Re: [PATCH 1/2] KEYS: asymmetric: enforce that sig algo matches key algo

[Jarkko Sakkinen <jarkko@kernel.org>]

On Mon, Jan 31, 2022 at 04:34:13PM -0800, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> Most callers of public_key_verify_signature(), including most indirect
> callers via verify_signature() as well as pkcs7_verify_sig_chain(),
> don't check that public_key_signature::pkey_algo matches
> public_key::pkey_algo.  These should always match.  However, a malicious
> signature could intentionally declare an unintended algorithm.  It is
> essential that such signatures be rejected outright, or that the
> algorithm of the *key* be used -- not the algorithm of the signature as
> that would allow attackers to choose the algorithm used.
> 
> Currently, public_key_verify_signature() correctly uses the key's
> algorithm when deciding which akcipher to allocate.  That's good.
> However, it uses the signature's algorithm when deciding whether to do
> the first step of SM2, which is incorrect.  Also, v4.19 and older
> kernels used the signature's algorithm for the entire process.
> 
> Prevent such errors by making public_key_verify_signature() enforce that
> the signature's algorithm matches the key's algorithm.
> 
> Also remove two checks of this done by callers, which are now redundant.
> 
> Cc: stable@vger.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
>  crypto/asymmetric_keys/pkcs7_verify.c    |  6 ------
>  crypto/asymmetric_keys/public_key.c      | 15 +++++++++++++++
>  crypto/asymmetric_keys/x509_public_key.c |  6 ------
>  3 files changed, 15 insertions(+), 12 deletions(-)
> 
> diff --git a/crypto/asymmetric_keys/pkcs7_verify.c b/crypto/asymmetric_keys/pkcs7_verify.c
> index 0b4d07aa88111..f94a1d1ad3a6c 100644
> --- a/crypto/asymmetric_keys/pkcs7_verify.c
> +++ b/crypto/asymmetric_keys/pkcs7_verify.c
> @@ -174,12 +174,6 @@ static int pkcs7_find_key(struct pkcs7_message *pkcs7,
>  		pr_devel("Sig %u: Found cert serial match X.509[%u]\n",
>  			 sinfo->index, certix);
>  
> -		if (strcmp(x509->pub->pkey_algo, sinfo->sig->pkey_algo) != 0) {
> -			pr_warn("Sig %u: X.509 algo and PKCS#7 sig algo don't match\n",
> -				sinfo->index);
> -			continue;
> -		}
> -
>  		sinfo->signer = x509;
>  		return 0;
>  	}
> diff --git a/crypto/asymmetric_keys/public_key.c b/crypto/asymmetric_keys/public_key.c
> index 4fefb219bfdc8..aba7113d86c76 100644
> --- a/crypto/asymmetric_keys/public_key.c
> +++ b/crypto/asymmetric_keys/public_key.c
> @@ -325,6 +325,21 @@ int public_key_verify_signature(const struct public_key *pkey,
>  	BUG_ON(!sig);
>  	BUG_ON(!sig->s);
>  
> +	/*
> +	 * The signature's claimed public key algorithm *must* match the key's
> +	 * actual public key algorithm.
> +	 *
> +	 * Small exception: ECDSA signatures don't specify the curve, but ECDSA
> +	 * keys do.  So the strings can mismatch slightly in that case:
> +	 * "ecdsa-nist-*" for the key, but "ecdsa" for the signature.
> +	 */
> +	if (!sig->pkey_algo)
> +		return -EINVAL;
> +	if (strcmp(pkey->pkey_algo, sig->pkey_algo) != 0 &&
> +	    (strncmp(pkey->pkey_algo, "ecdsa-", 6) != 0 ||
> +	     strcmp(sig->pkey_algo, "ecdsa") != 0))
> +		return -EKEYREJECTED;
> +
>  	ret = software_key_determine_akcipher(sig->encoding,
>  					      sig->hash_algo,
>  					      pkey, alg_name);
> diff --git a/crypto/asymmetric_keys/x509_public_key.c b/crypto/asymmetric_keys/x509_public_key.c
> index fe14cae115b51..71cc1738fbfd2 100644
> --- a/crypto/asymmetric_keys/x509_public_key.c
> +++ b/crypto/asymmetric_keys/x509_public_key.c
> @@ -128,12 +128,6 @@ int x509_check_for_self_signed(struct x509_certificate *cert)
>  			goto out;
>  	}
>  
> -	ret = -EKEYREJECTED;
> -	if (strcmp(cert->pub->pkey_algo, cert->sig->pkey_algo) != 0 &&
> -	    (strncmp(cert->pub->pkey_algo, "ecdsa-", 6) != 0 ||
> -	     strcmp(cert->sig->pkey_algo, "ecdsa") != 0))
> -		goto out;
> -
>  	ret = public_key_verify_signature(cert->pub, cert->sig);
>  	if (ret < 0) {
>  		if (ret == -ENOPKG) {
> -- 
> 2.35.1
> 

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

David, do you want to pick this?

BR, Jarkko