linux-kernel - Re: [PATCH 1/4] crypto: xts - restrict key lengths to approved values in FIPS mode

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Date:   Wed, 09 Nov 2022 11:06:17 +0100
From:   Nicolai Stange <nstange@...e.de>
To:     "Elliott, Robert (Servers)" <elliott@....com>
Cc:     Nicolai Stange <nstange@...e.de>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Vladis Dronov <vdronov@...hat.com>,
        Stephan Mueller <smueller@...onox.de>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] crypto: xts - restrict key lengths to approved
 values in FIPS mode

"Elliott, Robert (Servers)" <elliott@....com> writes:

>> diff --git a/include/crypto/xts.h b/include/crypto/xts.h
> ...
>> @@ -35,6 +35,13 @@ static inline int xts_verify_key(struct crypto_skcipher
>> *tfm,
>>  	if (keylen % 2)
>>  		return -EINVAL;
>> 
>> +	/*
>> +	 * In FIPS mode only a combined key length of either 256 or
>> +	 * 512 bits is allowed, c.f. FIPS 140-3 IG C.I.
>> +	 */
>> +	if (fips_enabled && keylen != 32 && keylen != 64)
>> +		return -EINVAL;
>> +
>>  	/* ensure that the AES and tweak key are not identical */
>>  	if ((fips_enabled || (crypto_skcipher_get_flags(tfm) &
>>  			      CRYPTO_TFM_REQ_FORBID_WEAK_KEYS)) &&
>> --
>> 2.38.0
>
> There's another function in the same file called xts_check_key() 
> that is used by some of the hardware drivers:

Right, thanks for spotting.

AFAICT, xts_check_key() is the older of the two variants,
xts_verify_key() had been introduced with commit f1c131b45410 ("crypto:
xts - Convert to skcipher"). There had initially only been a single
call from generic crypto/xts.c and the main difference to
xts_check_key() had been that it took a crypto_skcipher for its tfm
argument rather than a plain crypto_tfm as xts_check_key() did.

It seems that over time, xts crypto drivers adopted the newer
xts_verify_key() variant then.

>
> arch/s390/crypto/paes_s390.c:    * xts_check_key verifies the key length is not odd and makes
>  [that references it in the comment but actually calls xts_verify_key in the code]
> drivers/crypto/axis/artpec6_crypto.c:   ret = xts_check_key(&cipher->base, key, keylen);
> drivers/crypto/cavium/cpt/cptvf_algs.c: err = xts_check_key(tfm, key, keylen);
> drivers/crypto/cavium/nitrox/nitrox_skcipher.c: ret = xts_check_key(tfm, key, keylen);
> drivers/crypto/ccree/cc_cipher.c:           xts_check_key(tfm, key, keylen)) {
> drivers/crypto/marvell/octeontx/otx_cptvf_algs.c:       ret = xts_check_key(crypto_skcipher_tfm(tfm), key, keylen);
> drivers/crypto/marvell/octeontx2/otx2_cptvf_algs.c:     ret = xts_check_key(crypto_skcipher_tfm(tfm), key, keylen);
> drivers/crypto/atmel-aes.c:     err = xts_check_key(crypto_skcipher_tfm(tfm), key, keylen);
>
> It already has one check qualified by fips_enabled:
>
>         /* ensure that the AES and tweak key are not identical */
>         if (fips_enabled && !crypto_memneq(key, key + (keylen / 2), keylen / 2))
>                 return -EINVAL;
>
> Should that implement the same key length restrictions?

>From a quick glance, all of the above drivers merely convert some
crypto_skcipher to a crypto_tfm before passing it to xts_check_key().

So I think these should all be made to call xts_verify_key() directly
instead, the former xts_check_key() could then get dropped. But that's
more of a cleanup IMO and would probably deserve a separate patch series
on its own.

Thanks!

Nicolai

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