userns: allow sethostname in a container

Changelog:
Feb 23: let clone_uts_ns() handle setting uts->user_ns
To do so we need to pass in the task_struct who'll
get the utsname, so we can get its user_ns.
Feb 23: As per Oleg's coment, just pass in tsk, instead of two
of its members.

Signed-off-by: Serge E. Hallyn <[e-mail]>
Acked-by: "Eric W. Biederman" <[e-mail]>
Acked-by: Daniel Lezcano <[e-mail]>
Acked-by: David Howells <[e-mail]>
Cc: James Morris <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/utsname.h

···
 }
 
 extern struct uts_namespace *copy_utsname(unsigned long flags,
-					struct uts_namespace *ns);
+					  struct task_struct *tsk);
 extern void free_uts_ns(struct kref *kref);
 
 static inline void put_uts_ns(struct uts_namespace *ns)
···
 }
 
 static inline struct uts_namespace *copy_utsname(unsigned long flags,
-					struct uts_namespace *ns)
+						 struct task_struct *tsk)
 {
 	if (flags & CLONE_NEWUTS)
 		return ERR_PTR(-EINVAL);
 
-	return ns;
+	return tsk->nsproxy->uts_ns;
 }
 #endif
 

kernel/nsproxy.c

···
 		goto out_ns;
 	}
 
-	new_nsp->uts_ns = copy_utsname(flags, tsk->nsproxy->uts_ns);
+	new_nsp->uts_ns = copy_utsname(flags, tsk);
 	if (IS_ERR(new_nsp->uts_ns)) {
 		err = PTR_ERR(new_nsp->uts_ns);
 		goto out_uts;
-	}
-	if (new_nsp->uts_ns != tsk->nsproxy->uts_ns) {
-		put_user_ns(new_nsp->uts_ns->user_ns);
-		new_nsp->uts_ns->user_ns = task_cred_xxx(tsk, user)->user_ns;
-		get_user_ns(new_nsp->uts_ns->user_ns);
 	}
 
 	new_nsp->ipc_ns = copy_ipcs(flags, tsk->nsproxy->ipc_ns);

kernel/sys.c

···
 	int errno;
 	char tmp[__NEW_UTS_LEN];
 
-	if (!capable(CAP_SYS_ADMIN))
+	if (!ns_capable(current->nsproxy->uts_ns->user_ns, CAP_SYS_ADMIN))
 		return -EPERM;
 	if (len < 0 || len > __NEW_UTS_LEN)
 		return -EINVAL;

kernel/utsname.c

···
  * @old_ns: namespace to clone
  * Return NULL on error (failure to kmalloc), new ns otherwise
  */
-static struct uts_namespace *clone_uts_ns(struct uts_namespace *old_ns)
+static struct uts_namespace *clone_uts_ns(struct task_struct *tsk,
+					  struct uts_namespace *old_ns)
 {
 	struct uts_namespace *ns;
 
···
 
 	down_read(&uts_sem);
 	memcpy(&ns->name, &old_ns->name, sizeof(ns->name));
-	ns->user_ns = old_ns->user_ns;
-	get_user_ns(ns->user_ns);
+	ns->user_ns = get_user_ns(task_cred_xxx(tsk, user)->user_ns);
 	up_read(&uts_sem);
 	return ns;
 }
···
  * utsname of this process won't be seen by parent, and vice
  * versa.
  */
-struct uts_namespace *copy_utsname(unsigned long flags, struct uts_namespace *old_ns)
+struct uts_namespace *copy_utsname(unsigned long flags,
+				   struct task_struct *tsk)
 {
+	struct uts_namespace *old_ns = tsk->nsproxy->uts_ns;
 	struct uts_namespace *new_ns;
 
 	BUG_ON(!old_ns);
···
 	if (!(flags & CLONE_NEWUTS))
 		return old_ns;
 
-	new_ns = clone_uts_ns(old_ns);
+	new_ns = clone_uts_ns(tsk, old_ns);
 
 	put_uts_ns(old_ns);
 	return new_ns;