Re: [PATCH] arm64: Fix compile error with KVM and !HARDEN_BRANCH_PREDICTOR

From: Corey Minyard <minyard@acm.org>
To: Marc Zyngier <maz@kernel.org>
Cc: Stefan Wahren <stefan.wahren@i2se.com>,
	Corey Minyard <cminyard@mvista.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Andre Przywara <andre.przywara@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] arm64: Fix compile error with KVM and !HARDEN_BRANCH_PREDICTOR
Date: Tue, 3 Dec 2019 13:16:08 -0600	[thread overview]
Message-ID: <20191203191608.GA2896@minyard.net> (raw)
In-Reply-To: <8274815953233fb2d522f216b36d7247@www.loen.fr>

On Tue, Dec 03, 2019 at 06:45:46PM +0000, Marc Zyngier wrote:
> On 2019-12-03 18:35, Ard Biesheuvel wrote:
> 
> [...]
> 
> > OK, I have prepared a branch that carries all these patches in the
> > right order with the upstream commit reference added to the commit
> > log. I'll send this out for review first, before actually sending it
> > to -stable, unless anyone feels that this is a bad idea.
> > 
> > arm64: KVM: Move BP hardening vectors into .hyp.text section
> > arm64: insn: Add N immediate encoding
> > arm64: insn: Add encoder for bitwise operations using literals
> > arm64: KVM: Dynamically patch the kernel/hyp VA mask
> > arm64: cpufeatures: Drop the ARM64_HYP_OFFSET_LOW feature flag
> > arm64; insn: Add encoder for the EXTR instruction
> > arm64: insn: Allow ADD/SUB (immediate) with LSL #12
> > arm64: KVM: Dynamically compute the HYP VA mask
> > arm64: KVM: Introduce EL2 VA randomisation
> > arm64: KVM: Move stashing of x0/x1 into the vector code itself
> > arm64: KVM: Reserve 4 additional instructions in the BPI template
> > arm64: KVM: Allow far branches from vector slots to the main vectors
> > arm64: Make BP hardening slot counter available
> > arm64: Move the content of bpi.S to hyp-entry.S
> 
> My personal take on this is that it is *a lot* of code that affects
> a very critical path, and I'm not completely sure that it is what
> we want for something as dated as 4.14.

That was my thought, too.  I was just trying to fix a simple compile
error.

> 
> The only valuable reason I can think of (aside of the vulnerability
> mitigation aspect) would be if we need to backport more code to
> the exception entry path for EL2. I don't know if there is such
> need yet.

If that is the case, IMHO this should be backported as part of other
things that require it.

I don't know this code that well, but these changes seem to me more
setup for allowing other things to be done.

-corey

> 
>         M.
> -- 
> Jazz is not dead. It just smells funny...

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