Re: [PATCH v3 08/11] PM: hibernate: Use TPM-backed keys to encrypt image

Jarkko Sakkinen
2022-09-30T21:35:57Z

On Tue, Sep 27, 2022 at 09:49:19AM -0700, Evan Green wrote:
> When using encrypted hibernate images, have the TPM create a key for us
> and seal it. By handing back a sealed blob instead of the raw key, we
> prevent usermode from being able to decrypt and tamper with the
> hibernate image on a different machine.
>
> We'll also go through the motions of having PCR23 set to a known value at
> the time of key creation and unsealing. Currently there's nothing that
> enforces the contents of PCR23 as a condition to unseal the key blob,
> that will come in a later change.
>
> Sourced-from: Matthew Garrett <mjg59@google.com>
> Signed-off-by: Evan Green <evgreen@chromium.org>
>
> ---
> Matthew's incarnation of this patch is at:
> https://patchwork.kernel.org/project/linux-pm/patch/20210220013255.1083202-9-matthewgarrett@google.com/
>
> Changes in v3:
> - ENCRYPTED_HIBERNATION needs TRUSTED_KEYS builtin for
> key_type_trusted.
> - Remove KEYS dependency since it's covered by TRUSTED_KEYS (Kees)
>
> Changes in v2:
> - Rework load/create_kernel_key() to eliminate a label (Andrey)
> - Call put_device() needed from calling tpm_default_chip().
>
> kernel/power/Kconfig | 1 +
> kernel/power/snapenc.c | 207 +++++++++++++++++++++++++++++++++++++++--
> kernel/power/user.h | 1 +
> 3 files changed, 200 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/power/Kconfig b/kernel/power/Kconfig
> index cd574af0b43379..2f8acbd87b34dc 100644
> --- a/kernel/power/Kconfig
> +++ b/kernel/power/Kconfig
> @@ -96,6 +96,7 @@ config ENCRYPTED_HIBERNATION
> bool "Encryption support for userspace snapshots"
> depends on HIBERNATION_SNAPSHOT_DEV
> depends on CRYPTO_AEAD2=y
> + depends on TRUSTED_KEYS=y
> default n
> help
> Enable support for kernel-based encryption of hibernation snapshots
> diff --git a/kernel/power/snapenc.c b/kernel/power/snapenc.c
> index cb90692d6ab83a..90079f6d4f184b 100644
> --- a/kernel/power/snapenc.c
> +++ b/kernel/power/snapenc.c
> @@ -4,13 +4,23 @@
> #include <linux/crypto.h>
> #include <crypto/aead.h>
> #include <crypto/gcm.h>
> +#include <keys/trusted-type.h>
> +#include <linux/key-type.h>
> #include <linux/random.h>
> #include <linux/mm.h>
> +#include <linux/tpm.h>
> #include <linux/uaccess.h>
>
> #include "power.h"
> #include "user.h"
>
> +/* sha256("To sleep, perchance to dream") */
> +static struct tpm_digest known_digest = { .alg_id = TPM_ALG_SHA256,
> + .digest = {0x92, 0x78, 0x3d, 0x79, 0x2d, 0x00, 0x31, 0xb0, 0x55, 0xf9,
> + 0x1e, 0x0d, 0xce, 0x83, 0xde, 0x1d, 0xc4, 0xc5, 0x8e, 0x8c,
> + 0xf1, 0x22, 0x38, 0x6c, 0x33, 0xb1, 0x14, 0xb7, 0xec, 0x05,
> + 0x5f, 0x49}};
> +
> /* Encrypt more data from the snapshot into the staging area. */
> static int snapshot_encrypt_refill(struct snapshot_data *data)
> {
> @@ -313,6 +323,12 @@ void snapshot_teardown_encryption(struct snapshot_data *data)
> {
> int i;
>
> + if (data->key) {

Would be a helpful to have perhaps inline comment before the check.

Just stating this because I did not exactly follow why the null
check was needed (but do believe that there are good reasons to
do it).

> + key_revoke(data->key);
> + key_put(data->key);
> + data->key = NULL;
> + }
> +
> if (data->aead_req) {
> aead_request_free(data->aead_req);
> data->aead_req = NULL;
> @@ -381,11 +397,83 @@ static int snapshot_setup_encryption_common(struct snapshot_data *data)
> return rc;
> }
>
> +static int snapshot_create_kernel_key(struct snapshot_data *data)
> +{
> + const struct cred *cred = current_cred();
> + struct tpm_digest *digests = NULL;
> + struct tpm_chip *chip;
> + struct key *key = NULL;
> + int ret, i;
> + /* Create a key sealed by the SRK. */
> + char *keyinfo = "new\t32\tkeyhandle=0x81000000";

Again, I'd consider put this declaration as first.

> +
> + chip = tpm_default_chip();
> + if (!chip)
> + return -ENODEV;
> +
> + if (!(tpm_is_tpm2(chip))) {
> + ret = -ENODEV;
> + goto out_dev;
> + }
> +
> + ret = tpm_pcr_reset(chip, 23);
> + if (ret)
> + goto out;
> +
> + digests = kcalloc(chip->nr_allocated_banks, sizeof(struct tpm_digest),
> + GFP_KERNEL);
> + if (!digests) {
> + ret = -ENOMEM;
> + goto out;
> + }
> +
> + for (i = 0; i <= chip->nr_allocated_banks; i++) {
> + digests[i].alg_id = chip->allocated_banks[i].alg_id;
> + if (digests[i].alg_id == known_digest.alg_id)
> + memcpy(&digests[i], &known_digest, sizeof(known_digest));
> + }
> +
> + ret = tpm_pcr_extend(chip, 23, digests);
> + if (ret != 0)
> + goto out;
> +
> + key = key_alloc(&key_type_trusted, "swsusp", GLOBAL_ROOT_UID,
> + GLOBAL_ROOT_GID, cred, 0, KEY_ALLOC_NOT_IN_QUOTA,
> + NULL);
> +
> + if (IS_ERR(key)) {
> + ret = PTR_ERR(key);
> + key = NULL;
> + goto out;
> + }
> +
> + ret = key_instantiate_and_link(key, keyinfo, strlen(keyinfo) + 1, NULL,
> + NULL);

Generally speaking, even if it somehow would be "safe", not strlen()
thank you.

AFAIK, keyinfo is a constant so you could just as well use sizeof().
And then you would not need "+ 1".

> + if (ret != 0)
> + goto out;
> +
> + data->key = key;
> + key = NULL;
> +
> +out:
> + if (key) {
> + key_revoke(key);
> + key_put(key);
> + }
> +
> + kfree(digests);
> + tpm_pcr_reset(chip, 23);
> +
> +out_dev:
> + put_device(&chip->dev);
> + return ret;
> +}
> +
> int snapshot_get_encryption_key(struct snapshot_data *data,
> struct uswsusp_key_blob __user *key)
> {
> - u8 aead_key[SNAPSHOT_ENCRYPTION_KEY_SIZE];
> u8 nonce[USWSUSP_KEY_NONCE_SIZE];
> + struct trusted_key_payload *payload;
> int rc;
> /* Don't pull a random key from a world that can be reset. */
> if (data->ready)
> @@ -399,21 +487,28 @@ int snapshot_get_encryption_key(struct snapshot_data *data,
> get_random_bytes(nonce, sizeof(nonce));
> memcpy(&data->nonce_low, &nonce[0], sizeof(data->nonce_low));
> memcpy(&data->nonce_high, &nonce[8], sizeof(data->nonce_high));
> - /* Build a random key */
> - get_random_bytes(aead_key, sizeof(aead_key));
> - rc = crypto_aead_setkey(data->aead_tfm, aead_key, sizeof(aead_key));
> +
> + /* Create a kernel key, and set it. */
> + rc = snapshot_create_kernel_key(data);
> + if (rc)
> + goto fail;
> +
> + payload = data->key->payload.data[0];
> + /* Install the key */
> + rc = crypto_aead_setkey(data->aead_tfm, payload->key, SNAPSHOT_ENCRYPTION_KEY_SIZE);
> if (rc)
> goto fail;
>
> - /* Hand the key back to user mode (to be changed!) */
> - rc = put_user(sizeof(struct uswsusp_key_blob), &key->blob_len);
> + /* Hand the key back to user mode in sealed form. */
> + rc = put_user(payload->blob_len, &key->blob_len);
> if (rc)
> goto fail;
>
> - rc = copy_to_user(&key->blob, &aead_key, sizeof(aead_key));
> + rc = copy_to_user(&key->blob, &payload->blob, payload->blob_len);
> if (rc)
> goto fail;
>
> + /* The nonce just gets handed back in the clear. */
> rc = copy_to_user(&key->nonce, &nonce, sizeof(nonce));
> if (rc)
> goto fail;
> @@ -425,10 +520,99 @@ int snapshot_get_encryption_key(struct snapshot_data *data,
> return rc;
> }
>
> +static int snapshot_load_kernel_key(struct snapshot_data *data,
> + struct uswsusp_key_blob *blob)

Bad alignment.

> +{
> +
> + const struct cred *cred = current_cred();
> + char *keytemplate = "load\t%s\tkeyhandle=0x81000000";

Ditto.

> + struct tpm_digest *digests = NULL;
> + char *blobstring = NULL;
> + char *keyinfo = NULL;
> + struct tpm_chip *chip;
> + struct key *key = NULL;
> + int i, ret;
> +
> + chip = tpm_default_chip();
> + if (!chip)
> + return -ENODEV;
> +
> + if (!(tpm_is_tpm2(chip))) {
> + ret = -ENODEV;
> + goto out_dev;
> + }
> +
> + ret = tpm_pcr_reset(chip, 23);
> + if (ret)
> + goto out;
> +
> + digests = kcalloc(chip->nr_allocated_banks, sizeof(struct tpm_digest),
> + GFP_KERNEL);
> + if (!digests)
> + goto out;
> +
> + for (i = 0; i <= chip->nr_allocated_banks; i++) {
> + digests[i].alg_id = chip->allocated_banks[i].alg_id;
> + if (digests[i].alg_id == known_digest.alg_id)
> + memcpy(&digests[i], &known_digest, sizeof(known_digest));
> + }
> +
> + ret = tpm_pcr_extend(chip, 23, digests);
> + if (ret != 0)
> + goto out;
> +
> + blobstring = kmalloc(blob->blob_len * 2, GFP_KERNEL);
> + if (!blobstring) {
> + ret = -ENOMEM;
> + goto out;
> + }
> +
> + bin2hex(blobstring, blob->blob, blob->blob_len);
> + keyinfo = kasprintf(GFP_KERNEL, keytemplate, blobstring);
> + if (!keyinfo) {
> + ret = -ENOMEM;
> + goto out;
> + }
> +
> + key = key_alloc(&key_type_trusted, "swsusp", GLOBAL_ROOT_UID,
> + GLOBAL_ROOT_GID, cred, 0, KEY_ALLOC_NOT_IN_QUOTA,
> + NULL);
> +
> + if (IS_ERR(key)) {
> + ret = PTR_ERR(key);
> + key = NULL;
> + goto out;
> + }
> +
> + ret = key_instantiate_and_link(key, keyinfo, strlen(keyinfo) + 1, NULL,
> + NULL);

Ditto.

> + if (ret != 0)
> + goto out;
> +
> + data->key = key;
> + key = NULL;
> +
> +out:
> + if (key) {
> + key_revoke(key);
> + key_put(key);
> + }
> +
> + kfree(keyinfo);
> + kfree(blobstring);
> + kfree(digests);
> + tpm_pcr_reset(chip, 23);
> +
> +out_dev:
> + put_device(&chip->dev);
> + return ret;
> +}
> +
> int snapshot_set_encryption_key(struct snapshot_data *data,
> struct uswsusp_key_blob __user *key)
> {
> struct uswsusp_key_blob blob;
> + struct trusted_key_payload *payload;
> int rc;
>
> /* It's too late if data's been pushed in. */
> @@ -444,13 +628,18 @@ int snapshot_set_encryption_key(struct snapshot_data *data,
> if (rc)
> goto crypto_setup_fail;
>
> - if (blob.blob_len != sizeof(struct uswsusp_key_blob)) {
> + if (blob.blob_len > sizeof(key->blob)) {
> rc = -EINVAL;
> goto crypto_setup_fail;
> }
>
> + rc = snapshot_load_kernel_key(data, &blob);
> + if (rc)
> + goto crypto_setup_fail;
> +
> + payload = data->key->payload.data[0];
> rc = crypto_aead_setkey(data->aead_tfm,
> - blob.blob,
> + payload->key,
> SNAPSHOT_ENCRYPTION_KEY_SIZE);
>
> if (rc)
> diff --git a/kernel/power/user.h b/kernel/power/user.h
> index 6823e2eba7ec53..591b30bb213349 100644
> --- a/kernel/power/user.h
> +++ b/kernel/power/user.h
> @@ -31,6 +31,7 @@ struct snapshot_data {
> uint64_t crypt_total;
> uint64_t nonce_low;
> uint64_t nonce_high;
> + struct key *key;
> #endif
>
> };
> --
> 2.31.0
>

BR, Jarkko