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

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(-)

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

On Mon, Feb 21, 2022 at 02:43:21AM +0100, Jarkko Sakkinen wrote:
> 
> Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
> 
> David, do you want to pick this?
> 

No response from David, so I think you need to take these.

- Eric