Re: [RFC PATCH] KEYS: Double max_size to make keyctl pkey_verify work

[Vitaly Chikunov <vt@altlinux.org>]

On Wed, Feb 02, 2022 at 10:15:24PM -0500, Stefan Berger wrote:
> 
> On 2/2/22 01:59, Vitaly Chikunov wrote:
> > Rarely used `keyctl pkey_verify' can verify raw signatures, but was
> > failing, because ECDSA/EC-RDSA signature sizes are twice key sizes which
> > does not pass in/out sizes check in keyctl_pkey_params_get_2.
> > This in turn because these values cannot be distinguished by a single
> > `max_size' callback return value.
> > Also, `keyctl pkey_query` displays incorrect `max_sig_size' about these
> > algorithms.
> > 
> > Signed-off-by: Vitaly Chikunov <vt@altlinux.org>
> > ---
> >   crypto/asymmetric_keys/public_key.c | 15 +++++++++++++--
> >   1 file changed, 13 insertions(+), 2 deletions(-)
> > 
> > diff --git a/crypto/asymmetric_keys/public_key.c b/crypto/asymmetric_keys/public_key.c
> > index 4fefb219bfdc..3ffbab07ed2a 100644
> > --- a/crypto/asymmetric_keys/public_key.c
> > +++ b/crypto/asymmetric_keys/public_key.c
> > @@ -143,8 +143,19 @@ static int software_key_query(const struct kernel_pkey_params *params,
> >   	len = crypto_akcipher_maxsize(tfm);
> >   	info->key_size = len * 8;
> > -	info->max_data_size = len;
> > -	info->max_sig_size = len;
> > +	if (strcmp(alg_name, "ecrdsa") == 0 ||
> > +	    strncmp(alg_name, "ecdsa-", 6) == 0) {
> > +		/*
> > +		 * For these algos sig size is twice key size.
> > +		 * keyctl uses max_sig_size as minimum input size, and
> > +		 * max_data_size as minimum output size for a signature.
> > +		 */
> > +		info->max_data_size = len * 2;
> > +		info->max_sig_size = len * 2;
> I don't know about the data size but following my tests this is not enough
> for ECDSA signature size. In ECDSA case the r and s components of the
> signature are encode in asn.1, not 'raw'. So there are 2 bytes at the
> beginning for sequence identifier , 2 bytes asn.1 for the r component, 1
> additional 0-byte to make the r component always a positive number, then the
> r component, then 2 bytes asn.1 for the s component, 1 addition 0-byte to
> make the s component a positive number, then the s component. Phew.
> 
> info->max_sig_size = 2 + (2 + 1 + len) * 2;
> 
> so for NIST P384 it's: 2 + (2+1+48) * 2 = 104
> 
> Then it works for me as well.

Well, another solution, without changing API, is that max_size() should
return bigger size (to fit encoded signature), but in that case keyctl
will think wrongly about key_size.

Just for reference, keyctl_pkey_params_get_2 check that needs to be
passed:

        case KEYCTL_PKEY_VERIFY:
                if (uparams.in_len  > info.max_sig_size ||
                    uparams.out_len > info.max_data_size)
                        return -EINVAL;

So we can return arbitrarily big value, in theory.

Thanks,

> 
> 
> > +	} else {
> > +		info->max_data_size = len;
> > +		info->max_sig_size = len;
> > +	}
> >   	info->max_enc_size = len;
> >   	info->max_dec_size = len;
> >   	info->supported_ops = (KEYCTL_SUPPORTS_ENCRYPT |