linux-kernel - Re: [PATCH v2 1/3] arm64: crypto/aes-ce-ccm: Cleanup hwcap check

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Message-ID: <42a11fce-d231-2317-3c8e-a9044bbe5f62@arm.com>
Date:   Tue, 1 Nov 2016 14:18:12 +0000
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <marc.zyngier@....com>
Subject: Re: [PATCH v2 1/3] arm64: crypto/aes-ce-ccm: Cleanup hwcap check

On 01/11/16 14:03, Ard Biesheuvel wrote:
> Hi Suzuki,
>
> On 31 October 2016 at 16:03, Suzuki K Poulose <suzuki.poulose@....com> wrote:
>> Use the module_cpu_feature_match to make sure the system has
>> HWCAP_AES to use the module.
>>
>> Cc: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>> ---
>>  arch/arm64/crypto/aes-ce-ccm-glue.c | 5 ++---
>>  1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm64/crypto/aes-ce-ccm-glue.c b/arch/arm64/crypto/aes-ce-ccm-glue.c
>> index f4bf2f2..fa82eaa 100644
>> --- a/arch/arm64/crypto/aes-ce-ccm-glue.c
>> +++ b/arch/arm64/crypto/aes-ce-ccm-glue.c
...
>> -module_init(aes_mod_init);
>> +module_cpu_feature_match(AES, aes_mod_init);
>
> I don't think this change is correct. This will result in the AES
> instruction dependency to be exposed via the module alias, causing the
> module to be loaded automatically as soon as udev detects that the CPU
> implements those instructions. For plain AES, that makes sense, but
> AES in CCM mode is specific to CCMP (WPA2) on mac80211 controllers
> that have no hardware AES support, and to IPsec VPN. For this reason,
> the algo type is exposed via the module alias instead (i.e,
> 'ccm(aes)'), which will result in the module being loaded as soon as
> the crypto algo manager instantiates the transform. On CPUs that don't
> implement the AES instructions, this will fail, and it will fall back
> to the generic CCM driver instead.

Ah, thanks for the explanation. I will drop it.

Cheers
Suzuki