get rid of unprotected dereferencing of mnt->mnt_ns

It's safe only under namespace_sem or vfsmount_lock; all places
in fs/namespace.c that want mnt->mnt_ns->user_ns actually want to use
current->nsproxy->mnt_ns->user_ns (note the calls of check_mnt() in
there).

Cc: [e-mail]
Signed-off-by: Al Viro <[e-mail]>

fs/namespace.c

···
 	return retval;
 }
 
+/* 
+ * Is the caller allowed to modify his namespace?
+ */
+static inline bool may_mount(void)
+{
+	return ns_capable(current->nsproxy->mnt_ns->user_ns, CAP_SYS_ADMIN);
+}
+
 /*
  * Now umount can handle mount points as well as block devices.
  * This is important for filesystems which use unnamed block devices.
···
 	if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW))
 		return -EINVAL;
 
+	if (!may_mount())
+		return -EPERM;
+
 	if (!(flags & UMOUNT_NOFOLLOW))
 		lookup_flags |= LOOKUP_FOLLOW;
 
···
 	if (path.dentry != path.mnt->mnt_root)
 		goto dput_and_out;
 	if (!check_mnt(mnt))
-		goto dput_and_out;
-
-	retval = -EPERM;
-	if (!ns_capable(mnt->mnt_ns->user_ns, CAP_SYS_ADMIN))
 		goto dput_and_out;
 
 	retval = do_umount(mnt, flags);
···
 
 static int mount_is_safe(struct path *path)
 {
-	if (ns_capable(real_mount(path->mnt)->mnt_ns->user_ns, CAP_SYS_ADMIN))
+	if (may_mount())
 		return 0;
 	return -EPERM;
 #ifdef notyet
···
 	int type;
 	int err = 0;
 
-	if (!ns_capable(mnt->mnt_ns->user_ns, CAP_SYS_ADMIN))
+	if (!may_mount())
 		return -EPERM;
 
 	if (path->dentry != path->mnt->mnt_root)
···
 	struct mount *p;
 	struct mount *old;
 	int err = 0;
-	if (!ns_capable(real_mount(path->mnt)->mnt_ns->user_ns, CAP_SYS_ADMIN))
+	if (!may_mount())
 		return -EPERM;
 	if (!old_name || !*old_name)
 		return -EINVAL;
···
 			int mnt_flags, const char *name, void *data)
 {
 	struct file_system_type *type;
-	struct user_namespace *user_ns;
+	struct user_namespace *user_ns = current->nsproxy->mnt_ns->user_ns;
 	struct vfsmount *mnt;
 	int err;
 
 	if (!fstype)
 		return -EINVAL;
 
-	/* we need capabilities... */
-	user_ns = real_mount(path->mnt)->mnt_ns->user_ns;
-	if (!ns_capable(user_ns, CAP_SYS_ADMIN))
+	if (!may_mount())
 		return -EPERM;
 
 	type = get_fs_type(fstype);
···
 	struct mount *new_mnt, *root_mnt;
 	int error;
 
-	if (!ns_capable(current->nsproxy->mnt_ns->user_ns, CAP_SYS_ADMIN))
+	if (!may_mount())
 		return -EPERM;
 
 	error = user_path_dir(new_root, &new);