Re: [PATCH testsuite] policy/test_overlayfs.te: allow mounter to create whiteouts

[Ondrej Mosnacek <omosnace@redhat.com>]

On Wed, Jun 3, 2020 at 3:20 PM Ondrej Mosnacek <omosnace@redhat.com> wrote:
> On Wed, Jun 3, 2020 at 3:12 PM Stephen Smalley
> <stephen.smalley.work@gmail.com> wrote:
> > On Wed, Jun 3, 2020 at 4:23 AM Ondrej Mosnacek <omosnace@redhat.com> wrote:
> > >
> > > On Tue, Jun 2, 2020 at 7:42 PM Stephen Smalley
> > > <stephen.smalley.work@gmail.com> wrote:
> > > > This is required to pass the tests for kernels that include
> > > > commit a3c751a50fe6 ("vfs: allow unprivileged whiteout creation"),
> > > > which changed vfs_mknod() to permit whiteout creation without
> > > > requiring CAP_MKNOD and then switched vfs_whiteout() to use vfs_mknod()
> > > > rather than calling i_op->mknod() directly, which was originally done
> > > > to avoid such checking.  However, vfs_mknod() still calls the LSM hook
> > > > and therefore applies SELinux checks on whiteout creation.  Since
> > > > vfs_whiteout() now calls vfs_mknod(), SELinux :chr_file create permission
> > > > is now required for such whiteout creation by overlayfs.  Skipping the LSM
> > > > hook call or SELinux check entirely seems unsafe since we otherwise would
> > > > never check whether the process was allowed to create a file in that label;
> > > > even though the whiteout device cannot be read/written, it can be used as
> > > > a channel wrt its existence and attributes.
> > > >
> > > > See the discussion in:
> > > > https://lore.kernel.org/linux-fsdevel/20200409212859.GH28467@miu.piliscsaba.redhat.com/
> > > >
> > > > Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
> > > > ---
> > > >  policy/test_overlayfs.te | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > Thanks, this looks good and fixes the failure for me.
> > >
> > > >
> > > > diff --git a/policy/test_overlayfs.te b/policy/test_overlayfs.te
> > > > index b29621e..c844b82 100644
> > > > --- a/policy/test_overlayfs.te
> > > > +++ b/policy/test_overlayfs.te
> > > > @@ -88,7 +88,7 @@ manage_dirs_pattern(test_overlay_mounter_t, test_overlay_mounter_files_t, test_o
> > > >  #
> > > >  # Needed to remove a transition file
> > >
> > > Just please also update this comment ("...to create and remove..." or similar).
> >
> > That wouldn't be correct.  The additional :chr_file create permission
> > is needed to remove the transition file because removing a file from
> > overlayfs creates a whiteout device file.  It isn't to create the
> > transition file.
>
> Ah, I see... In that case the patch is fine as-is, so:
>
> Acked-by: Ondrej Mosnacek <omosnace@redhat.com>

This patch is now applied.

-- 
Ondrej Mosnacek
Software Engineer, Platform Security - SELinux kernel
Red Hat, Inc.