Hi Kees,
On Mon, Apr 18, 2022 at 03:14:14PM -0700, Kees Cook wrote:
> [oops, resending to actual CC list]
>
> On Mon, Apr 18, 2022 at 02:15:56PM -0700, Matthias Kaehlcke wrote:
> > This series extends LoadPin to allow loading of kernel files
> > from trusted dm-verity devices. It adds the concept of
> > trusted verity devices to LoadPin. Userspace can use the
> > new systl file 'loadpin/trusted_verity_root_digests' to
> > provide LoadPin with a list of root digests from dm-verity
> > devices that LoadPin should consider as trusted. When a
> > kernel file is read LoadPin first checks (as usual) whether
> > the file is located on the pinned root, if so the file can
> > be loaded. Otherwise, if the verity extension is enabled,
> > LoadPin determines whether the file is located on a verity
> > backed device and whether the root digest of that device
> > is in the list of trusted digests. The file can be loaded
> > if the verity device has a trusted root digest.
> >
> > The list of trusted root digests can only be written once
> > (typically at boot time), to limit the possiblity of
> > attackers setting up rogue verity devices and marking them
> > as trusted.
> Thanks for working all this out! Where does the list of trusted
> roothashes come from? I assume some chain of trust exists. Is the list
> maybe already stored on the rootfs?
Yes, at least the content of the list comes from the rootfs. The
userspace part is still TBD (also pending on the evolution of this
patchset), having the list pre-formatted in a single file on the
rootfs should be fine.
> It'd be nice if there was some way to pass the trust chain to LoadPin
> more directly.
I imagine you envision LoadPin reading the file itself, instead of
just processing the content. That should be doable. One option would
be to pass the path of the file with the hashes through the sysctl
file and use kernel_read_file_from_path() to read it if the path is
in the pinned root (or maybe even in any trusted file system ;-)
On Mon, Apr 18, 2022 at 03:43:27PM -0700, Matthias Kaehlcke wrote:
> Hi Kees,
>
> On Mon, Apr 18, 2022 at 03:14:14PM -0700, Kees Cook wrote:
> > [oops, resending to actual CC list]
> >
> > On Mon, Apr 18, 2022 at 02:15:56PM -0700, Matthias Kaehlcke wrote:
> > > This series extends LoadPin to allow loading of kernel files
> > > from trusted dm-verity devices. It adds the concept of
> > > trusted verity devices to LoadPin. Userspace can use the
> > > new systl file 'loadpin/trusted_verity_root_digests' to
> > > provide LoadPin with a list of root digests from dm-verity
> > > devices that LoadPin should consider as trusted. When a
> > > kernel file is read LoadPin first checks (as usual) whether
> > > the file is located on the pinned root, if so the file can
> > > be loaded. Otherwise, if the verity extension is enabled,
> > > LoadPin determines whether the file is located on a verity
> > > backed device and whether the root digest of that device
> > > is in the list of trusted digests. The file can be loaded
> > > if the verity device has a trusted root digest.
> > >
> > > The list of trusted root digests can only be written once
> > > (typically at boot time), to limit the possiblity of
> > > attackers setting up rogue verity devices and marking them
> > > as trusted.
>
>
> > Thanks for working all this out! Where does the list of trusted
> > roothashes come from? I assume some chain of trust exists. Is the list
> > maybe already stored on the rootfs?
>
> Yes, at least the content of the list comes from the rootfs. The
> userspace part is still TBD (also pending on the evolution of this
> patchset), having the list pre-formatted in a single file on the
> rootfs should be fine.
Ah-ha, that's perfect.
> > It'd be nice if there was some way to pass the trust chain to LoadPin
> > more directly.
>
> I imagine you envision LoadPin reading the file itself, instead of
> just processing the content. That should be doable. One option would
> be to pass the path of the file with the hashes through the sysctl
> file and use kernel_read_file_from_path() to read it if the path is
> in the pinned root (or maybe even in any trusted file system ;-)
It could be a boot param or a Kconfig too. But yeah, having LoadPin able
to use itself to validate the file path would be much nicer.
--
Kees Cook