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>

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index cc4f453..8178156 100644 (file)
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -36,6 +36,9 @@ static inline void put_user_ns(struct user_namespace *ns)
                kref_put(&ns->kref, free_user_ns);
 }
 
+uid_t user_ns_map_uid(struct user_namespace *to, const struct cred *cred, uid_t uid);
+gid_t user_ns_map_gid(struct user_namespace *to, const struct cred *cred, gid_t gid);
+
 #else
 
 static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
@@ -52,6 +55,17 @@ static inline void put_user_ns(struct user_namespace *ns)
 {
 }
 
+static inline uid_t user_ns_map_uid(struct user_namespace *to,
+       const struct cred *cred, uid_t uid)
+{
+       return uid;
+}
+static inline gid_t user_ns_map_gid(struct user_namespace *to,
+       const struct cred *cred, gid_t gid)
+{
+       return gid;
+}
+
 #endif
 
 #endif /* _LINUX_USER_H */

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index b2d70d3..2591583 100644 (file)
--- 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;
+}