ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman)]

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 unprivileged
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 setgroups before writing to
gid_map.

Cc: stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Reviewed-by: Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@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 b507f9af7ff2..3b29b9a52332 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -826,6 +826,11 @@ 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 (!(ns->flags & USERNS_SETGROUPS_ALLOWED) &&
+			    gid_eq(gid, cred->egid))
+				return true;
 		}
 	}
 
-- 
1.9.1