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

From: Eric Biggers <ebiggers@kernel.org>
To: Vitaly Chikunov <vt@altlinux.org>
Cc: keyrings@vger.kernel.org, Jarkko Sakkinen <jarkko@kernel.org>,
	David Howells <dhowells@redhat.com>,
	linux-crypto@vger.kernel.org, linux-integrity@vger.kernel.org,
	Stefan Berger <stefanb@linux.ibm.com>,
	Gilad Ben-Yossef <gilad@benyossef.com>,
	Tianjia Zhang <tianjia.zhang@linux.alibaba.com>,
	Mimi Zohar <zohar@linux.ibm.com>,
	stable@vger.kernel.org
Subject: Re: [PATCH 1/2] KEYS: asymmetric: enforce that sig algo matches key algo
Date: Tue, 1 Feb 2022 19:10:33 -0800	[thread overview]
Message-ID: <Yfn2KZgjuFRSJzSj@sol.localdomain> (raw)
In-Reply-To: <20220202025230.hrfochvm3uyuh2wm@altlinux.org>

On Wed, Feb 02, 2022 at 05:52:30AM +0300, Vitaly Chikunov wrote:
> Eric,
> 
> 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.
> 
> Why should they match?

For the reasons I explain in the rest of the commit message.  To summarize: to
have a valid signature verification scheme the algorithm must be fixed by the
key, and not attacker-controlled.

> 
> public_key_signature is the data prepared to verify the cert's
> signature. The cert's signature algorithm could be different from the
> public key algorithm defined in the cert itself. They should match only
> for self-signed certs. For example, you should be able to sign RSA
> public key with ECDSA signature and vice versa. Or 256-bit EC-RDSA with
> 512-bit EC-RDSA. This check will prevent this.

That has nothing to do with this patch, as this patch is only dealing with the
signature.  A cert's public key algorithm can be different, and that is fine.

> > 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;
> 
> This seem incorrect too, as sig->pkey_algo could be NULL for direct
> signature verification calls. For example, for keyctl pkey_verify.

We can make it optional if some callers aren't providing it.  Of course, such
callers wouldn't be able to verify ECDSA signatures.

- Eric