From: Christian Brauner <christian.brauner@xxxxxxxxxx> commit 2ca4dcc4909d787ee153272f7efc2bff3b498720 upstream. 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 <hch@xxxxxx> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx CC: linux-fsdevel@xxxxxxxxxxxxxxx Suggested-by: Seth Forshee <seth.forshee@xxxxxxxxxxxxx> Signed-off-by: Christian Brauner <christian.brauner@xxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- fs/namespace.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) --- a/fs/namespace.c +++ b/fs/namespace.c @@ -3853,8 +3853,12 @@ static int can_idmap_mount(const struct 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. */
