tpm: disable the TPM if NULL name changes

Update tpm2_load_context() to return -EINVAL on integrity failures and
use this as a signal when loading the NULL context that something
might be wrong. If the signal fails, check the name of the NULL
primary against the one stored in the chip data and if there is a
mismatch disable the TPM because it is likely to have suffered a reset
attack.

Signed-off-by: James Bottomley <[e-mail]>
Reviewed-by: Jarkko Sakkinen <[e-mail]>
Tested-by: Jarkko Sakkinen <[e-mail]>
Signed-off-by: Jarkko Sakkinen <[e-mail]>

drivers/char/tpm/tpm-chip.c

···
 {
 	int rc = -EIO;
 
+	if (chip->flags & TPM_CHIP_FLAG_DISABLE)
+		return rc;
+
 	get_device(&chip->dev);
 
 	down_read(&chip->ops_sem);

drivers/char/tpm/tpm2-sessions.c

···
 /* maximum number of names the TPM must remember for authorization */
 #define AUTH_MAX_NAMES	3
 
+static int tpm2_create_primary(struct tpm_chip *chip, u32 hierarchy,
+			       u32 *handle, u8 *name);
+
 /*
  * This is the structure that carries all the auth information (like
  * session handle, nonces, session key and auth) from use to use it is
···
 	return 0;
 }
 
+static int tpm2_load_null(struct tpm_chip *chip, u32 *null_key)
+{
+	int rc;
+	unsigned int offset = 0; /* dummy offset for null seed context */
+	u8 name[SHA256_DIGEST_SIZE + 2];
+
+	rc = tpm2_load_context(chip, chip->null_key_context, &offset,
+			       null_key);
+	if (rc != -EINVAL)
+		return rc;
+
+	/* an integrity failure may mean the TPM has been reset */
+	dev_err(&chip->dev, "NULL key integrity failure!\n");
+	/* check the null name against what we know */
+	tpm2_create_primary(chip, TPM2_RH_NULL, NULL, name);
+	if (memcmp(name, chip->null_key_name, sizeof(name)) == 0)
+		/* name unchanged, assume transient integrity failure */
+		return rc;
+	/*
+	 * Fatal TPM failure: the NULL seed has actually changed, so
+	 * the TPM must have been illegally reset.  All in-kernel TPM
+	 * operations will fail because the NULL primary can't be
+	 * loaded to salt the sessions, but disable the TPM anyway so
+	 * userspace programmes can't be compromised by it.
+	 */
+	dev_err(&chip->dev, "NULL name has changed, disabling TPM due to interference\n");
+	chip->flags |= TPM_CHIP_FLAG_DISABLE;
+
+	return rc;
+}
+
 /**
  * tpm2_start_auth_session() - create a HMAC authentication session with the TPM
  * @chip: the TPM chip structure to create the session with
···
 	struct tpm_buf buf;
 	struct tpm2_auth *auth = chip->auth;
 	int rc;
-	/* null seed context has no offset, but we must provide one */
-	unsigned int offset = 0;
-	u32 nullkey;
+	u32 null_key;
 
-	rc = tpm2_load_context(chip, chip->null_key_context, &offset,
-			       &nullkey);
+	rc = tpm2_load_null(chip, &null_key);
 	if (rc)
 		goto out;
 
···
 		goto out;
 
 	/* salt key handle */
-	tpm_buf_append_u32(&buf, nullkey);
+	tpm_buf_append_u32(&buf, null_key);
 	/* bind key handle */
 	tpm_buf_append_u32(&buf, TPM2_RH_NULL);
 	/* nonce caller */
···
 	tpm_buf_append_u16(&buf, TPM_ALG_SHA256);
 
 	rc = tpm_transmit_cmd(chip, &buf, 0, "start auth session");
-	tpm2_flush_context(chip, nullkey);
+	tpm2_flush_context(chip, null_key);
 
 	if (rc == TPM2_RC_SUCCESS)
 		rc = tpm2_parse_start_auth_session(auth, &buf);
···
  * @buf:	The response buffer from the chip
  * @handle:	pointer to be filled in with the return handle of the primary
  * @hierarchy:	The hierarchy the primary was created for
+ * @name:	pointer to be filled in with the primary key name
  *
  * Return:
  * * 0		- OK
···
  * * TPM_RC	- A TPM error
  */
 static int tpm2_parse_create_primary(struct tpm_chip *chip, struct tpm_buf *buf,
-				     u32 *handle, u32 hierarchy)
+				     u32 *handle, u32 hierarchy, u8 *name)
 {
 	struct tpm_header *head = (struct tpm_header *)buf->data;
 	off_t offset_r = TPM_HEADER_SIZE, offset_t;
 	u16 len = TPM_HEADER_SIZE;
 	u32 total_len = be32_to_cpu(head->length);
-	u32 val, param_len;
+	u32 val, param_len, keyhandle;
 
-	*handle = tpm_buf_read_u32(buf, &offset_r);
+	keyhandle = tpm_buf_read_u32(buf, &offset_r);
+	if (handle)
+		*handle = keyhandle;
+	else
+		tpm2_flush_context(chip, keyhandle);
+
 	param_len = tpm_buf_read_u32(buf, &offset_r);
 	/*
 	 * param_len doesn't include the header, but all the other
···
 		return -EINVAL;
 	len = tpm_buf_read_u16(buf, &offset_r);
 	offset_t = offset_r;
-	/* now we have the public area, compute the name of the object */
-	put_unaligned_be16(TPM_ALG_SHA256, chip->null_key_name);
-	sha256(&buf->data[offset_r], len, chip->null_key_name + 2);
+	if (name) {
+		/*
+		 * now we have the public area, compute the name of
+		 * the object
+		 */
+		put_unaligned_be16(TPM_ALG_SHA256, name);
+		sha256(&buf->data[offset_r], len, name + 2);
+	}
 
 	/* validate the public key */
 	val = tpm_buf_read_u16(buf, &offset_t);
···
  * @chip:      the TPM chip to create under
  * @hierarchy: The hierarchy handle to create under
  * @handle:    The returned volatile handle on success
+ * @name:      The name of the returned key
  *
  * For platforms that might not have a persistent primary, this can be
  * used to create one quickly on the fly (it uses Elliptic Curve not
···
  * * TPM_RC	- A TPM error
  */
 static int tpm2_create_primary(struct tpm_chip *chip, u32 hierarchy,
-			       u32 *handle)
+			       u32 *handle, u8 *name)
 {
 	int rc;
 	struct tpm_buf buf;
···
 			      "attempting to create NULL primary");
 
 	if (rc == TPM2_RC_SUCCESS)
-		rc = tpm2_parse_create_primary(chip, &buf, handle, hierarchy);
+		rc = tpm2_parse_create_primary(chip, &buf, handle, hierarchy,
+					       name);
 
 	tpm_buf_destroy(&buf);
 
···
 	u32 null_key;
 	int rc;
 
-	rc = tpm2_create_primary(chip, TPM2_RH_NULL, &null_key);
+	rc = tpm2_create_primary(chip, TPM2_RH_NULL, &null_key,
+				 chip->null_key_name);
 
 	if (rc == TPM2_RC_SUCCESS) {
 		unsigned int offset = 0; /* dummy offset for null key context */

drivers/char/tpm/tpm2-space.c

···
 		*handle = 0;
 		tpm_buf_destroy(&tbuf);
 		return -ENOENT;
+	} else if (tpm2_rc_value(rc) == TPM2_RC_INTEGRITY) {
+		tpm_buf_destroy(&tbuf);
+		return -EINVAL;
 	} else if (rc > 0) {
 		dev_warn(&chip->dev, "%s: failed with a TPM error 0x%04X\n",
 			 __func__, rc);

include/linux/tpm.h

···
 	TPM2_RC_SUCCESS		= 0x0000,
 	TPM2_RC_HASH		= 0x0083, /* RC_FMT1 */
 	TPM2_RC_HANDLE		= 0x008B,
+	TPM2_RC_INTEGRITY	= 0x009F,
 	TPM2_RC_INITIALIZE	= 0x0100, /* RC_VER1 */
 	TPM2_RC_FAILURE		= 0x0101,
 	TPM2_RC_DISABLED	= 0x0120,
···
 	TPM_CHIP_FLAG_FIRMWARE_UPGRADE		= BIT(7),
 	TPM_CHIP_FLAG_SUSPENDED			= BIT(8),
 	TPM_CHIP_FLAG_HWRNG_DISABLED		= BIT(9),
+	TPM_CHIP_FLAG_DISABLE			= BIT(10),
 };
 
 #define to_tpm_chip(d) container_of(d, struct tpm_chip, dev)
···
 
 static inline u32 tpm2_rc_value(u32 rc)
 {
-	return (rc & BIT(7)) ? rc & 0xff : rc;
+	return (rc & BIT(7)) ? rc & 0xbf : rc;
 }
 
 #if defined(CONFIG_TCG_TPM) || defined(CONFIG_TCG_TPM_MODULE)