Re: [PATCH v8 2/3] tpm: Rollback tpm2_load_null()

[James Bottomley]

On Mon, 2024-10-28 at 07:50 +0200, Jarkko Sakkinen wrote:
[...]
> --- a/drivers/char/tpm/tpm2-sessions.c
> +++ b/drivers/char/tpm/tpm2-sessions.c
> @@ -915,33 +915,37 @@ static int tpm2_parse_start_auth_session(struct
> tpm2_auth *auth,
>  
>  static int tpm2_load_null(struct tpm_chip *chip, u32 *null_key)
>  {
> -       int rc;
>         unsigned int offset = 0; /* dummy offset for null seed
> context */
>         u8 name[SHA256_DIGEST_SIZE + 2];
> +       u32 tmp_null_key;
> +       int rc;
>  
>         rc = tpm2_load_context(chip, chip->null_key_context, &offset,
> -                              null_key);
> -       if (rc != -EINVAL)
> -               return rc;
> +                              &tmp_null_key);
> +       if (rc != -EINVAL) {
> +               if (!rc)
> +                       *null_key = tmp_null_key;
> +               goto err;
> +       }
>  
> -       /* an integrity failure may mean the TPM has been reset */
> -       dev_err(&chip->dev, "NULL key integrity failure!\n");
> -       /* check the null name against what we know */
> -       tpm2_create_primary(chip, TPM2_RH_NULL, NULL, name);
> -       if (memcmp(name, chip->null_key_name, sizeof(name)) == 0)
> -               /* name unchanged, assume transient integrity failure
> */
> -               return rc;
> -       /*
> -        * Fatal TPM failure: the NULL seed has actually changed, so
> -        * the TPM must have been illegally reset.  All in-kernel TPM
> -        * operations will fail because the NULL primary can't be
> -        * loaded to salt the sessions, but disable the TPM anyway so
> -        * userspace programmes can't be compromised by it.
> -        */
> -       dev_err(&chip->dev, "NULL name has changed, disabling TPM due
> to interference\n");
> +       /* Try to re-create null key, given the integrity failure: */
> +       rc = tpm2_create_primary(chip, TPM2_RH_NULL, &tmp_null_key,
> name);
> +       if (rc)
> +               goto err;

>From a security point of view, this probably isn't such a good idea:
the reason the context load failed above is likely the security
condition we're checking for: the null seed changed because an
interposer did a reset.  That means that if the interposer knows about
this error leg, it would simply error out the create primary here and
the TPM wouldn't be disabled.

Regards,

James