linux-kernel - Re: [PATCH 10/11] arm64/kvm: context-switch ptrauth registers

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Message-ID: <20170801143227.GC11534@arm.com>
Date:   Tue, 1 Aug 2017 15:32:27 +0100
From:   Will Deacon <will.deacon@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Christoffer Dall <cdall@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, arnd@...db.de,
        catalin.marinas@....com, christoffer.dall@...aro.org,
        Dave.Martin@....com, jiong.wang@....com,
        kvmarm@...ts.cs.columbia.edu, linux-arch@...r.kernel.org,
        marc.zyngier@....com, suzuki.poulose@....com, yao.qi@....com,
        linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH 10/11] arm64/kvm: context-switch ptrauth registers

On Tue, Aug 01, 2017 at 03:26:07PM +0100, Mark Rutland wrote:
> On Tue, Aug 01, 2017 at 01:00:14PM +0200, Christoffer Dall wrote:
> > On Wed, Jul 19, 2017 at 05:01:31PM +0100, Mark Rutland wrote:
> > > When pointer authentication is supported, a guest may wish to use it.
> > > This patch adds the necessary KVM infrastructure for this to work, with
> > > a semi-lazy context switch of the pointer auth state.
> > > 
> > > When we schedule a vcpu, we disable guest usage of pointer
> > > authentication instructions and accesses to the keys. While these are
> > > disabled, we avoid context-switching the keys. When we trap the guest
> > > trying to use pointer authentication functionality, we change to eagerly
> > > context-switching the keys, and enable the feature. The next time the
> > > vcpu is scheduled out/in, we start again.
> > > 
> > > This is sufficient for systems with uniform pointer authentication
> > > support. For systems with mismatched support, it will be necessary to
> > 
> > What is mismatched support?  You mean systems where one CPU has ptrauth
> > and another one doesn't (or if they both have it but in different ways)?
> 
> Both! Any case where the support is not uniform across all CPUs.
> 
> A CPU can implement address auth and/or generic auth, and either may use
> an architected algorithm or an IMP DEF algorithm.
> 
> Even if all CPUs report an IMP DEF algorithm, the particular algorithm
> may differ across CPUs.

I know you don't like it, but I think we should resort to MIDR at that point
because otherwise IMP DEF algorithms will never be used by Linux and people
will complain.

Will