linux-kernel - Re: [PATCH v3 05/21] arm64: kvm: deal with kernel symbols outside of linear mapping

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Date:	Tue, 12 Jan 2016 12:36:57 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	linux-arm-kernel@...ts.infradead.org,
	kernel-hardening@...ts.openwall.com, will.deacon@....com,
	catalin.marinas@....com, leif.lindholm@...aro.org,
	keescook@...omium.org, linux-kernel@...r.kernel.org,
	stuart.yoder@...escale.com, bhupesh.sharma@...escale.com,
	arnd@...db.de, marc.zyngier@....com, christoffer.dall@...aro.org
Subject: Re: [PATCH v3 05/21] arm64: kvm: deal with kernel symbols outside of
 linear mapping

On Mon, Jan 11, 2016 at 02:18:58PM +0100, Ard Biesheuvel wrote:
> KVM on arm64 uses a fixed offset between the linear mapping at EL1 and
> the HYP mapping at EL2. Before we can move the kernel virtual mapping
> out of the linear mapping, we have to make sure that references to kernel
> symbols that are accessed via the HYP mapping are translated to their
> linear equivalent.
> 
> To prevent inadvertent direct references from sneaking in later, change
> the type of all extern declarations to HYP kernel symbols to the opaque
> 'struct kvm_ksym', which does not decay to a pointer type like char arrays
> and function references. This is not bullet proof, but at least forces the
> user to take the address explicitly rather than referencing it directly.
>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>

Cool feature!

> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> index 5e377101f919..e3865845d3e1 100644
> --- a/arch/arm64/include/asm/kvm_asm.h
> +++ b/arch/arm64/include/asm/kvm_asm.h
> @@ -105,24 +105,29 @@
>  #ifndef __ASSEMBLY__
>  struct kvm;
>  struct kvm_vcpu;
> +struct kvm_ksym;

So that one doesn't have to trawl git logs, it might be worth a comment
as to the purpose of struct kvm_ksym (and thus why we never need to
actually define it).

Either way:

Reviewed-by: Mark Rutland <mark.rutland@....com>

Mark.