Re: [PATCH v6 22/43] KVM: arm64: Validate register access for a Realm VM

[Gavin Shan <gshan@redhat.com>]

On 12/13/24 1:55 AM, 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 accessed.
> 
> Signed-off-by: Steven Price <steven.price@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 | 43 ++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 43 insertions(+)
> 
> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> index 12dad841f2a5..1ee2fe072f1a 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]):
> +	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;
> @@ -115,6 +133,9 @@ static int core_reg_size_from_offset(const struct kvm_vcpu *vcpu, u64 off)
>   	if (vcpu_has_sve(vcpu) && core_reg_offset_is_vreg(off))
>   		return -EINVAL;
>   
> +	if (kvm_is_realm(vcpu->kvm) && !kvm_realm_validate_core_reg(off))
> +		return -EPERM;
> +
>   	return size;
>   }
>   
> @@ -783,12 +804,34 @@ 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:

It looks the core registers in kvm_arm_set_reg() has been validated for twice.

   ioctl(KVM_SET_ONE_REG)
     kvm_arm_set_reg
       validate_realm_set_reg
         kvm_realm_validate_core_reg        // 1
       set_core_reg
         core_reg_offset_from_id
         core_reg_addr
           core_reg_offset_from_id
           core_reg_size_from_offset
             kvm_realm_validate_core_reg    // 2
         copy_from_user

Besides, there are other types of registers that can be accessed by KVM_{GET, SET}_ONE_REG:
firmware and bitmap registers, SVE registers, timer registers, system registers. Need we
to hide any of them from the user space? As I can understand, the SVE registers are owned
by RMM at least and won't be exposed to user space.

Thanks,
Gavin