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Message-ID: <624d1add-c421-4782-9cbc-5cc8f1c0ce51@arm.com>
Date: Mon, 19 May 2025 18:56:31 +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 v8 22/43] KVM: arm64: Validate register access for a Realm
VM
On 16/04/2025 14:41, 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.
>
> Signed-off-by: Steven Price <steven.price@....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 | 40 ++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 40 insertions(+)
>
> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> index 2196979a24a3..ff0306650b39 100644
> --- a/arch/arm64/kvm/guest.c
> +++ b/arch/arm64/kvm/guest.c
> @@ -73,6 +73,24 @@ 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.
> + */
> + switch (off) {
> + case KVM_REG_ARM_CORE_REG(regs.regs[0]) ...
> + KVM_REG_ARM_CORE_REG(regs.regs[30]):
May be add :
/* PC can only be set before the Realm is ACTIVATED */
Either ways:
Reviewed-by: Suzuki K Poulose <suzuki.poulose@....com>
> + 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 +801,34 @@ 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:
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