Re: [PATCH 1/2] KVM: arm64: PSCI: Narrow input registers when using 32bit functions

From: Alexandru Elisei <alexandru.elisei@arm.com>
To: Marc Zyngier <maz@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org
Subject: Re: [PATCH 1/2] KVM: arm64: PSCI: Narrow input registers when using 32bit functions
Date: Fri, 3 Apr 2020 15:02:28 +0100	[thread overview]
Message-ID: <a7b7d9d7-120f-2c33-b803-64b999b5a839@arm.com> (raw)
In-Reply-To: <20200401165816.530281-2-maz@kernel.org>

Hi,

On 4/1/20 5:58 PM, Marc Zyngier wrote:
> When a guest delibarately uses an SSMC32 function number (which is allowed),

s/SSMC32/SMC32

> we should make sure we drop the top 32bits from the input arguments, as they
> could legitimately be junk.
>
> Reported-by: Christoffer Dall <christoffer.dall@arm.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  virt/kvm/arm/psci.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
>
> diff --git a/virt/kvm/arm/psci.c b/virt/kvm/arm/psci.c
> index 17e2bdd4b76f..69ff4a51ceb5 100644
> --- a/virt/kvm/arm/psci.c
> +++ b/virt/kvm/arm/psci.c
> @@ -187,6 +187,18 @@ static void kvm_psci_system_reset(struct kvm_vcpu *vcpu)
>  	kvm_prepare_system_event(vcpu, KVM_SYSTEM_EVENT_RESET);
>  }
>  
> +static void kvm_psci_narrow_to_32bit(struct kvm_vcpu *vcpu)
> +{
> +	int i;
> +
> +	/*
> +	 * Zero the input registers' upper 32 bits. They will be fully
> +	 * zeroed on exit, so we're fine changing them in place.
> +	 */
> +	for (i = 1; i < 4; i++)
> +		vcpu_set_reg(vcpu, i, (u32)vcpu_get_reg(vcpu, i));

One minor suggestion, it could be lower_32_bits instead, but that's down to
personal preference and entirely up to you.

> +}
> +
>  static int kvm_psci_0_2_call(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm *kvm = vcpu->kvm;
> @@ -211,12 +223,16 @@ static int kvm_psci_0_2_call(struct kvm_vcpu *vcpu)
>  		val = PSCI_RET_SUCCESS;
>  		break;
>  	case PSCI_0_2_FN_CPU_ON:
> +		kvm_psci_narrow_to_32bit(vcpu);
> +		fallthrough;
>  	case PSCI_0_2_FN64_CPU_ON:
>  		mutex_lock(&kvm->lock);
>  		val = kvm_psci_vcpu_on(vcpu);
>  		mutex_unlock(&kvm->lock);
>  		break;
>  	case PSCI_0_2_FN_AFFINITY_INFO:
> +		kvm_psci_narrow_to_32bit(vcpu);
> +		fallthrough;
>  	case PSCI_0_2_FN64_AFFINITY_INFO:
>  		val = kvm_psci_vcpu_affinity_info(vcpu);
>  		break;

From ARM DEN 0022D, those are indeed the only functions with ids that differ from
SMC32 to SMC64, and have arguments that KVM doesn't ignore (like it does with
CPU_SUSPEND).

I also had a look at smccc_get_arg{1,2,3}, because they read the register values
and return an unsigned long. smccc_get_arg1 is called after the registers have
been narrowed, or the result is cast into an u32 when called before that.
smccc_get_arg{2,3} are always called as part of the individual PSCI function
implementations, which come after the arguments have been narrowed. With that:

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

Thanks,
Alex
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