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Date:   Sat, 20 Feb 2021 01:32:55 +0000
From:   Matthew Garrett <matthewgarrett@...gle.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-integrity@...r.kernel.org, linux-pm@...r.kernel.org,
        keyrings@...r.kernel.org, zohar@...ux.ibm.com, jejb@...ux.ibm.com,
        jarkko@...nel.org, corbet@....net, rjw@...ysocki.net,
        Matthew Garrett <matthewgarrett@...gle.com>,
        Matthew Garrett <mjg59@...gle.com>
Subject: [PATCH 9/9] pm: hibernate: seal the encryption key with a PCR policy

The key blob is not secret, and by default the TPM will happily unseal
it regardless of system state. We can protect against that by sealing
the secret with a PCR policy - if the current PCR state doesn't match,
the TPM will refuse to release the secret. For now let's just seal it to
PCR 23. In the long term we may want a more flexible policy around this,
such as including PCR 7.

Signed-off-by: Matthew Garrett <mjg59@...gle.com>
---
 include/linux/tpm.h |   4 ++
 kernel/power/tpm.c  | 161 ++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 160 insertions(+), 5 deletions(-)

diff --git a/include/linux/tpm.h b/include/linux/tpm.h
index f6970986b097..2e0141978c87 100644
--- a/include/linux/tpm.h
+++ b/include/linux/tpm.h
@@ -225,18 +225,22 @@ enum tpm2_command_codes {
 	TPM2_CC_CONTEXT_LOAD	        = 0x0161,
 	TPM2_CC_CONTEXT_SAVE	        = 0x0162,
 	TPM2_CC_FLUSH_CONTEXT	        = 0x0165,
+	TPM2_CC_START_AUTH_SESSION      = 0x0176,
 	TPM2_CC_VERIFY_SIGNATURE        = 0x0177,
 	TPM2_CC_GET_CAPABILITY	        = 0x017A,
 	TPM2_CC_GET_RANDOM	        = 0x017B,
 	TPM2_CC_PCR_READ	        = 0x017E,
+	TPM2_CC_POLICY_PCR              = 0x017F,
 	TPM2_CC_PCR_EXTEND	        = 0x0182,
 	TPM2_CC_EVENT_SEQUENCE_COMPLETE = 0x0185,
 	TPM2_CC_HASH_SEQUENCE_START     = 0x0186,
+	TPM2_CC_POLICY_GET_DIGEST       = 0x0189,
 	TPM2_CC_CREATE_LOADED           = 0x0191,
 	TPM2_CC_LAST		        = 0x0193, /* Spec 1.36 */
 };
 
 enum tpm2_permanent_handles {
+	TPM2_RH_NULL            = 0x40000007,
 	TPM2_RS_PW		= 0x40000009,
 };
 
diff --git a/kernel/power/tpm.c b/kernel/power/tpm.c
index 34e6cfb98ce4..5de27c2f08be 100644
--- a/kernel/power/tpm.c
+++ b/kernel/power/tpm.c
@@ -125,6 +125,118 @@ static int swsusp_enc_dec(struct trusted_key_payload *payload, char *buf,
 	return ret;
 }
 
+static int tpm_setup_policy(struct tpm_chip *chip, int *session_handle)
+{
+	struct tpm_header *head;
+	struct tpm_buf buf;
+	char nonce[32] = {0x00};
+	int rc;
+
+	rc = tpm_buf_init(&buf, TPM2_ST_NO_SESSIONS,
+			  TPM2_CC_START_AUTH_SESSION);
+	if (rc)
+		return rc;
+
+	/* Decrypt key */
+	tpm_buf_append_u32(&buf, TPM2_RH_NULL);
+
+	/* Auth entity */
+	tpm_buf_append_u32(&buf, TPM2_RH_NULL);
+
+	/* Nonce - blank is fine here */
+	tpm_buf_append_u16(&buf, sizeof(nonce));
+	tpm_buf_append(&buf, nonce, sizeof(nonce));
+
+	/* Encrypted secret - empty */
+	tpm_buf_append_u16(&buf, 0);
+
+	/* Policy type - session */
+	tpm_buf_append_u8(&buf, 0x01);
+
+	/* Encryption type - NULL */
+	tpm_buf_append_u16(&buf, TPM_ALG_NULL);
+
+	/* Hash type - SHA256 */
+	tpm_buf_append_u16(&buf, TPM_ALG_SHA256);
+
+	rc = tpm_send(chip, buf.data, tpm_buf_length(&buf));
+
+	if (rc)
+		goto out;
+
+	head = (struct tpm_header *)buf.data;
+
+	if (be32_to_cpu(head->length) != sizeof(struct tpm_header) +
+	    sizeof(int) + sizeof(u16) + sizeof(nonce)) {
+		rc = -EINVAL;
+		goto out;
+	}
+
+	*session_handle = be32_to_cpu(*(int *)&buf.data[10]);
+	memcpy(nonce, &buf.data[16], sizeof(nonce));
+
+	tpm_buf_destroy(&buf);
+
+	rc = tpm_buf_init(&buf, TPM2_ST_NO_SESSIONS, TPM2_CC_POLICY_PCR);
+	if (rc)
+		return rc;
+
+	tpm_buf_append_u32(&buf, *session_handle);
+
+	/* PCR digest - read from the PCR, we'll verify creation data later */
+	tpm_buf_append_u16(&buf, 0);
+
+	/* One PCR */
+	tpm_buf_append_u32(&buf, 1);
+
+	/* SHA256 banks */
+	tpm_buf_append_u16(&buf, TPM_ALG_SHA256);
+
+	/* Select PCR 23 */
+	tpm_buf_append_u32(&buf, 0x03000080);
+
+	rc = tpm_send(chip, buf.data, tpm_buf_length(&buf));
+
+	if (rc)
+		goto out;
+
+out:
+	tpm_buf_destroy(&buf);
+	return rc;
+}
+
+static int tpm_policy_get_digest(struct tpm_chip *chip, int handle,
+				 char *digest)
+{
+	struct tpm_header *head;
+	struct tpm_buf buf;
+	int rc;
+
+	rc = tpm_buf_init(&buf, TPM2_ST_NO_SESSIONS, TPM2_CC_POLICY_GET_DIGEST);
+	if (rc)
+		return rc;
+
+	tpm_buf_append_u32(&buf, handle);
+
+	rc = tpm_send(chip, buf.data, tpm_buf_length(&buf));
+
+	if (rc)
+		goto out;
+
+	head = (struct tpm_header *)buf.data;
+	if (be32_to_cpu(head->length) != sizeof(struct tpm_header) +
+	    sizeof(u16) + SHA256_DIGEST_SIZE) {
+		rc = -EINVAL;
+		goto out;
+	}
+
+	memcpy(digest, &buf.data[12], SHA256_DIGEST_SIZE);
+out:
+	tpm_buf_destroy(&buf);
+
+	return rc;
+}
+
 static int tpm_certify_creationdata(struct tpm_chip *chip,
 				    struct trusted_key_payload *payload)
 {
@@ -182,11 +294,14 @@ int swsusp_encrypt_digest(struct swsusp_header *header)
 	const struct cred *cred = current_cred();
 	struct trusted_key_payload *payload;
 	struct tpm_digest *digests = NULL;
+	char policy[SHA256_DIGEST_SIZE];
+	char *policydigest = NULL;
 	struct tpm_chip *chip;
 	struct key *key;
+	int session_handle;
 	int ret, i;
-
-	char *keyinfo = "new\t32\tkeyhandle=0x81000001\tcreationpcrs=0x00800000";
+	char *keyinfo = NULL;
+	char *keytemplate = "new\t32\tkeyhandle=0x81000001\tcreationpcrs=0x00800000\tpolicydigest=%s";
 
 	chip = tpm_default_chip();
 
@@ -213,10 +328,35 @@ int swsusp_encrypt_digest(struct swsusp_header *header)
 			memcpy(&digests[i], &digest, sizeof(digest));
 	}
 
+	policydigest = kmalloc(SHA256_DIGEST_SIZE * 2 + 1, GFP_KERNEL);
+	if (!policydigest) {
+		ret = -ENOMEM;
+		goto reset;
+	}
+
 	ret = tpm_pcr_extend(chip, 23, digests);
 	if (ret != 0)
 		goto reset;
 
+	ret = tpm_setup_policy(chip, &session_handle);
+
+	if (ret != 0)
+		goto reset;
+
+	ret = tpm_policy_get_digest(chip, session_handle, policy);
+
+	if (ret != 0)
+		goto reset;
+
+	bin2hex(policydigest, policy, SHA256_DIGEST_SIZE);
+	policydigest[64] = '\0';
+
+	keyinfo = kasprintf(GFP_KERNEL, keytemplate, policydigest);
+	if (!keyinfo) {
+		ret = -ENOMEM;
+		goto reset;
+	}
+
 	key = key_alloc(&key_type_trusted, "swsusp", GLOBAL_ROOT_UID,
 			GLOBAL_ROOT_GID, cred, 0, KEY_ALLOC_NOT_IN_QUOTA,
 			NULL);
@@ -228,6 +368,7 @@ int swsusp_encrypt_digest(struct swsusp_header *header)
 
 	ret = key_instantiate_and_link(key, keyinfo, strlen(keyinfo) + 1, NULL,
 				       NULL);
+
 	if (ret < 0)
 		goto error;
 
@@ -244,6 +385,8 @@ int swsusp_encrypt_digest(struct swsusp_header *header)
 	key_revoke(key);
 	key_put(key);
 reset:
+	kfree(keyinfo);
+	kfree(policydigest);
 	kfree(digests);
 	tpm_pcr_reset(chip, 23);
 	return ret;
@@ -252,13 +395,14 @@ int swsusp_encrypt_digest(struct swsusp_header *header)
 int swsusp_decrypt_digest(struct swsusp_header *header)
 {
 	const struct cred *cred = current_cred();
-	char *keytemplate = "load\t%s\tkeyhandle=0x81000001";
+	char *keytemplate = "load\t%s\tkeyhandle=0x81000001\tpolicyhandle=0x%x";
 	struct trusted_key_payload *payload;
 	struct tpm_digest *digests = NULL;
 	char certhash[SHA256_DIGEST_SIZE];
 	char *blobstring = NULL;
 	char *keyinfo = NULL;
 	struct tpm_chip *chip;
+	int session_handle;
 	struct key *key;
 	int i, ret;
 
@@ -291,15 +435,22 @@ int swsusp_decrypt_digest(struct swsusp_header *header)
 	if (ret != 0)
 		goto reset;
 
-	blobstring = kmalloc(header->blob_len * 2, GFP_KERNEL);
+	ret = tpm_setup_policy(chip, &session_handle);
+
+	if (ret != 0)
+		goto reset;
+
+	blobstring = kmalloc(header->blob_len * 2 + 1, GFP_KERNEL);
 	if (!blobstring) {
 		ret = -ENOMEM;
 		goto reset;
 	}
 
 	bin2hex(blobstring, header->blob, header->blob_len);
+	blobstring[header->blob_len * 2] = '\0';
 
-	keyinfo = kasprintf(GFP_KERNEL, keytemplate, blobstring);
+	keyinfo = kasprintf(GFP_KERNEL, keytemplate, blobstring,
+			    session_handle);
 	if (!keyinfo) {
 		ret = -ENOMEM;
 		goto reset;
-- 
2.30.0.617.g56c4b15f3c-goog

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