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

From: Vitaly Chikunov <vt@altlinux.org>
To: Eric Biggers <ebiggers@kernel.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: Wed, 2 Feb 2022 08:20:14 +0300	[thread overview]
Message-ID: <20220202052014.fflcmgpykwbfzdt4@altlinux.org> (raw)
In-Reply-To: <Yfn2KZgjuFRSJzSj@sol.localdomain>

On Tue, Feb 01, 2022 at 07:10:33PM -0800, Eric Biggers wrote:
> 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.

You are right and I was mistaken about that (obscured by keyctl
pkey_verify error and self-signed keys verification). Then it's all
good!

I also tested these patches to work well with rsa-ecdsa and ecrdsa
certificates using keyctl restrict_keyring.

Thanks,

> 
> > > 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