netdev - Re: [PATCH v4] mac80211: move extra crypto data off the stack

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Date:   Mon, 17 Oct 2016 14:06:47 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     "<linux-wireless@...r.kernel.org>" <linux-wireless@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        "<netdev@...r.kernel.org>" <netdev@...r.kernel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH v4] mac80211: move extra crypto data off the stack

On 17 October 2016 at 11:02, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
>
>
>> On 17 Oct 2016, at 10:54, Johannes Berg <johannes@...solutions.net> wrote:
>>
>>
>>>> Well, if your other patch to make it OK to be on-stack would be
>>>> applied instead, this wouldn't make much sense either :)
>>>>
>>>
>>> Yes but that one only fixes ccm not gcm
>>
>> Yes, but we can do the same for GCM, no?
>>
>
> No, not really. ccm happens to use aes with the same key for the mac and the encryption. gcm uses an another algo entirely for the mac
>
>>>> In this particular patch, we could reduce the size of the struct,
>>>> but I
>>>> don't actually think it'll make a difference to go from 48 to 36
>>>> bytes,
>>>> given alignment etc., so I think I'll just leave it as is.
>>>>
>>>
>>> I understand you are in a hurry, but this is unlikely to be the only
>>> such issue. I will propose an 'auxdata' feature for the crypto api
>>> that can be used here, but also for any other occurrence where client
>>> data assoiciated with the request can no longer be allocated on the
>>> stack
>>
>> No objections. I'll merge this anyway today I think, reverting is easy
>> later.
>>
>
> ok fair enough

Actually, while I think it will be worthwhile going forward to
implement such an 'auxiliary data' feature in a generic way, I still
think we should address the issue at hand without too much
complication.

If we pedal back to the version of 'mac80211: move struct aead_req off
the stack' that uses kzalloc() instead of aead_request_alloc(), we can
simply add some space for aad[] and/or zero[], and get rid of the kmem
cache entirely.

If you're past this point already, i won't bother but otherwise I can
rework 'mac80211: move struct aead_req off the stack' so that the
other patch is no longer required (and IIRC, this is actually
something you proposed yourself a couple of iterations ago?)