On Tue Jan 2, 2024 at 7:04 PM EET, James Bottomley wrote:
> If some entity is snooping the TPM bus, they can see the random
> numbers we're extracting from the TPM and do prediction attacks
> against their consumers. Foil this attack by using response
> encryption to prevent the attacker from seeing the random sequence.
>
> Signed-off-by: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
> ---
> drivers/char/tpm/tpm2-cmd.c | 21 +++++++++++++++++----
> 1 file changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
> index a53a843294ed..0cdf892ec2a7 100644
> --- a/drivers/char/tpm/tpm2-cmd.c
> +++ b/drivers/char/tpm/tpm2-cmd.c
> @@ -292,25 +292,35 @@ int tpm2_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
> if (!num_bytes || max > TPM_MAX_RNG_DATA)
> return -EINVAL;
>
> - err = tpm_buf_init(&buf, 0, 0);
> + err = tpm2_start_auth_session(chip);
> if (err)
> return err;
>
> + err = tpm_buf_init(&buf, 0, 0);
> + if (err) {
> + tpm2_end_auth_session(chip);
> + return err;
> + }
> +
> do {
> - tpm_buf_reset(&buf, TPM2_ST_NO_SESSIONS, TPM2_CC_GET_RANDOM);
> + tpm_buf_reset(&buf, TPM2_ST_SESSIONS, TPM2_CC_GET_RANDOM);
> + tpm_buf_append_hmac_session_opt(chip, &buf, TPM2_SA_ENCRYPT
> + | TPM2_SA_CONTINUE_SESSION,
> + NULL, 0);
> tpm_buf_append_u16(&buf, num_bytes);
> + tpm_buf_fill_hmac_session(chip, &buf);
> err = tpm_transmit_cmd(chip, &buf,
> offsetof(struct tpm2_get_random_out,
> buffer),
> "attempting get random");
> + err = tpm_buf_check_hmac_response(chip, &buf, err);
> if (err) {
> if (err > 0)
> err = -EIO;
> goto out;
> }
>
> - out = (struct tpm2_get_random_out *)
> - &buf.data[TPM_HEADER_SIZE];
> + out = (struct tpm2_get_random_out *)tpm_buf_parameters(&buf);
> recd = min_t(u32, be16_to_cpu(out->size), num_bytes);
> if (tpm_buf_length(&buf) <
> TPM_HEADER_SIZE +
> @@ -327,9 +337,12 @@ int tpm2_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
> } while (retries-- && total < max);
>
> tpm_buf_destroy(&buf);
> + tpm2_end_auth_session(chip);
> +
> return total ? total : -EIO;
> out:
> tpm_buf_destroy(&buf);
> + tpm2_end_auth_session(chip);
> return err;
> }
>
Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
BR, Jarkko
