[PATCH v2] 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 a kernel
heap memory exposure and over-read.

This fixes tpm1_get_random() and tpm2_get_random() to mask positive TPM
errors into -EIO while still passing through any negative errors.

[   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")
Fixes: ce63c05b664e ("tpm: migrate tpm2_get_random() to use struct tpm_buf")
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>
---
v2: - also fix tpm2 implementation (Jason Gunthorpe)
---
 drivers/char/tpm/tpm1-cmd.c | 7 +++++--
 drivers/char/tpm/tpm2-cmd.c | 7 +++++--
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/drivers/char/tpm/tpm1-cmd.c b/drivers/char/tpm/tpm1-cmd.c
index 85dcf2654d11..8e17d4f30aeb 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);
@@ -559,6 +559,9 @@ int tpm1_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
 	rc = total ? (int)total : -EIO;
 out:
 	tpm_buf_destroy(&buf);
+fail:
+	if (rc > 0)
+		rc = -EIO;
 	return rc;
 }
 
diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
index e74c5b7b64bf..dd0677f50f9c 100644
--- a/drivers/char/tpm/tpm2-cmd.c
+++ b/drivers/char/tpm/tpm2-cmd.c
@@ -301,7 +301,7 @@ struct tpm2_get_random_out {
  *
  * Return:
  *   size of the buffer on success,
- *   -errno otherwise
+ *   -errno otherwise ((positive TPM return codes are masked to -EIO)
  */
 int tpm2_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
 {
@@ -319,7 +319,7 @@ int tpm2_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
 
 	err = tpm_buf_init(&buf, 0, 0);
 	if (err)
-		return err;
+		goto fail;
 
 	do {
 		tpm_buf_reset(&buf, TPM2_ST_NO_SESSIONS, TPM2_CC_GET_RANDOM);
@@ -352,6 +352,9 @@ int tpm2_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
 	return total ? total : -EIO;
 out:
 	tpm_buf_destroy(&buf);
+fail:
+	if (err > 0)
+		erro = -EIO;
 	return err;
 }
 
-- 
2.17.1


-- 
Kees Cook