linux-kernel - Re: [PATCH net-next v3 02/17] zinc: introduce minimal cryptography library

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Date:   Mon, 17 Sep 2018 07:51:54 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Andrew Lunn <andrew@...n.ch>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Eric Biggers <ebiggers@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Samuel Neves <sneves@....uc.pt>,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>
Subject: Re: [PATCH net-next v3 02/17] zinc: introduce minimal cryptography library



> On Sep 16, 2018, at 10:26 PM, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:

> 
> As far as I can tell (i.e., as a user not a network dev), WireGuard is
> an excellent piece of code, and I would like to see it merged. I also
> think there is little disagreement about the quality of the proposed
> algorithm implementations and the usefulness of having a set of easy
> to use solid crypto primitives in addition to or complementing the
> current crypto API.
> 
> I do have some concerns over how the code is organized though:
> 
> * simd_relax() is currently not called by the crypto routines
> themselves. This means that the worst case scheduling latency is
> unbounded, which is unacceptable for the -rt kernel. The worst case
> scheduling latency should never be proportional to the input size.
> (Apologies for not spotting that earlier)
> 
> * Using a cute name for the crypto library [that will end up being the
> preferred choice for casual use only] may confuse people, given that
> we have lots of code in crypto/ already. I'd much prefer using, e.g.,
> crypto/lib and crypto/api (regardless of where the arch specific
> pieces live)
> 
> * I'd prefer the arch specific pieces to live under arch, but I can
> live with keeping them in a single place, as long as the arch
> maintainers have some kind of jurisdiction over them. I also think
> there should be some overlap between the maintainership
> responsibilities of the two generic pieces (api and lib).
> 
> * (Nit) The GCC command line -include'd .h files contain variable and
> function definitions so they are actually .c files.

Hmm. I would suggest just getting rid of the -include magic entirely.  The resulting ifdef will be more comprehensible.


> * The current organization of the code puts all available (for the
> arch) versions of all routines into a single module, which can only be
> built in once we update random.c to use Zinc's chacha20 routines. This
> bloats the core kernel (which is a huge deal for embedded systems that
> have very strict boot requirements). It also makes it impossible to
> simply blacklist a module if you, for instance, prefer to use the
> [potentially more power efficient] scalar code over the SIMD code when
> using a distro kernel.

I think the module organization needs to change. It needs to be possible to have chacha20 built in but AES or whatever as a module.

> 
> [To summarize the 4 points above, I'd much rather see a more
> conventional organization where different parts are provided by
> different modules. I don't think the argument that inlining is needed
> for performance is actually valid, given that we have branch
> predictors and static keys, and the arch SIMD code is built as
> separate object files anyway]

I might have agreed before Spectre :(. Unfortunately, unless we do some magic, I think the code would look something like:

if (static_branch_likely(have_simd)) arch_chacha20();

...where arch_chacha20 is a *pointer*. And that will generate a retpoline and run very, very slowly.  (I just rewrote some of the x86 entry code to eliminate one retpoline. I got a 5% speedup on some tests according to the kbuild bot.)

So, if we really wanted modules, we’d need a new dynamic patching mechanism.

I would suggest instead adding two boot (or debugfs) options:

simd=off: disables simd_get using a static branch.

crypto_chacha20_nosimd: Does what it sounds like.