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.
[1] https://patchwork.kernel.org/project/linux-pm/cover/20210220013255.1083202-1-matthewgarrett@xxxxxxxxxx/
[2] https://mjg59.dreamwidth.org/58077.html
Changes in v2:
- Fixed sparse warnings
- Adjust hash len by 2 due to new ASN.1 storage, and add underflow
check.
- Rework load/create_kernel_key() to eliminate a label (Andrey)
- Call put_device() needed from calling tpm_default_chip().
- Add missing static on snapshot_encrypted_byte_count()
- Fold in only the used kernel key bytes to the user key.
- Make the user key length 32 (Eric)
- Use CRYPTO_LIB_SHA256 for less boilerplate (Eric)
- Fixed some sparse warnings
- Use CRYPTO_LIB_SHA256 to get rid of sha256_data() (Eric)
- Adjusted offsets due to new ASN.1 format, and added a creation data
length check.
- Fix sparse warnings
- Fix session type comment (Andrey)
- Eliminate extra label in get/create_kernel_key() (Andrey)
- Call tpm_try_get_ops() before calling tpm2_flush_context().
Evan Green (7):
security: keys: trusted: Include TPM2 creation data
security: keys: trusted: Verify creation data
PM: hibernate: Add kernel-based encryption
PM: hibernate: Use TPM-backed keys to encrypt image
PM: hibernate: Mix user key in encrypted hibernate
PM: hibernate: Verify the digest encryption key
PM: hibernate: seal the encryption key with a PCR policy
Matthew Garrett (3):
tpm: Add support for in-kernel resetting of PCRs
tpm: Allow PCR 23 to be restricted to kernel-only use
security: keys: trusted: Allow storage of PCR values in creation data
Documentation/power/userland-swsusp.rst | 8 +
.../security/keys/trusted-encrypted.rst | 4 +
drivers/char/tpm/Kconfig | 10 +
drivers/char/tpm/tpm-dev-common.c | 8 +
drivers/char/tpm/tpm-interface.c | 28 +
drivers/char/tpm/tpm.h | 23 +
drivers/char/tpm/tpm1-cmd.c | 69 ++
drivers/char/tpm/tpm2-cmd.c | 58 +
drivers/char/tpm/tpm2-space.c | 2 +-
include/keys/trusted-type.h | 9 +
include/linux/tpm.h | 12 +
include/uapi/linux/suspend_ioctls.h | 28 +-
kernel/power/Kconfig | 16 +
kernel/power/Makefile | 1 +
kernel/power/power.h | 1 +
kernel/power/snapenc.c | 1037 +++++++++++++++++
kernel/power/snapshot.c | 5 +
kernel/power/user.c | 44 +-
kernel/power/user.h | 114 ++
security/keys/trusted-keys/tpm2key.asn1 | 5 +-
security/keys/trusted-keys/trusted_tpm1.c | 9 +
security/keys/trusted-keys/trusted_tpm2.c | 304 ++++-
22 files changed, 1754 insertions(+), 41 deletions(-)
create mode 100644 kernel/power/snapenc.c
create mode 100644 kernel/power/user.h
