linux-kernel - Re: [PATCH v5 23/43] KVM: arm64: Validate register access for a Realm VM

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Message-ID: <de0ebef7-8be3-444f-99ee-5e9b6f9140f7@arm.com>
Date: Thu, 17 Oct 2024 16:32:12 +0100
From: Suzuki K Poulose <suzuki.poulose@....com>
To: Steven Price <steven.price@....com>, kvm@...r.kernel.org,
 kvmarm@...ts.linux.dev
Cc: Catalin Marinas <catalin.marinas@....com>, Marc Zyngier <maz@...nel.org>,
 Will Deacon <will@...nel.org>, James Morse <james.morse@....com>,
 Oliver Upton <oliver.upton@...ux.dev>, Zenghui Yu <yuzenghui@...wei.com>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Joey Gouly <joey.gouly@....com>, Alexandru Elisei
 <alexandru.elisei@....com>, Christoffer Dall <christoffer.dall@....com>,
 Fuad Tabba <tabba@...gle.com>, linux-coco@...ts.linux.dev,
 Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>,
 Gavin Shan <gshan@...hat.com>, Shanker Donthineni <sdonthineni@...dia.com>,
 Alper Gun <alpergun@...gle.com>, "Aneesh Kumar K . V"
 <aneesh.kumar@...nel.org>
Subject: Re: [PATCH v5 23/43] KVM: arm64: Validate register access for a Realm
 VM

Hi Steven

On 04/10/2024 16:27, Steven Price wrote:
> The RMM only allows setting the lower GPRS (x0-x7) and PC for a realm
> guest. Check this in kvm_arm_set_reg() so that the VMM can receive a
> suitable error return if other registers are accessed.
> 
> Signed-off-by: Steven Price <steven.price@....com>
> ---
>   arch/arm64/kvm/guest.c | 26 ++++++++++++++++++++++++++
>   1 file changed, 26 insertions(+)
> 
> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> index 962f985977c2..c23b9480ceb0 100644
> --- a/arch/arm64/kvm/guest.c
> +++ b/arch/arm64/kvm/guest.c
> @@ -783,12 +783,38 @@ int kvm_arm_get_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>   	return kvm_arm_sys_reg_get_reg(vcpu, reg);
>   }
>   
> +/*
> + * The RMI ABI only enables setting the lower GPRs (x0-x7) and PC.

This is true only for REC_CREATE ? But when we handle SMCCC calls in the
userspace, we may need to allow setting x0-x17 and we should accommodate
for that here ?

Otherwise looks good to me.

Suzuki

> + * All other registers are reset to architectural or otherwise defined reset
> + * values by the RMM, except for a few configuration fields that correspond to
> + * Realm parameters.
> + */
> +static bool validate_realm_set_reg(struct kvm_vcpu *vcpu,
> +				   const struct kvm_one_reg *reg)
> +{
> +	if ((reg->id & KVM_REG_ARM_COPROC_MASK) == KVM_REG_ARM_CORE) {
> +		u64 off = core_reg_offset_from_id(reg->id);
> +
> +		switch (off) {
> +		case KVM_REG_ARM_CORE_REG(regs.regs[0]) ...
> +		     KVM_REG_ARM_CORE_REG(regs.regs[7]):

> +		case KVM_REG_ARM_CORE_REG(regs.pc):
> +			return true;
> +		}
> +	}
> +
> +	return false;
> +}
> +
>   int kvm_arm_set_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>   {
>   	/* We currently use nothing arch-specific in upper 32 bits */
>   	if ((reg->id & ~KVM_REG_SIZE_MASK) >> 32 != KVM_REG_ARM64 >> 32)
>   		return -EINVAL;
>   
> +	if (kvm_is_realm(vcpu->kvm) && !validate_realm_set_reg(vcpu, reg))
> +		return -EINVAL;
> +
>   	switch (reg->id & KVM_REG_ARM_COPROC_MASK) {
>   	case KVM_REG_ARM_CORE:	return set_core_reg(vcpu, reg);
>   	case KVM_REG_ARM_FW: