Re: [PATCH v5 1/3] overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode_fh

[Mark Salyzyn]

On 08/28/2018 10:34 AM, Amir Goldstein wrote:
> On Tue, Aug 28, 2018 at 7:53 PM Mark Salyzyn <salyzyn@xxxxxxxxxxx> wrote:
> > Assumption never checked, should fail if the mounter creds are not
> > sufficient.
> >
> > Signed-off-by: Mark Salyzyn <salyzyn@xxxxxxxxxxx>
> > Cc: Miklos Szeredi <miklos@xxxxxxxxxx>
> > Cc: Jonathan Corbet <corbet@xxxxxxx>
> > Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
> > Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> > Cc: Amir Goldstein <amir73il@xxxxxxxxx>
> > Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> > Cc: Stephen Smalley <sds@xxxxxxxxxxxxx>
> > Cc: linux-unionfs@xxxxxxxxxxxxxxx
> > Cc: linux-doc@xxxxxxxxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> >
> > 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.

<sigh> I can only truly test this on 4.14 (android's current top of 
tree) and on Hikey with that. Lack of due diligence for Top of Linux.
> > +       }
> > +
> >          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().

Ahhhh
> 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.
NFS export/import blocked on Android devices.
> Thanks,
> Amir.