[PATCH v2 03/10] ovl: check privs before decoding file handle

[Amir Goldstein]

On Mon, Dec 7, 2020 at 6:36 PM Miklos Szeredi <mszeredi at redhat.com> wrote:
>
> CAP_DAC_READ_SEARCH is required by open_by_handle_at(2) so check it in
> ovl_decode_real_fh() as well to prevent privilege escalation for
> unprivileged overlay mounts.
>
> Signed-off-by: Miklos Szeredi <mszeredi at redhat.com>
> ---
>  fs/overlayfs/namei.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
> index a6162c4076db..82a55fdb1e7a 100644
> --- a/fs/overlayfs/namei.c
> +++ b/fs/overlayfs/namei.c
> @@ -156,6 +156,9 @@ struct dentry *ovl_decode_real_fh(struct ovl_fh *fh, struct vfsmount *mnt,
>         struct dentry *real;
>         int bytes;
>
> +       if (!capable(CAP_DAC_READ_SEARCH))
> +               return NULL;
> +

If the mounter is not capable in init ns, ovl_check_origin() and
ovl_verify_index()
will not function as expected and this will break index and nfs export features.
So I think we need to also check capability in ovl_can_decode_fh(), to auto
disable those features.

Thanks,
Amir.