user_ns: Introduce user_nsmap_uid and user_ns_map_gid.

Define what happens when a we view a uid from one user_namespace
in another user_namepece.

- If the user namespaces are the same no mapping is necessary.

- For most cases of difference use overflowuid and overflowgid,
  the uid and gid currently used for 16bit apis when we have a 32bit uid
  that does fit in 16bits.  Effectively the situation is the same,
  we want to return a uid or gid that is not assigned to any user.

- For the case when we happen to be mapping the uid or gid of the
  creator of the target user namespace use uid 0 and gid as confusing
  that user with root is not a problem.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: Serge E. Hallyn <serue@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 44 insertions, 0 deletions

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index b2d70d38dff..25915832291 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -9,6 +9,7 @@
 #include <linux/nsproxy.h>
 #include <linux/slab.h>
 #include <linux/user_namespace.h>
+#include <linux/highuid.h>
 #include <linux/cred.h>
 
 /*
@@ -82,3 +83,46 @@ void free_user_ns(struct kref *kref)
         schedule_work(&ns->destroyer);
 }
 EXPORT_SYMBOL(free_user_ns);
+
+uid_t user_ns_map_uid(struct user_namespace *to, const struct cred *cred, uid_t uid)
+{
+        struct user_namespace *tmp;
+
+        if (likely(to == cred->user->user_ns))
+                return uid;
+
+
+        /* Is cred->user the creator of the target user_ns
+         * or the creator of one of it's parents?
+         */
+        for ( tmp = to; tmp != &init_user_ns;
+              tmp = tmp->creator->user_ns ) {
+                if (cred->user == tmp->creator) {
+                        return (uid_t)0;
+                }
+        }
+
+        /* No useful relationship so no mapping */
+        return overflowuid;
+}
+
+gid_t user_ns_map_gid(struct user_namespace *to, const struct cred *cred, gid_t gid)
+{
+        struct user_namespace *tmp;
+
+        if (likely(to == cred->user->user_ns))
+                return gid;
+
+        /* Is cred->user the creator of the target user_ns
+         * or the creator of one of it's parents?
+         */
+        for ( tmp = to; tmp != &init_user_ns;
+              tmp = tmp->creator->user_ns ) {
+                if (cred->user == tmp->creator) {
+                        return (gid_t)0;
+                }
+        }
+
+        /* No useful relationship so no mapping */
+        return overflowgid;
+}