linux-kernel - Re: [PATCH v2 03/18] crypto: dh - optimize domain parameter serialization for well-known groups

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Date:   Mon, 13 Dec 2021 11:06:36 +0100
From:   Nicolai Stange <nstange@...e.de>
To:     Hannes Reinecke <hare@...e.de>
Cc:     Nicolai Stange <nstange@...e.de>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Stephan Müller <smueller@...onox.de>,
        Torsten Duwe <duwe@...e.de>, Zaibo Xu <xuzaibo@...wei.com>,
        Giovanni Cabiddu <giovanni.cabiddu@...el.com>,
        David Howells <dhowells@...hat.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
        qat-linux@...el.com, keyrings@...r.kernel.org
Subject: Re: [PATCH v2 03/18] crypto: dh - optimize domain parameter serialization for well-known groups

Hannes Reinecke <hare@...e.de> writes:

> On 12/9/21 10:03 AM, Nicolai Stange wrote:
>> diff --git a/crypto/dh_helper.c b/crypto/dh_helper.c
>> index aabc91e4f63f..9f21204e5dee 100644
>> --- a/crypto/dh_helper.c
>> +++ b/crypto/dh_helper.c
>> @@ -45,18 +72,24 @@ int crypto_dh_encode_key(char *buf, unsigned int len, const struct dh *params)
>>  		.type = CRYPTO_KPP_SECRET_TYPE_DH,
>>  		.len = len
>>  	};
>> +	int group_id;
>>  
>>  	if (unlikely(!len))
>>  		return -EINVAL;
>>  
>>  	ptr = dh_pack_data(ptr, end, &secret, sizeof(secret));
>> +	group_id = (int)params->group_id;
>> +	ptr = dh_pack_data(ptr, end, &group_id, sizeof(group_id));
>
> Me being picky again.
> To my knowledge, 'int' doesn't have a fixed width, but is rather only
> guaranteed to hold certain values.
> So as soon as one relies on any fixed size (as this one does) I tend to
> use fixed size type like 'u32' to make it absolutely clear what is to be
> expected here.
>
> But the I don't know the conventions in the crypto code; if an 'int' is
> assumed to be 32 bits throughout the crypto code I guess we should be fine.

Yes, I thought about this, too. However, the other, already existing
fields like ->key_size and ->p_size are getting serialized as unsigned
ints and I decided to stick to that for ->group_id as well. Except for
the testmgr vectors, the encoding is internal to the
crypto_dh_encode_key() and crypto_dh_decode_key() pair anyway -- all
that would happen if sizeof(int) != 4 is that the tests would fail.

So, IMO, making the serialization of struct dh to use u32 throughout is
not really in scope for this series and would probably deserve a patch
on its own, if desired.

Thanks,

Nicolai

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