linux-kernel - Re: Why the auxiliary cipher in gss_krb5

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Date:   Tue, 08 Dec 2020 14:13:48 +0000
From:   David Howells <dhowells@...hat.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     dhowells@...hat.com, 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?

Ard Biesheuvel <ardb@...nel.org> wrote:

> 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.

I never said it wasn't possible.  But doing a pair of order-1 allocations from
there might have a significant detrimental effect on performance - in which
case Trond and co. will say "no".

Remember: to crypt 1MiB of data on a 64-bit machine requires 2 x minimum 8KiB
scatterlist arrays.  That's assuming the pages in the middle are contiguous,
which might not be the case for a direct I/O read/write.  So for the DIO case,
it could be involve an order-2 allocation (or chaining of single pages).

David