Message-ID: <CAMj1kXHVq1NWA28jKxBrHHi1JOPoGXEamC7uMgTOmFwzmcYxRA@mail.gmail.com>
Date: Mon, 15 Dec 2025 16:54:34 +0900
From: Ard Biesheuvel <ardb@...nel.org>
To: Eric Biggers <ebiggers@...nel.org>
Cc: Diederik de Haas <diederik@...ow-tech.com>, linux-crypto@...r.kernel.org, 
	linux-kernel@...r.kernel.org, "Jason A . Donenfeld" <Jason@...c4.com>, 
	Herbert Xu <herbert@...dor.apana.org.au>, linux-arm-kernel@...ts.infradead.org, 
	stable@...r.kernel.org
Subject: Re: [PATCH] crypto: arm64/ghash - Fix incorrect output from ghash-neon

On Fri, 12 Dec 2025 at 06:40, Eric Biggers <ebiggers@...nel.org> wrote:
>
> On Wed, Dec 10, 2025 at 06:31:44PM +0900, Ard Biesheuvel wrote:
> > On Wed, 10 Dec 2025 at 18:22, Diederik de Haas <diederik@...ow-tech.com> wrote:
> > >
> > > On Tue Dec 9, 2025 at 11:34 PM CET, Eric Biggers wrote:
> > > > Commit 9a7c987fb92b ("crypto: arm64/ghash - Use API partial block
> > > > handling") made ghash_finup() pass the wrong buffer to
> > > > ghash_do_simd_update().  As a result, ghash-neon now produces incorrect
> > > > outputs when the message length isn't divisible by 16 bytes.  Fix this.
> > >
> > > I was hoping to not have to do a 'git bisect', but this is much better
> > > :-D I can confirm that this patch fixes the error I was seeing, so
> > >
> > > Tested-by: Diederik de Haas <diederik@...ow-tech.com>
> > >
> > > > (I didn't notice this earlier because this code is reached only on CPUs
> > > > that support NEON but not PMULL.  I haven't yet found a way to get
> > > > qemu-system-aarch64 to emulate that configuration.)
> > >
> > > https://www.qemu.org/docs/master/system/arm/raspi.html indicates it can
> > > emulate various Raspberry Pi models. I've only tested it with RPi 3B+
> > > (bc of its wifi+bt chip), but I wouldn't be surprised if all RPi models
> > > would have this problem? Dunno if QEMU emulates that though.
> > >
> >
> > All 64-bit RPi models except the RPi5 are affected by this, as those
> > do not implement the crypto extensions. So I would expect QEMU to do
> > the same.
> >
> > It would be nice, though, if we could emulate this on the mach-virt
> > machine model too. It should be fairly trivial to do, so if there is
> > demand for this I can look into it.
>
> I'm definitely interested in it.  I'm already testing multiple "-cpu"
> options, and it's easy to add more.
>
> With qemu-system-aarch64 I'm currently only using "-M virt", since the
> other machine models I've tried don't boot with arm64 defconfig,
> including "-M raspi3b" and "-M raspi4b".
>
> There may be some tricks I'm missing.  Regardless, expanding the
> selection of available CPUs for "-M virt" would be helpful.  Either by
> adding "real" CPUs that have "interesting" combinations of features, or
> by just allowing turning features off like
> "-cpu max,aes=off,pmull=off,sha256=off".  (Certain features like sve can
> already be turned off in that way, but not the ones relevant to us.)
>

There are some architectural rules around which combinations of crypto
extensions are permitted:
- PMULL implies AES, and there is no way for the ID registers to
describe a CPU that has PMULL but not AES
- SHA256 implies SHA1 (but the ID register fields are independent)
- SHA3 and SHA512 both imply SHA256+SHA1
- SVE versions are not allowed to be implemented unless the plain NEON
version is implemented as well
-  FEAT_Crypto has different meanings for v8.0, v8.2 and v9.x

So it would be much easier, also in terms of future maintenance, to
have a simple 'crypto=off' setting that applies to all emulated CPU
models, given that disabling all crypto on any given compliant CPU
will never result in something that the architecture does not permit.

Would that work for you?

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