linux-kernel - Re: [PATCH] lib/crypto: arm/curve25519: Disable on CPU_BIG

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Message-ID: <20251104173749.GA1780@sol>
Date: Tue, 4 Nov 2025 09:37:49 -0800
From: Eric Biggers <ebiggers@...nel.org>
To: linux-crypto@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.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] lib/crypto: arm/curve25519: Disable on CPU_BIG_ENDIAN

On Mon, Nov 03, 2025 at 09:49:06PM -0800, Eric Biggers wrote:
> On big endian arm kernels, the arm optimized Curve25519 code produces
> incorrect outputs and fails the Curve25519 test.  This has been true
> ever since this code was added.
> 
> It seems that hardly anyone (or even no one?) actually uses big endian
> arm kernels.  But as long as they're ostensibly supported, we should
> disable this code on them so that it's not accidentally used.
> 
> Note: for future-proofing, use !CPU_BIG_ENDIAN instead of
> CPU_LITTLE_ENDIAN.  Both of these are arch-specific options that could
> get removed in the future if big endian support gets dropped.
> 
> Fixes: d8f1308a025f ("crypto: arm/curve25519 - wire up NEON implementation")
> Cc: stable@...r.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@...nel.org>
> ---
> 
> This patch is targeting libcrypto-fixes
> 
>  lib/crypto/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/crypto/Kconfig b/lib/crypto/Kconfig
> index 8886055e938f..16859c6226dd 100644
> --- a/lib/crypto/Kconfig
> +++ b/lib/crypto/Kconfig
> @@ -62,11 +62,11 @@ config CRYPTO_LIB_CURVE25519
>  	  of the functions from <crypto/curve25519.h>.
>  
>  config CRYPTO_LIB_CURVE25519_ARCH
>  	bool
>  	depends on CRYPTO_LIB_CURVE25519 && !UML && !KMSAN
> -	default y if ARM && KERNEL_MODE_NEON
> +	default y if ARM && KERNEL_MODE_NEON && !CPU_BIG_ENDIAN
>  	default y if PPC64 && CPU_LITTLE_ENDIAN
>  	default y if X86_64
>  
>  config CRYPTO_LIB_CURVE25519_GENERIC
>  	bool
> 
> base-commit: 1af424b15401d2be789c4dc2279889514e7c5c94
> -- 
> 2.51.2
> 

Applied to https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git/log/?h=libcrypto-fixes

- Eric