linux-kernel - Re: [RFC V1 3/7] crypto: ghash - Optimized GHASH computations

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Date:   Fri, 15 Jan 2021 21:07:52 -0800
From:   "Dey, Megha" <megha.dey@...el.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ravi.v.shankar@...el.com, tim.c.chen@...el.com,
        andi.kleen@...el.com, dave.hansen@...el.com,
        wajdi.k.feghali@...el.com, greg.b.tucker@...el.com,
        robert.a.kasten@...el.com, rajendrakumar.chinnaiyan@...el.com,
        tomasz.kantecki@...el.com, ryan.d.saffores@...el.com,
        ilya.albrekht@...el.com, kyung.min.park@...el.com,
        Tony Luck <tony.luck@...el.com>, ira.weiny@...el.com
Subject: Re: [RFC V1 3/7] crypto: ghash - Optimized GHASH computations


On 1/15/2021 5:43 PM, Eric Biggers wrote:
> On Fri, Jan 15, 2021 at 04:14:40PM -0800, Dey, Megha wrote:
>>> Hello Megha,
>>>
>>> What is the purpose of this separate GHASH module? GHASH is only used
>>> in combination with AES-CTR to produce GCM, and this series already
>>> contains a GCM driver.
>>>
>>> Do cores exist that implement PCLMULQDQ but not AES-NI?
>>>
>>> If not, I think we should be able to drop this patch (and remove the
>>> existing PCLMULQDQ GHASH driver as well)
>> AFAIK, dm-verity (authenticated but not encrypted file system) is one use
>> case for authentication only.
>>
>> Although I am not sure if GHASH is specifically used for this or SHA?
>>
>> Also, I do not know of any cores that implement PCLMULQDQ and not AES-NI.
>>
> dm-verity only uses unkeyed hash algorithms.  So no, it doesn't use GHASH.

Hmm, I see. If that is the case, I am not aware of any other use case 
apart from GCM.

I see that the existing GHASH module in the kernel from 2009. I am not 
sure if there was a use case then, which now is no longer valid.

There many be out-of-tree kernel modules which may be using it but again 
its only speculation.

So, in the next version should I remove the existing GHASH module? (And 
of course remove this patch as well?)

-Megha

>
> - Eric