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

[Jason Gunthorpe]

On Mon, Apr 01, 2019 at 11:32:11AM -0700, Kees Cook wrote:
> 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@xxxxxxxxx>
> Reported-by: Craig Robson <craig@xxxxxxxxx>
> Fixes: 7aee9c52d7ac ("tpm: tpm1: rewrite tpm1_get_random() using tpm_buf structure")
> Cc: Laura Abbott <labbott@xxxxxxxxxx>
> Cc: Tomas Winkler <tomas.winkler@xxxxxxxxx>
> Cc: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>  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
> +++ 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;

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

I think you actually want this:

diff --git a/drivers/char/tpm/tpm1-cmd.c b/drivers/char/tpm/tpm1-cmd.c
index 85dcf2654d1102..a01e6fba1aacb6 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
  */
 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];
 
diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
index e74c5b7b64bfbd..9fa498b4cf8816 100644
--- a/drivers/char/tpm/tpm2-cmd.c
+++ b/drivers/char/tpm/tpm2-cmd.c
@@ -328,8 +328,10 @@ int tpm2_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
                                       offsetof(struct tpm2_get_random_out,
                                                buffer),
                                       "attempting get random");
-               if (err)
+               if (err) {
+                       rc = -EIO;
                        goto out;
+               }
 
                out = (struct tpm2_get_random_out *)
                        &buf.data[TPM_HEADER_SIZE];