From:   Pascal Van Leeuwen <pvanleeuwen@insidesecure.com>
To:     Eric Biggers <ebiggers@kernel.org>
CC:     "linux-crypto@vger.kernel.org" <linux-crypto@vger.kernel.org>,
        Herbert Xu <herbert@gondor.apana.org.au>
Subject: RE: Question regarding crypto scatterlists / testmgr
Thread-Topic: Question regarding crypto scatterlists / testmgr
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Date:   Wed, 17 Apr 2019 21:16:54 +0000
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> -----Original Message-----
> From: Eric Biggers [mailto:ebiggers@kernel.org]
> Sent: Wednesday, April 17, 2019 10:24 PM
> To: Pascal Van Leeuwen <pvanleeuwen@insidesecure.com>
> Cc: linux-crypto@vger.kernel.org; Herbert Xu
> <herbert@gondor.apana.org.au>
> Subject: Re: Question regarding crypto scatterlists / testmgr
>
> Hi Pascal,
>
> On Wed, Apr 17, 2019 at 07:51:08PM +0000, Pascal Van Leeuwen wrote:
> > Hi,
> >
> > I'm trying to fix the inside-secure driver to pass all testmgr
> > tests and I have one final issue remaining with the AEAD ciphers.
> > As it was not clear at all what the exact problem was, I spent
> > some time reverse engineering testmgr and I got the distinct
> > impression that it is using scatter particles that cross page
> > boundaries. On purpose, even.
> >
> > While the inside-secure driver is built on the premise that
> > scatter particles are continuous in device space. As I can't
> > think of any reason why you would want to scatter/gather other
> > than to handle virtual-to-physical address translation ...
> > In any case, this should affect all other other operations as
> > well, but maybe those just got "lucky" by getting particles
> > that were still contiguous in device space, despite the page
> > crossing (to *really* verify this, you would have to fully
> > randomize your page allocation!)
> >
> > Anyway, assuming that I *should* be able to handle particles
> > that are *not* contiguous in device space, then there should
> > probably already exist some function in the kernel API that
> > converts a scatterlist with non-contiguous particles into a
> > scatterlist with contiguous particles, taking into account the
> > presence of an IOMMU? Considering pretty much every device
> > driver would need to do that?
> > Does anyone know which function(s) to use for that?
> >
> > Regards,
> > Pascal van Leeuwen
> > Silicon IP Architect, Multi-Protocol Engines @ Inside Secure
> >
>
> Indeed, since v5.1, testmgr tests scatterlist elements that cross a
> page.
> However, the pages are guaranteed to be *physically* contiguous.  Does
> dma_map_sg() not handle this?
>
I'm not entirely sure and the API documentation is not particularly
clear on *what* dma_map_sg() actually does, but I highly doubt it
considering the particle count is only an input parameter (i.e. it
can't output an increase in particles that would be required).
So I think it just ensures the pages are actually flushed to memory
and accessible by the device (in case an IOMMU interferes) and not
much than that.

In any case, scatter particles to be used by hardware should *not*
cross any physical page boundaries.
But also see the thread I had on this with Ard - seems like the crypto
API already has some mechanism for enforcing this but it's not enabled
for AEAD ciphers?

>
> BTW, this isn't just a theoretical case.  Many crypto API users do
> crypto on
> kmalloced buffers, and those can cross a page boundary, especially if
> they are
> large.  All software crypto algorithms handle this case.
>
Software sits behind the CPU's MMU and sees virtual memory as
contiguous. It does not need to "handle" anything, it gets it for free.
Hardware does not have that luxury, unless you have a functioning IOMMU
but that is still pretty rare.
So for hardware, you need to break down your buffers until individual
pages and stitch those together. That's the main use case of a scatter
list and it requires the particles to NOT cross physical pages.

> The fact that these types of issues are just being considered now
> certainly
> isn't raising my confidence in the hardware crypto drivers in the
> kernel...
>
Actually, this is *not* a problem with the hardware drivers. It's a
problem with the API and/or how you are trying to use it. Hardware
does NOT see the nice contiguous virtual memory that SW sees.

If the driver may expect to receive particles that cross page
boundaries - if that's the spec - fine, but then it will have to
break those down into individual pages by itself. However, whomever
created the inside-secure driver was under the impression that this
was not supposed to be the case. And I don't know who's right or
wrong there, but from a side discussion with Ard I got the impression
that the Crypto API should fix this up before it reaches the driver.

Regards,

Pascal van Leeuwen
Silicon IP Architect, Multi-Protocol Engines @ Inside Secure