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Date:   Tue, 8 Dec 2020 15:04:05 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Chuck Lever <chuck.lever@...cle.com>,
        Bruce Fields <bfields@...ldses.org>,
        CIFS <linux-cifs@...r.kernel.org>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "open list:BPF JIT for MIPS (32-BIT AND 64-BIT)" 
        <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, linux-afs@...ts.infradead.org
Subject: Re: Why the auxiliary cipher in gss_krb5_crypto.c?

On Tue, 8 Dec 2020 at 14:25, David Howells <dhowells@...hat.com> wrote:
>
> I wonder - would it make sense to reserve two arrays of scatterlist structs
> and a mutex per CPU sufficient to map up to 1MiB of pages with each array
> while the krb5 service is in use?
>
> That way sunrpc could, say, grab the mutex, map the input and output buffers,
> do the entire crypto op in one go and then release the mutex - at least for
> big ops, small ops needn't use this service.
>
> For rxrpc/afs's use case this would probably be overkill - it's doing crypto
> on each packet, not on whole operations - but I could still make use of it
> there.
>
> However, that then limits the maximum size of an op to 1MiB, plus dangly bits
> on either side (which can be managed with chained scatterlist structs) and
> also limits the number of large simultaneous krb5 crypto ops we can do.
>

Apparently, it is permitted for gss_krb5_cts_crypt() to do a
kmalloc(GFP_NOFS) in the context from where gss_krb5_aes_encrypt() is
being invoked, and so I don't see why it wouldn't be possible to
simply kmalloc() a scatterlist[] of the appropriate size, populate it
with all the pages, bufs and whatever else gets passed into the
skcipher, and pass it into the skcipher in one go.

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