[PATCH] tpm: Salt tpm_get_random() result with get_random_bytes()

[Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>]

Salt the result that comes from the TPM RNG with random bytes from the
kernel RNG. This will allow to use tpm_get_random() as a substitute for
get_random_bytes().  TPM could have a bug (making results predicatable),
backdoor or even an inteposer in the bus. Salting gives protections
against these concerns.

Cc: David Safford <david.safford@xxxxxx>
Cc: Pascal Van Leeuwen <pvanleeuwen@xxxxxxxxxxxxxx>
Cc: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>
---
 drivers/char/tpm/tpm-interface.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
index 7f105490604c..a135b1cd5a17 100644
--- a/drivers/char/tpm/tpm-interface.c
+++ b/drivers/char/tpm/tpm-interface.c
@@ -22,6 +22,7 @@
 #include <linux/poll.h>
 #include <linux/slab.h>
 #include <linux/mutex.h>
+#include <linux/random.h>
 #include <linux/spinlock.h>
 #include <linux/suspend.h>
 #include <linux/freezer.h>
@@ -431,16 +432,24 @@ int tpm_pm_resume(struct device *dev)
 EXPORT_SYMBOL_GPL(tpm_pm_resume);
 
 /**
- * tpm_get_random() - get random bytes from the TPM's RNG
+ * tpm_get_random() - Get random bytes from the TPM's RNG
  * @chip:	a &struct tpm_chip instance, %NULL for the default chip
  * @out:	destination buffer for the random bytes
  * @max:	the max number of bytes to write to @out
  *
- * Return: number of random bytes read or a negative error value.
+ * Get random bytes from the TPM's RNG and salt the result with the same amount
+ * of bytes from the kernel RNG. Salting allows to call this function as a
+ * substitute for get_random_bytes() where appropriate.
+ *
+ * Return:
+ *   number of random bytes on success,
+ *   -errno on error
  */
 int tpm_get_random(struct tpm_chip *chip, u8 *out, size_t max)
 {
+	u8 salt[TPM_MAX_RNG_DATA];
 	int rc;
+	int i;
 
 	if (!out || max > TPM_MAX_RNG_DATA)
 		return -EINVAL;
@@ -455,6 +464,14 @@ int tpm_get_random(struct tpm_chip *chip, u8 *out, size_t max)
 		rc = tpm1_get_random(chip, out, max);
 
 	tpm_put_ops(chip);
+
+	if (rc > 0) {
+		get_random_bytes(salt, rc);
+
+		for (i = 0; i < rc; i++)
+			out[i] ^= salt[i];
+	}
+
 	return rc;
 }
 EXPORT_SYMBOL_GPL(tpm_get_random);
-- 
2.20.1