Re: [PATCH] KVM: arm64: Fully zero the vcpu state on reset

From: Alexandru Elisei <alexandru.elisei@arm.com>
To: Marc Zyngier <maz@kernel.org>,
	linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org,
	kvmarm@lists.cs.columbia.edu
Cc: James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	kernel-team@android.com, stable@vger.kernel.org
Subject: Re: [PATCH] KVM: arm64: Fully zero the vcpu state on reset
Date: Thu, 8 Apr 2021 16:36:40 +0100	[thread overview]
Message-ID: <c34842b2-215a-e9d0-7d9d-a8cd64ed1171@arm.com> (raw)
In-Reply-To: <20210407181308.2265808-1-maz@kernel.org>

Hi Marc,

On 4/7/21 7:13 PM, Marc Zyngier wrote:
> On vcpu reset, we expect all the registers to be brought back
> to their initial state, which happens to be a bunch of zeroes.
>
> However, some recent commit broke this, and is now leaving a bunch
> of registers (such as a FP state) with whatever was left by the
> guest. My bad.
>
> Just zero the whole vcpu context on reset. It is more than we
> strictly need, but at least we won't miss anything. This also
> zeroes the __hyp_running_vcpu pointer, which is always NULL
> for a vcpu anyway.

Had a look at struct kvm_cpu_context and indeed the only field which doesn't
represent a guest register is __hyp_running_vcpu. Did a grep for all the places
where __hyp_running_vcpu is used, and indeed the assumption is that for a guest
the pointer is NULL, as __sysreg_restore_el1_state() relies on it.

>
> Cc: stable@vger.kernel.org
> Fixes: e47c2055c68e ("KVM: arm64: Make struct kvm_regs userspace-only")
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/kvm/reset.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> index bd354cd45d28..ef1c49a1a3ad 100644
> --- a/arch/arm64/kvm/reset.c
> +++ b/arch/arm64/kvm/reset.c
> @@ -240,8 +240,8 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
>  		break;
>  	}
>  
> -	/* Reset core registers */
> -	memset(vcpu_gp_regs(vcpu), 0, sizeof(*vcpu_gp_regs(vcpu)));
> +	/* Zero all registers */
> +	memset(&vcpu->arch.ctxt, 0, sizeof(vcpu->arch.ctxt));

Checked that code earlier in the function does not touch the guest registers from
vcpu->arch.ctxt, to make sure we're not overwriting other reset values by mistake.
Looks good to me:

Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>

Thanks,

Alex

>  	vcpu_gp_regs(vcpu)->pstate = pstate;
>  
>  	/* Reset system registers */