tpm: Actually fail on TPM errors during "get random"

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 <[email protected]>

Reported-by Craig Robson <[email protected]>

Fixes 7aee9c52d7ac ("tpm: tpm1: rewrite tpm1_get_random() using tpm_buf structure")

Cc Laura Abbott <[email protected]>

Cc Tomas Winkler <[email protected]>

Cc Jarkko Sakkinen <[email protected]>

Cc [email protected]

Signed-off-by Kees Cook <[email protected]>

 drivers/char/tpm/tpm1-cmd.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

drivers/char/tpm/tpm1-cmd.c +2 -1

index 85dcf2654d11..faeb78ecf960 100644
+++ 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)
 {

Quoted 37 lines · Kees Cook

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 <[email protected]>

Reported-by Craig Robson <[email protected]>

Fixes 7aee9c52d7ac ("tpm: tpm1: rewrite tpm1_get_random() using tpm_buf structure")

Cc Laura Abbott <[email protected]>

Cc Tomas Winkler <[email protected]>

Cc Jarkko Sakkinen <[email protected]>

Cc [email protected]

Signed-off-by Kees Cook <[email protected]>

 drivers/char/tpm/tpm1-cmd.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

drivers/char/tpm/tpm1-cmd.c +2 -1

index 85dcf2654d11..faeb78ecf960 100644
+++ 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)
 

drivers/char/tpm/tpm1-cmd.c int tpm1_get_random(struct tpm_chip *chip, u8 *dest, size_t max) · line 527

Quoted 4 lines

drivers/char/tpm/tpm1-cmd.c +1 -1

      rc = tpm_buf_init(&buf, TPM_TAG_RQU_COMMAND, TPM_ORD_GET_RANDOM);
      if (rc)
-             return rc;
+             goto fail;

?? Does tpm_buf_init() return positive?? Shouldn't..

I think you actually want this:

drivers/char/tpm/tpm1-cmd.c +4 -2

index 85dcf2654d1102..a01e6fba1aacb6 100644
@@ -510,7 +510,7 @@ struct tpm1_get_random_out {
  *
  * Return:
  * *  number of bytes read
- * * -errno or a TPM return code otherwise
+ * * -errno
  */
 int tpm1_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
 {

@@ -531,8 +531,10 @@ int tpm1_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
 
                rc = tpm_transmit_cmd(chip, &buf, sizeof(out->rng_data_len),
                                      "attempting get random");
-               if (rc)
+               if (rc) {
+                       rc = -EIO
                        goto out;
+               }
 
                out = (struct tpm1_get_random_out *)&buf.data[TPM_HEADER_SIZE];
 

drivers/char/tpm/tpm2-cmd.c +0 -0

index e74c5b7b64bfbd..9fa498b4cf8816 100644
@@ -328,8 +328,10 @@ int tpm2_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
                                       offsetof(struct tpm2_get_random_out,

drivers/char/tpm/tpm1-cmd.c +2 -2

@@ -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;
 

drivers/char/tpm/tpm1-cmd.c +0 -0

 	do {
 		tpm_buf_append_u32(&buf, num_bytes);

drivers/char/tpm/tpm1-cmd.c +4 -0

+fail:
+	if (rc < 0)
+		return rc;
+	return -EIO;
 }
 

#define TPM_ORD_PCRREAD 21

Signature · 1 line

2.17.1

On Mon, 2019-04-01 at 11:32 -0700, Kees Cook wrote:
[...]

Quoted 23 lines

drivers/char/tpm/tpm1-cmd.c +2 -2

@@ -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);

drivers/char/tpm/tpm1-cmd.c int tpm1_get_random(struct tpm_chip *chip, u8 · line 563

Quoted 5 lines

drivers/char/tpm/tpm1-cmd.c +0 -1

@@ -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;

You can't remove this otherwise the only return will ever be a failure.

I think what you're trying to catch is tpm_transmit_cmd returning a positive failure, So you need to check the output of tpm_transmit_cmd as well and goto failure leaving the above return in place.