[PATCH v4 08/11] PM: hibernate: Use TPM-backed keys to encrypt image

Evan Green
2022-11-03T18:05:07Z

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 v4:
- s/tpm_pcr_reset/tpm2_pcr_reset/ due to change in other patch
- Variable ordering and whitespace fixes (Jarkko)
- Add NULL check explanation in teardown (Jarkko)
- Change strlen+1 to sizeof for static buffer (Jarkko)
- Fix nr_allocated_banks loop overflow (found via KASAN)

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 | 211 +++++++++++++++++++++++++++++++++++++++--
kernel/power/user.h | 1 +
3 files changed, 204 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 f215df16dad4d3..7ff4fc66f7500c 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)
{
@@ -314,6 +324,16 @@ void snapshot_teardown_encryption(struct snapshot_data *data)
{
int i;

+ /*
+ * Do NULL checks so this function can safely be called from error paths
+ * and other places where this context may not be fully set up.
+ */
+ if (data->key) {
+ 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;
@@ -382,10 +402,82 @@ static int snapshot_setup_encryption_common(struct snapshot_data *data)
return rc;
}

+static int snapshot_create_kernel_key(struct snapshot_data *data)
+{
+ /* Create a key sealed by the SRK. */
+ char *keyinfo = "new\t32\tkeyhandle=0x81000000";
+ const struct cred *cred = current_cred();
+ struct tpm_digest *digests = NULL;
+ struct key *key = NULL;
+ struct tpm_chip *chip;
+ int ret, i;
+
+ chip = tpm_default_chip();
+ if (!chip)
+ return -ENODEV;
+
+ if (!(tpm_is_tpm2(chip))) {
+ ret = -ENODEV;
+ goto out_dev;
+ }
+
+ ret = tpm2_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, sizeof(keyinfo), NULL,
+ NULL);
+ if (ret != 0)
+ goto out;
+
+ data->key = key;
+ key = NULL;
+
+out:
+ if (key) {
+ key_revoke(key);
+ key_put(key);
+ }
+
+ kfree(digests);
+ tpm2_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];
+ struct trusted_key_payload *payload;
u8 nonce[USWSUSP_KEY_NONCE_SIZE];
int rc;

@@ -401,21 +493,28 @@ 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;
@@ -427,10 +526,99 @@ 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)
+{
+
+ char *keytemplate = "load\t%s\tkeyhandle=0x81000000";
+ const struct cred *cred = current_cred();
+ struct tpm_digest *digests = NULL;
+ char *blobstring = NULL;
+ struct key *key = NULL;
+ struct tpm_chip *chip;
+ char *keyinfo = NULL;
+ int i, ret;
+
+ chip = tpm_default_chip();
+ if (!chip)
+ return -ENODEV;
+
+ if (!(tpm_is_tpm2(chip))) {
+ ret = -ENODEV;
+ goto out_dev;
+ }
+
+ ret = tpm2_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);
+ 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);
+ tpm2_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. */
@@ -446,13 +634,18 @@ 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 ac429782abff85..6c86fb64ebe13e 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

};
--
2.38.1.431.g37b22c650d-goog