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

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Message-ID: <20250624151031.GB111675@e124191.cambridge.arm.com>
Date: Tue, 24 Jun 2025 16:10:31 +0100
From: Joey Gouly <joey.gouly@....com>
To: Steven Price <steven.price@....com>
Cc: kvm@...r.kernel.org, kvmarm@...ts.linux.dev,
	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>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	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>,
	Emi Kisanuki <fj0570is@...itsu.com>
Subject: Re: [PATCH v9 22/43] KVM: arm64: Validate register access for a
 Realm VM

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@...hat.com>
> Reviewed-by: Suzuki K Poulose <suzuki.poulose@....com>
> Signed-off-by: Steven Price <steven.price@....com>

Reviewed-by: Joey Gouly <joey.gouly@....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
>