linux-kernel - Re: [PATCH v5 1/3] overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode

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Date:   Tue, 28 Aug 2018 20:34:40 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Mark Salyzyn <salyzyn@...roid.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Miklos Szeredi <miklos@...redi.hu>,
        Jonathan Corbet <corbet@....net>,
        Vivek Goyal <vgoyal@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v5 1/3] overlayfs: check CAP_DAC_READ_SEARCH before
 issuing exportfs_decode_fh

On Tue, Aug 28, 2018 at 7:53 PM Mark Salyzyn <salyzyn@...roid.com> wrote:
>
> Assumption never checked, should fail if the mounter creds are not
> sufficient.
>
> Signed-off-by: Mark Salyzyn <salyzyn@...roid.com>
> Cc: Miklos Szeredi <miklos@...redi.hu>
> Cc: Jonathan Corbet <corbet@....net>
> Cc: Vivek Goyal <vgoyal@...hat.com>
> Cc: Eric W. Biederman <ebiederm@...ssion.com>
> Cc: Amir Goldstein <amir73il@...il.com>
> Cc: Randy Dunlap <rdunlap@...radead.org>
> Cc: Stephen Smalley <sds@...ho.nsa.gov>
> Cc: linux-unionfs@...r.kernel.org
> Cc: linux-doc@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
>
> v5:
> - dependency of "overlayfs: override_creds=off option bypass creator_cred"
> ---
>  fs/overlayfs/namei.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
> index c993dd8db739..84982b6525fb 100644
> --- a/fs/overlayfs/namei.c
> +++ b/fs/overlayfs/namei.c
> @@ -193,6 +193,11 @@ struct dentry *ovl_decode_real_fh(struct ovl_fh *fh, struct vfsmount *mnt,
>         if (!uuid_equal(&fh->uuid, &mnt->mnt_sb->s_uuid))
>                 return NULL;
>
> +       if (!capable(CAP_DAC_READ_SEARCH)) {
> +               origin = ERR_PTR(-EPERM);
> +               goto out;

Which branch is this works based on?
I don't see any out label in current code.

> +       }
> +
>         bytes = (fh->len - offsetof(struct ovl_fh, fid));
>         real = exportfs_decode_fh(mnt, (struct fid *)fh->fid,
>                                   bytes >> 2, (int)fh->type,
> --

Please add same test in ovl_can_decode_fh().

Problem: none of the ovl_export_operations functions override creds.
I guess things are working now because nfsd is privileged enough.
IOW, the capability check you added doesn't check mounter creds
when coming from nfs export ops - I guess that is not what you want
although you probably don'r enable nfs export.

Thanks,
Amir.