[PATCH] tpm: Actually fail on TPM errors during "get random"

[Kees Cook <keescook@chromium.org>]

A "get random" may fail with a TPM error, but those codes were returned
as-is to the caller, which assumed the result was the number of bytes
that had been written to the target buffer, which could lead to an kernel
heap memory exposure and over-read.

This fixes tpm1_get_random() to mask positive TPM errors into -EIO, as
before.

[   18.092103] tpm tpm0: A TPM error (379) occurred attempting get random
[   18.092106] usercopy: Kernel memory exposure attempt detected from SLUB object 'kmalloc-64' (offset 0, size 379)!

Link: https://bugzilla.redhat.com/show_bug.cgi?id=1650989
Reported-by: Phil Baker <baker1tex@gmail.com>
Reported-by: Craig Robson <craig@zhatt.com>
Fixes: 7aee9c52d7ac ("tpm: tpm1: rewrite tpm1_get_random() using tpm_buf structure")
Cc: Laura Abbott <labbott@redhat.com>
Cc: Tomas Winkler <tomas.winkler@intel.com>
Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 drivers/char/tpm/tpm1-cmd.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/char/tpm/tpm1-cmd.c b/drivers/char/tpm/tpm1-cmd.c
index 85dcf2654d11..faeb78ecf960 100644
--- a/drivers/char/tpm/tpm1-cmd.c
+++ b/drivers/char/tpm/tpm1-cmd.c
@@ -510,7 +510,7 @@ struct tpm1_get_random_out {
  *
  * Return:
  * *  number of bytes read
- * * -errno or a TPM return code otherwise
+ * * -errno (positive TPM return codes are masked to -EIO)
  */
 int tpm1_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
 {
@@ -524,7 +524,7 @@ int tpm1_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
 
 	rc = tpm_buf_init(&buf, TPM_TAG_RQU_COMMAND, TPM_ORD_GET_RANDOM);
 	if (rc)
-		return rc;
+		goto fail;
 
 	do {
 		tpm_buf_append_u32(&buf, num_bytes);
@@ -559,7 +559,10 @@ int tpm1_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
 	rc = total ? (int)total : -EIO;
 out:
 	tpm_buf_destroy(&buf);
-	return rc;
+fail:
+	if (rc < 0)
+		return rc;
+	return -EIO;
 }
 
 #define TPM_ORD_PCRREAD 21
-- 
2.17.1


-- 
Kees Cook