Re: [PATCH 0/2] crypto: add new driver for Marvell CESA

Boris Brezillon
2015-04-10T15:11:58Z

Hi Jason,

On Fri, 10 Apr 2015 13:50:56 +0000
Jason Cooper <jason@lakedaemon.net> wrote:

> Hey Boris,
>
> On Thu, Apr 09, 2015 at 04:58:41PM +0200, Boris Brezillon wrote:
> > I know we usually try to adapt existing drivers instead of replacing them
> > by new ones, but after trying to refactor the mv_cesa driver I realized it
> > would take longer than writing an new one from scratch.
>
> I'm sorry, but this makes me *very* uncomfortable. Any organization
> worth it's salt will do a very careful audit of code touching
> cryptographic material and sensitive (decrypted) data. From that point
> on, small audits of the changes to the code allow the organization to
> build a comfort level with kernel updates. iow, following the git
> history of the driver.
>
> By apply this series, we are basically forcing those organizations to
> either a) stop updating, or b) expend significant resources to do
> another full audit.
>
> In short, this series breaks the audit chain for the mv_cesa driver.
>
> Maybe I'm the only person with this level of paranoia. If so, I'm sure
> others will override me.
>
> From my POV, it looks like the *only* reason we've chosen this route is
> developer convenience. I don't think that's sufficient reason to break
> the change history of a driver handling sensitive data.

Well, I understand you concern, but if you read carefully the old and
new drivers, you'll notice that they are completely different (the only
thing I kept are the macro definitions).
I really tried to adapt the existing driver to add the missing
features (especially the support for TDMA), but all my attempts
ended up introducing hackish code (not even talking about the
performance penalty of this approach). Is that really what we want ?
How would you make such big changes on the existing driver (I mean, the
core infrastructure dealing with crypto requests is completely
different) ?

I have another solution though: keep the existing driver for old
marvell SoCs (orion, kirkwood and dove), and add a new one for modern
SoCs (armada 370, XP, 375 and 38x), so that users of the mv_cesa driver
won't have to audit the new code.

>
> For an example of how I use the git history and binary differences to
> audit a series of changes to cryptographic code, please take a look at
> objdiff [1]. You can even duplicate my audit of my submission for the
> skein/threefish driver currently in the staging tree, starting at [2]
> and going up to [3].

Thanks for the pointers.

Best Regards,

Boris

--
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering