Re: [PATCH v2 1/3] KEYS: trusted_tpm1: Compare HMAC values in constant time

[Jarkko Sakkinen <jarkko@kernel.org>]

On Sat, Aug 09, 2025 at 10:19:39AM -0700, Eric Biggers wrote:
> To prevent timing attacks, HMAC value comparison needs to be constant
> time.  Replace the memcmp() with the correct function, crypto_memneq().
> 
> [For the Fixes commit I used the commit that introduced the memcmp().
> It predates the introduction of crypto_memneq(), but it was still a bug
> at the time even though a helper function didn't exist yet.]
> 
> Fixes: d00a1c72f7f4 ("keys: add new trusted key-type")
> Cc: stable@vger.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@kernel.org>
> ---
>  security/keys/trusted-keys/trusted_tpm1.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/security/keys/trusted-keys/trusted_tpm1.c b/security/keys/trusted-keys/trusted_tpm1.c
> index 89c9798d18007..e73f2c6c817a0 100644
> --- a/security/keys/trusted-keys/trusted_tpm1.c
> +++ b/security/keys/trusted-keys/trusted_tpm1.c
> @@ -5,10 +5,11 @@
>   *
>   * See Documentation/security/keys/trusted-encrypted.rst
>   */
>  
>  #include <crypto/hash_info.h>
> +#include <crypto/utils.h>
>  #include <linux/init.h>
>  #include <linux/slab.h>
>  #include <linux/parser.h>
>  #include <linux/string.h>
>  #include <linux/err.h>
> @@ -239,11 +240,11 @@ int TSS_checkhmac1(unsigned char *buffer,
>  			  TPM_NONCE_SIZE, enonce, TPM_NONCE_SIZE, ononce,
>  			  1, continueflag, 0, 0);
>  	if (ret < 0)
>  		goto out;
>  
> -	if (memcmp(testhmac, authdata, SHA1_DIGEST_SIZE))
> +	if (crypto_memneq(testhmac, authdata, SHA1_DIGEST_SIZE))
>  		ret = -EINVAL;
>  out:
>  	kfree_sensitive(sdesc);
>  	return ret;
>  }
> @@ -332,20 +333,20 @@ static int TSS_checkhmac2(unsigned char *buffer,
>  	ret = TSS_rawhmac(testhmac1, key1, keylen1, SHA1_DIGEST_SIZE,
>  			  paramdigest, TPM_NONCE_SIZE, enonce1,
>  			  TPM_NONCE_SIZE, ononce, 1, continueflag1, 0, 0);
>  	if (ret < 0)
>  		goto out;
> -	if (memcmp(testhmac1, authdata1, SHA1_DIGEST_SIZE)) {
> +	if (crypto_memneq(testhmac1, authdata1, SHA1_DIGEST_SIZE)) {
>  		ret = -EINVAL;
>  		goto out;
>  	}
>  	ret = TSS_rawhmac(testhmac2, key2, keylen2, SHA1_DIGEST_SIZE,
>  			  paramdigest, TPM_NONCE_SIZE, enonce2,
>  			  TPM_NONCE_SIZE, ononce, 1, continueflag2, 0, 0);
>  	if (ret < 0)
>  		goto out;
> -	if (memcmp(testhmac2, authdata2, SHA1_DIGEST_SIZE))
> +	if (crypto_memneq(testhmac2, authdata2, SHA1_DIGEST_SIZE))
>  		ret = -EINVAL;
>  out:
>  	kfree_sensitive(sdesc);
>  	return ret;
>  }
> -- 
> 2.50.1
> 

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>

BR, Jarkko