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

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Message-ID: <87h6z8e7jc.fsf@suse.de>
Date:   Wed, 09 Nov 2022 11:39:19 +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
>
> arch/s390/crypto/aes_s390.c has similar lines:
>
> static int xts_aes_set_key(struct crypto_skcipher *tfm, const u8 *in_key,
>                            unsigned int key_len)
> {
>         struct s390_xts_ctx *xts_ctx = crypto_skcipher_ctx(tfm);
>         unsigned long fc;
>         int err;
>
>         err = xts_fallback_setkey(tfm, in_key, key_len);
>         if (err)
>                 return err;
>
>         /* In fips mode only 128 bit or 256 bit keys are valid */
>         if (fips_enabled && key_len != 32 && key_len != 64)
>                 return -EINVAL;
>
>
> xts_fallback_setkey will now enforce that rule when setting up the
> fallback algorithm keys, which makes the xts_aes_set_key check
> unreachable.

Good finding!

>
> If that fallback setup were not present, then a call to xts_verify_key
> might be preferable to enforce any other rules like the WEAK_KEYS
> rule.
>

So if this patch here would get accepted, I'd propose to remove the then
dead code from aes_s390 afterwards and make an explicit call to
xts_verify_key() instead.

Or shall I split out the XTS patch from this series here and post these
two changes separately then? Herbert, any preferences?

Thanks!

Nicolai

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