tpm: Opt-in in disable PCR integrity protection

The initial HMAC session feature added TPM bus encryption and/or integrity
protection to various in-kernel TPM operations. This can cause performance
bottlenecks with IMA, as it heavily utilizes PCR extend operations.

In order to mitigate this performance issue, introduce a kernel
command-line parameter to the TPM driver for disabling the integrity
protection for PCR extend operations (i.e. TPM2_PCR_Extend).

Cc: James Bottomley <[email protected]>
Link: https://lore.kernel.org/linux-integrity/[email protected]/
Fixes: 6519fea ("tpm: add hmac checks to tpm2_pcr_extend()")
Tested-by: Mimi Zohar <[email protected]>
Co-developed-by: Roberto Sassu <[email protected]>
Signed-off-by: Roberto Sassu <[email protected]>
Co-developed-by: Mimi Zohar <[email protected]>
Signed-off-by: Mimi Zohar <[email protected]>
Signed-off-by: Jarkko Sakkinen <[email protected]>

Author: jarkkojs

Documentation/admin-guide/kernel-parameters.txt

@@ -6727,6 +6727,15 @@
 	torture.verbose_sleep_duration= [KNL]
 			Duration of each verbose-printk() sleep in jiffies.
 
+	tpm.disable_pcr_integrity= [HW,TPM]
+			Do not protect PCR registers from unintended physical
+			access, or interposers in the bus by the means of
+			having an integrity protected session wrapped around
+			TPM2_PCR_Extend command. Consider this in a situation
+			where TPM is heavily utilized by IMA, thus protection
+			causing a major performance hit, and the space where
+			machines are deployed is by other means guarded.
+
 	tpm_suspend_pcr=[HW,TPM]
 			Format: integer pcr id
 			Specify that at suspend time, the tpm driver

drivers/char/tpm/tpm-buf.c

@@ -146,6 +146,26 @@ void tpm_buf_append_u32(struct tpm_buf *buf, const u32 value)
 }
 EXPORT_SYMBOL_GPL(tpm_buf_append_u32);
 
+/**
+ * tpm_buf_append_handle() - Add a handle
+ * @chip:	&tpm_chip instance
+ * @buf:	&tpm_buf instance
+ * @handle:	a TPM object handle
+ *
+ * Add a handle to the buffer, and increase the count tracking the number of
+ * handles in the command buffer. Works only for command buffers.
+ */
+void tpm_buf_append_handle(struct tpm_chip *chip, struct tpm_buf *buf, u32 handle)
+{
+	if (buf->flags & TPM_BUF_TPM2B) {
+		dev_err(&chip->dev, "Invalid buffer type (TPM2B)\n");
+		return;
+	}
+
+	tpm_buf_append_u32(buf, handle);
+	buf->handles++;
+}
+
 /**
  * tpm_buf_read() - Read from a TPM buffer
  * @buf:	&tpm_buf instance

drivers/char/tpm/tpm2-cmd.c

@@ -14,6 +14,10 @@
 #include "tpm.h"
 #include <crypto/hash_info.h>
 
+static bool disable_pcr_integrity;
+module_param(disable_pcr_integrity, bool, 0444);
+MODULE_PARM_DESC(disable_pcr_integrity, "Disable integrity protection of TPM2_PCR_Extend");
+
 static struct tpm2_hash tpm2_hash_map[] = {
 	{HASH_ALGO_SHA1, TPM_ALG_SHA1},
 	{HASH_ALGO_SHA256, TPM_ALG_SHA256},
@@ -232,18 +236,26 @@ int tpm2_pcr_extend(struct tpm_chip *chip, u32 pcr_idx,
 	int rc;
 	int i;
 
-	rc = tpm2_start_auth_session(chip);
-	if (rc)
-		return rc;
+	if (!disable_pcr_integrity) {
+		rc = tpm2_start_auth_session(chip);
+		if (rc)
+			return rc;
+	}
 
 	rc = tpm_buf_init(&buf, TPM2_ST_SESSIONS, TPM2_CC_PCR_EXTEND);
 	if (rc) {
-		tpm2_end_auth_session(chip);
+		if (!disable_pcr_integrity)
+			tpm2_end_auth_session(chip);
 		return rc;
 	}
 
-	tpm_buf_append_name(chip, &buf, pcr_idx, NULL);
-	tpm_buf_append_hmac_session(chip, &buf, 0, NULL, 0);
+	if (!disable_pcr_integrity) {
+		tpm_buf_append_name(chip, &buf, pcr_idx, NULL);
+		tpm_buf_append_hmac_session(chip, &buf, 0, NULL, 0);
+	} else {
+		tpm_buf_append_handle(chip, &buf, pcr_idx);
+		tpm_buf_append_auth(chip, &buf, 0, NULL, 0);
+	}
 
 	tpm_buf_append_u32(&buf, chip->nr_allocated_banks);
 
@@ -253,9 +265,11 @@ int tpm2_pcr_extend(struct tpm_chip *chip, u32 pcr_idx,
 			       chip->allocated_banks[i].digest_size);
 	}
 
-	tpm_buf_fill_hmac_session(chip, &buf);
+	if (!disable_pcr_integrity)
+		tpm_buf_fill_hmac_session(chip, &buf);
 	rc = tpm_transmit_cmd(chip, &buf, 0, "attempting extend a PCR value");
-	rc = tpm_buf_check_hmac_response(chip, &buf, rc);
+	if (!disable_pcr_integrity)
+		rc = tpm_buf_check_hmac_response(chip, &buf, rc);
 
 	tpm_buf_destroy(&buf);
 

drivers/char/tpm/tpm2-sessions.c

@@ -237,9 +237,7 @@ void tpm_buf_append_name(struct tpm_chip *chip, struct tpm_buf *buf,
 #endif
 
 	if (!tpm2_chip_auth(chip)) {
-		tpm_buf_append_u32(buf, handle);
-		/* count the number of handles in the upper bits of flags */
-		buf->handles++;
+		tpm_buf_append_handle(chip, buf, handle);
 		return;
 	}
 
@@ -272,6 +270,31 @@ void tpm_buf_append_name(struct tpm_chip *chip, struct tpm_buf *buf,
 }
 EXPORT_SYMBOL_GPL(tpm_buf_append_name);
 
+void tpm_buf_append_auth(struct tpm_chip *chip, struct tpm_buf *buf,
+			 u8 attributes, u8 *passphrase, int passphrase_len)
+{
+	/* offset tells us where the sessions area begins */
+	int offset = buf->handles * 4 + TPM_HEADER_SIZE;
+	u32 len = 9 + passphrase_len;
+
+	if (tpm_buf_length(buf) != offset) {
+		/* not the first session so update the existing length */
+		len += get_unaligned_be32(&buf->data[offset]);
+		put_unaligned_be32(len, &buf->data[offset]);
+	} else {
+		tpm_buf_append_u32(buf, len);
+	}
+	/* auth handle */
+	tpm_buf_append_u32(buf, TPM2_RS_PW);
+	/* nonce */
+	tpm_buf_append_u16(buf, 0);
+	/* attributes */
+	tpm_buf_append_u8(buf, 0);
+	/* passphrase */
+	tpm_buf_append_u16(buf, passphrase_len);
+	tpm_buf_append(buf, passphrase, passphrase_len);
+}
+
 /**
  * tpm_buf_append_hmac_session() - Append a TPM session element
  * @chip: the TPM chip structure
@@ -309,26 +332,8 @@ void tpm_buf_append_hmac_session(struct tpm_chip *chip, struct tpm_buf *buf,
 #endif
 
 	if (!tpm2_chip_auth(chip)) {
-		/* offset tells us where the sessions area begins */
-		int offset = buf->handles * 4 + TPM_HEADER_SIZE;
-		u32 len = 9 + passphrase_len;
-
-		if (tpm_buf_length(buf) != offset) {
-			/* not the first session so update the existing length */
-			len += get_unaligned_be32(&buf->data[offset]);
-			put_unaligned_be32(len, &buf->data[offset]);
-		} else {
-			tpm_buf_append_u32(buf, len);
-		}
-		/* auth handle */
-		tpm_buf_append_u32(buf, TPM2_RS_PW);
-		/* nonce */
-		tpm_buf_append_u16(buf, 0);
-		/* attributes */
-		tpm_buf_append_u8(buf, 0);
-		/* passphrase */
-		tpm_buf_append_u16(buf, passphrase_len);
-		tpm_buf_append(buf, passphrase, passphrase_len);
+		tpm_buf_append_auth(chip, buf, attributes, passphrase,
+				    passphrase_len);
 		return;
 	}
 

include/linux/tpm.h

@@ -421,6 +421,7 @@ void tpm_buf_append_u32(struct tpm_buf *buf, const u32 value);
 u8 tpm_buf_read_u8(struct tpm_buf *buf, off_t *offset);
 u16 tpm_buf_read_u16(struct tpm_buf *buf, off_t *offset);
 u32 tpm_buf_read_u32(struct tpm_buf *buf, off_t *offset);
+void tpm_buf_append_handle(struct tpm_chip *chip, struct tpm_buf *buf, u32 handle);
 
 /*
  * Check if TPM device is in the firmware upgrade mode.
@@ -505,6 +506,8 @@ void tpm_buf_append_name(struct tpm_chip *chip, struct tpm_buf *buf,
 void tpm_buf_append_hmac_session(struct tpm_chip *chip, struct tpm_buf *buf,
 				 u8 attributes, u8 *passphrase,
 				 int passphraselen);
+void tpm_buf_append_auth(struct tpm_chip *chip, struct tpm_buf *buf,
+			 u8 attributes, u8 *passphrase, int passphraselen);
 static inline void tpm_buf_append_hmac_session_opt(struct tpm_chip *chip,
 						   struct tpm_buf *buf,
 						   u8 attributes,