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

[Suzuki K Poulose <suzuki.poulose@arm.com>]

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@arm.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: