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. [1] https://patchwork.kernel.org/project/linux-pm/cover/20210220013255.1083202-1-matthewgarrett@xxxxxxxxxx/ [2] https://mjg59.dreamwidth.org/58077.html Evan Green (6): 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 (4): tpm: Add support for in-kernel resetting of PCRs tpm: Allow PCR 23 to be restricted to kernel-only use security: keys: trusted: Parse out individual components of the key blob 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 | 15 + kernel/power/Makefile | 1 + kernel/power/power.h | 1 + kernel/power/snapenc.c | 1076 +++++++++++++++++ kernel/power/snapshot.c | 5 + kernel/power/user.c | 44 +- kernel/power/user.h | 114 ++ security/keys/trusted-keys/trusted_tpm1.c | 9 + security/keys/trusted-keys/trusted_tpm2.c | 164 ++- 21 files changed, 1670 insertions(+), 18 deletions(-) create mode 100644 kernel/power/snapenc.c create mode 100644 kernel/power/user.h -- 2.31.0
