Re: [PATCH v2 00/10] Encrypted Hibernation

Evan Green
2022-09-07T17:12:26Z

On Wed, Aug 31, 2022 at 11:35 AM Limonciello, Mario
<mario.limonciello@amd.com> wrote:
>
> On 8/23/2022 17:25, Evan Green wrote:
> > We are exploring enabling hibernation in some new scenarios. However,
> > our security team has a few requirements, listed below:
> > 1. The hibernate image must be encrypted with protection derived from
> > both the platform (eg TPM) and user authentication data (eg
> > password).
> > 2. Hibernation must not be a vector by which a malicious userspace can
> > escalate to the kernel.
> >
> > Requirement #1 can be achieved solely with uswsusp, however requirement
> > 2 necessitates mechanisms in the kernel to guarantee integrity of the
> > hibernate image. The kernel needs a way to authenticate that it generated
> > the hibernate image being loaded, and that the image has not been tampered
> > with. Adding support for in-kernel AEAD encryption with a TPM-sealed key
> > allows us to achieve both requirements with a single computation pass.
> >
> > Matthew Garrett published a series [1] that aligns closely with this
> > goal. His series utilized the fact that PCR23 is a resettable PCR that
> > can be blocked from access by usermode. The TPM can create a sealed key
> > tied to PCR23 in two ways. First, the TPM can attest to the value of
> > PCR23 when the key was created, which the kernel can use on resume to
> > verify that the kernel must have created the key (since it is the only
> > one capable of modifying PCR23). It can also create a policy that enforces
> > PCR23 be set to a specific value as a condition of unsealing the key,
> > preventing usermode from unsealing the key by talking directly to the
> > TPM.
> >
> > This series adopts that primitive as a foundation, tweaking and building
> > on it a bit. Where Matthew's series used the TPM-backed key to encrypt a
> > hash of the image, this series uses the key directly as a gcm(aes)
> > encryption key, which the kernel uses to encrypt and decrypt the
> > hibernate image in chunks of 16 pages. This provides both encryption and
> > integrity, which turns out to be a noticeable performance improvement over
> > separate passes for encryption and hashing.
> >
> > The series also introduces the concept of mixing user key material into
> > the encryption key. This allows usermode to introduce key material
> > based on unspecified external authentication data (in our case derived
> > from something like the user password or PIN), without requiring
> > usermode to do a separate encryption pass.
> >
> > Matthew also documented issues his series had [2] related to generating
> > fake images by booting alternate kernels without the PCR23 limiting.
> > With access to PCR23 on the same machine, usermode can create fake
> > hibernate images that are indistinguishable to the new kernel from
> > genuine ones. His post outlines a solution that involves adding more
> > PCRs into the creation data and policy, with some gyrations to make this
> > work well on a standard PC.
> >
> > Our approach would be similar: on our machines PCR 0 indicates whether
> > the system is booted in secure/verified mode or developer mode. By
> > adding PCR0 to the policy, we can reject hibernate images made in
> > developer mode while in verified mode (or vice versa).
> >
> > Additionally, mixing in the user authentication data limits both
> > data exfiltration attacks (eg a stolen laptop) and forged hibernation
> > image attacks to attackers that already know the authentication data (eg
> > user's password). This, combined with our relatively sealed userspace
> > (dm-verity on the rootfs), and some judicious clearing of the hibernate
> > image (such as across an OS update) further reduce the risk of an online
> > attack. The remaining attack space of a forgery from someone with
> > physical access to the device and knowledge of the authentication data
> > is out of scope for us, given that flipping to developer mode or
> > reflashing RO firmware trivially achieves the same thing.
> >
> > A couple of patches still need to be written on top of this series. The
> > generalized functionality to OR in additional PCRs via Kconfig (like PCR
> > 0 or 5) still needs to be added. We'll also need a patch that disallows
> > unencrypted forms of resume from hibernation, to fully close the door
> > to malicious userspace. However, I wanted to get this series out first
> > and get reactions from upstream before continuing to add to it.
>
> Something else to think about in this series is what happens with
> `hibernation_available` in kernel/power/hibernate.c. Currently if the
> system is locked down hibernate is disabled, but I would think that
> with a setup like that described here that should no longer be necessary.
>

Correct, I think that would be a reasonable followup to this series.

-Evan