[PATCH v3 5/7] fs: Treat foreign mounts as nosuid

[Andy Lutomirski]

On Wed, Sep 16, 2015 at 1:02 PM, Seth Forshee
<seth.forshee at canonical.com> wrote:
> From: Andy Lutomirski <luto at amacapital.net>
>
> If a process gets access to a mount from a different user
> namespace, that process should not be able to take advantage of
> setuid files or selinux entrypoints from that filesystem.  Prevent
> this by treating mounts from other mount namespaces and those not
> owned by current_user_ns() or an ancestor as nosuid.
>
> This will make it safer to allow more complex filesystems to be
> mounted in non-root user namespaces.
>
> This does not remove the need for MNT_LOCK_NOSUID.  The setuid,
> setgid, and file capability bits can no longer be abused if code in
> a user namespace were to clear nosuid on an untrusted filesystem,
> but this patch, by itself, is insufficient to protect the system
> from abuse of files that, when execed, would increase MAC privilege.
>
> As a more concrete explanation, any task that can manipulate a
> vfsmount associated with a given user namespace already has
> capabilities in that namespace and all of its descendents.  If they
> can cause a malicious setuid, setgid, or file-caps executable to
> appear in that mount, then that executable will only allow them to
> elevate privileges in exactly the set of namespaces in which they
> are already privileges.
>
> On the other hand, if they can cause a malicious executable to
> appear with a dangerous MAC label, running it could change the
> caller's security context in a way that should not have been
> possible, even inside the namespace in which the task is confined.
>
> As a hardening measure, this would have made CVE-2014-5207 much
> more difficult to exploit.
>
> Signed-off-by: Andy Lutomirski <luto at amacapital.net>
> Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
> ---
>  fs/exec.c                |  2 +-
>  fs/namespace.c           | 13 +++++++++++++
>  include/linux/mount.h    |  1 +
>  security/commoncap.c     |  2 +-
>  security/selinux/hooks.c |  2 +-
>  5 files changed, 17 insertions(+), 3 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index b06623a9347f..ea7311d72cc3 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1295,7 +1295,7 @@ static void bprm_fill_uid(struct linux_binprm *bprm)
>         bprm->cred->euid = current_euid();
>         bprm->cred->egid = current_egid();
>
> -       if (bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID)
> +       if (!mnt_may_suid(bprm->file->f_path.mnt))
>                 return;
>
>         if (task_no_new_privs(current))
> diff --git a/fs/namespace.c b/fs/namespace.c
> index da70f7c4ece1..2101ce7b96ab 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -3276,6 +3276,19 @@ found:
>         return visible;
>  }
>
> +bool mnt_may_suid(struct vfsmount *mnt)
> +{
> +       /*
> +        * Foreign mounts (accessed via fchdir or through /proc
> +        * symlinks) are always treated as if they are nosuid.  This
> +        * prevents namespaces from trusting potentially unsafe
> +        * suid/sgid bits, file caps, or security labels that originate
> +        * in other namespaces.
> +        */
> +       return !(mnt->mnt_flags & MNT_NOSUID) && check_mnt(real_mount(mnt)) &&
> +              in_userns(current_user_ns(), mnt->mnt_sb->s_user_ns);

Is check_mnt correct here?  If I read it correctly, this means that,
if I just unshare my userns and do nothing else (and, in particular,
don't unshare my mount namespace), then everything will have
mnt_may_suid return false.

--Andy