linux-kernel - Re: [PATCH] KVM: arm64: Use different pointer authentication keys for pKVM

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Message-ID: <ZHSJ38WATzgJF7SR@google.com>
Date:   Mon, 29 May 2023 11:17:51 +0000
From:   Mostafa Saleh <smostafa@...gle.com>
To:     Oliver Upton <oliver.upton@...ux.dev>
Cc:     maz@...nel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        tabba@...gle.com, kaleshsingh@...gle.com, will@...nel.org,
        catalin.marinas@....com, yuzenghui@...wei.com,
        suzuki.poulose@....com, james.morse@....com
Subject: Re: [PATCH] KVM: arm64: Use different pointer authentication keys
 for pKVM

Hi Oliver,

Thanks for reviewing the patch.

On Fri, May 26, 2023 at 08:47:52PM +0000, Oliver Upton wrote:
> On Tue, May 16, 2023 at 02:15:31PM +0000, Mostafa Saleh wrote:
> > When the kernel is compiled with CONFIG_ARM64_PTR_AUTH_KERNEL, it
> > uses Armv8.3-Pauth for return address protection for the kernel code
> > including nvhe code in EL2.
> > 
> > Same keys are used in both kernel(EL1) and nvhe code(EL2), this is
> > fine for nvhe but not when running in protected mode(pKVM) as the host
> > can't be trusted.
> 
> But we trust it enough to hand pKVM a fresh set of keys before firing
> off? I understand there is some degree of initialization required to get
> pKVM off the ground, but I question in this case if key handoff is
> strictly necessary.
>
> There are potentially other sources of random directly available at EL2,
> such as the SMCCC TRNG ABI or FEAT_RNG. Should pKVM prefer one of these
> random implementations and only fall back to host-provided keys if
> absolutely necessary?
> 
According to my understanding, the kernel is still completely trusted at
this point (it sets the initial page table for the hypervisor), so I
believe it should be fine to trust it for ptrauth keys. However, I agree,
it would be better if the hypervisor can get its own keys through
firmware/hardware if supported. I will add this in V2.

> > The keys for the hypervisor are generated from the kernel before it
> > de-privileges, each cpu has different keys, this relies on nvhe code
> > not being migratable while running.
> > 
> > This patch adds host/hyp save/restore for the keys.
> > For guest/hyp, they are already handled in common kvm code in
> > __guest_enter, where they are saved/restored if they are not
> > trapped.
> 
> Try to avoid "this patch" or any self-referential language in the
> changelog. Just directly state what the patch does:
> 
>   Similar to guest entry/exit, start context switching the pointer
>
I will update it in V2.

Thanks,
Mostafa