fs/mount_setattr: tighten permission checks

We currently don't have any filesystems that support idmapped mounts which are mountable inside a user namespace. That was a deliberate decision for now as a userns root can just mount the filesystem themselves. So enforce this restriction explicitly until there's a real use-case for this. This way we can notice it and will have a chance to adapt and audit our translation helpers and fstests appropriately if we need to support such filesystems. Cc: Christoph Hellwig <[email protected]> Cc: Al Viro <[email protected]> Cc: [email protected] CC: [email protected] Suggested-by: Seth Forshee <[email protected]> Signed-off-by: Christian Brauner <[email protected]>
tuxedocomputers · May 12, 2021 · 2ca4dcc · 2ca4dcc
1 parent 6efb943
commit 2ca4dcc
Showing 1 changed file with 5 additions and 1 deletion.
diff --git a/fs/namespace.c b/fs/namespace.c
@@ -3855,8 +3855,12 @@ static int can_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
 	if (!(m->mnt_sb->s_type->fs_flags & FS_ALLOW_IDMAP))
 		return -EINVAL;
 
+	/* Don't yet support filesystem mountable in user namespaces. */
+	if (m->mnt_sb->s_user_ns != &init_user_ns)
+		return -EINVAL;
+
 	/* We're not controlling the superblock. */
-	if (!ns_capable(m->mnt_sb->s_user_ns, CAP_SYS_ADMIN))
+	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
 	/* Mount has already been visible in the filesystem hierarchy. */