linux-kernel - Re: [PATCH] arm64: head: rely on CONFIG_RANDOM_TRUST

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Message-ID: <Yvz4ccSO+BhICmyE@arighi-desktop>
Date:   Wed, 17 Aug 2022 16:17:21 +0200
From:   Andrea Righi <andrea.righi@...onical.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: head: rely on CONFIG_RANDOM_TRUST_CPU

On Wed, Aug 17, 2022 at 02:16:28PM +0100, Mark Brown wrote:
> On Wed, Aug 17, 2022 at 11:46:18AM +0200, Andrea Righi wrote:
> > The CONFIG_ARCH_RANDOM .config option has been removed by
> > commit 9592eef7c16e ("random: remove CONFIG_ARCH_RANDOM").
> > 
> > Depend on CONFIG_RANDOM_TRUST_CPU to determine whether we can rely on
> > __arm64_rndr() to initialize the seed for kaslr.
> > 
> > Fixes: 9592eef7c16e ("random: remove CONFIG_ARCH_RANDOM")
> 
> > -#ifdef CONFIG_ARCH_RANDOM
> > +#ifdef CONFIG_RANDOM_TRUST_CPU
> >  		 if (!__early_cpu_has_rndr() ||
> >  		     !__arm64_rndr((unsigned long *)&seed))
> >  #endif
> 
> I think the sense here would be more that we should just unconditionally
> use RNDR if it's present - previously we'd use the result even if we
> didn't have strong trust in the CPU implementation and I don't see why
> we'd want to change that.

Makes sense, in that case it'd be better to just remove the guard
completely, as mentioned by Will.

Thanks,
-Andrea