[PATCH v9 22/43] KVM: arm64: Validate register access for a Realm VM

[Joey Gouly]

On Wed, Jun 11, 2025 at 11:48:19AM +0100, Steven Price wrote:
> The RMM only allows setting the GPRS (x0-x30) 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 written to.
> 
> The RMM makes similar restrictions for reading of the guest's registers
> (this is *confidential* compute after all), however we don't impose the
> restriction here. This allows the VMM to read (stale) values from the
> registers which might be useful to read back the initial values even if
> the RMM doesn't provide the latest version. For migration of a realm VM,
> a new interface will be needed so that the VMM can receive an
> (encrypted) blob of the VM's state.
> 
> Reviewed-by: Gavin Shan <gshan at redhat.com>
> Reviewed-by: Suzuki K Poulose <suzuki.poulose at arm.com>
> Signed-off-by: Steven Price <steven.price at arm.com>

Reviewed-by: Joey Gouly <joey.gouly at arm.com>

> ---
> Changes since v5:
>  * Upper GPRS can be set as part of a HOST_CALL return, so fix up the
>    test to allow them.
> ---
>  arch/arm64/kvm/guest.c | 41 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 41 insertions(+)
> 
> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> index 2196979a24a3..a114b9e15eec 100644
> --- a/arch/arm64/kvm/guest.c
> +++ b/arch/arm64/kvm/guest.c
> @@ -73,6 +73,25 @@ static u64 core_reg_offset_from_id(u64 id)
>  	return id & ~(KVM_REG_ARCH_MASK | KVM_REG_SIZE_MASK | KVM_REG_ARM_CORE);
>  }
>  
> +static bool kvm_realm_validate_core_reg(u64 off)
> +{
> +	/*
> +	 * Note that GPRs can only sometimes be controlled by the VMM.
> +	 * For PSCI only X0-X6 are used, higher registers are ignored (restored
> +	 * from the REC).
> +	 * For HOST_CALL all of X0-X30 are copied to the RsiHostCall structure.
> +	 * For emulated MMIO X0 is always used.
> +	 * PC can only be set before the realm is activated.
> +	 */
> +	switch (off) {
> +	case KVM_REG_ARM_CORE_REG(regs.regs[0]) ...
> +	     KVM_REG_ARM_CORE_REG(regs.regs[30]):
> +	case KVM_REG_ARM_CORE_REG(regs.pc):
> +		return true;
> +	}
> +	return false;
> +}
> +
>  static int core_reg_size_from_offset(const struct kvm_vcpu *vcpu, u64 off)
>  {
>  	int size;
> @@ -783,12 +802,34 @@ 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 some GPRs and PC. The selection of GPRs
> + * that are available depends on the Realm state and the reason for the last
> + * exit.  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);
> +
> +		return kvm_realm_validate_core_reg(off);
> +	}
> +
> +	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:
> -- 
> 2.43.0
>