[PATCH 5/5] KVM: arm64: Disable privileged hypercalls after pKVM finalisation

Thu Sep 23 05:56:21 PDT 2021

On Thu, 23 Sep 2021 12:22:56 +0100,
Will Deacon <will at kernel.org> wrote:
> 
> After pKVM has been 'finalised' using the __pkvm_prot_finalize hypercall,
> the calling CPU will have a Stage-2 translation enabled to prevent access
> to memory pages owned by EL2.
> 
> Although this forms a significant part of the process to deprivilege the
> host kernel, we also need to ensure that the hypercall interface is
> reduced so that the EL2 code cannot, for example, be re-initialised using
> a new set of vectors.
> 
> Re-order the hypercalls so that only a suffix remains available after
> finalisation of pKVM.
> 
> Cc: Marc Zyngier <maz at kernel.org>
> Cc: Quentin Perret <qperret at google.com>
> Signed-off-by: Will Deacon <will at kernel.org>
> ---
>  arch/arm64/include/asm/kvm_asm.h   | 43 ++++++++++++++++--------------
>  arch/arm64/kvm/hyp/nvhe/hyp-main.c | 26 +++++++++++-------
>  2 files changed, 39 insertions(+), 30 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> index e86045ac43ba..68630fd382c5 100644
> --- a/arch/arm64/include/asm/kvm_asm.h
> +++ b/arch/arm64/include/asm/kvm_asm.h
> @@ -43,27 +43,30 @@
>  
>  #define KVM_HOST_SMCCC_FUNC(name) KVM_HOST_SMCCC_ID(__KVM_HOST_SMCCC_FUNC_##name)
>  
> +/* Hypercalls available only prior to pKVM finalisation */
>  #define __KVM_HOST_SMCCC_FUNC___kvm_hyp_init			0
> -#define __KVM_HOST_SMCCC_FUNC___kvm_vcpu_run			1
> -#define __KVM_HOST_SMCCC_FUNC___kvm_flush_vm_context		2
> -#define __KVM_HOST_SMCCC_FUNC___kvm_tlb_flush_vmid_ipa		3
> -#define __KVM_HOST_SMCCC_FUNC___kvm_tlb_flush_vmid		4
> -#define __KVM_HOST_SMCCC_FUNC___kvm_flush_cpu_context		5
> -#define __KVM_HOST_SMCCC_FUNC___kvm_timer_set_cntvoff		6
> -#define __KVM_HOST_SMCCC_FUNC___kvm_enable_ssbs			7
> -#define __KVM_HOST_SMCCC_FUNC___vgic_v3_get_gic_config		8
> -#define __KVM_HOST_SMCCC_FUNC___vgic_v3_read_vmcr		9
> -#define __KVM_HOST_SMCCC_FUNC___vgic_v3_write_vmcr		10
> -#define __KVM_HOST_SMCCC_FUNC___vgic_v3_init_lrs		11
> -#define __KVM_HOST_SMCCC_FUNC___kvm_get_mdcr_el2		12
> -#define __KVM_HOST_SMCCC_FUNC___vgic_v3_save_aprs		13
> -#define __KVM_HOST_SMCCC_FUNC___vgic_v3_restore_aprs		14
> -#define __KVM_HOST_SMCCC_FUNC___pkvm_init			15
> -#define __KVM_HOST_SMCCC_FUNC___pkvm_host_share_hyp		16
> -#define __KVM_HOST_SMCCC_FUNC___pkvm_create_private_mapping	17
> -#define __KVM_HOST_SMCCC_FUNC___pkvm_cpu_set_vector		18
> -#define __KVM_HOST_SMCCC_FUNC___pkvm_prot_finalize		19
> -#define __KVM_HOST_SMCCC_FUNC___kvm_adjust_pc			20
> +#define __KVM_HOST_SMCCC_FUNC___kvm_get_mdcr_el2		1
> +#define __KVM_HOST_SMCCC_FUNC___pkvm_init			2
> +#define __KVM_HOST_SMCCC_FUNC___pkvm_create_private_mapping	3
> +#define __KVM_HOST_SMCCC_FUNC___pkvm_cpu_set_vector		4
> +#define __KVM_HOST_SMCCC_FUNC___kvm_enable_ssbs			5
> +#define __KVM_HOST_SMCCC_FUNC___vgic_v3_init_lrs		6
> +#define __KVM_HOST_SMCCC_FUNC___vgic_v3_get_gic_config		7
> +#define __KVM_HOST_SMCCC_FUNC___pkvm_prot_finalize		8
> +
> +/* Hypercalls available after pKVM finalisation */
> +#define __KVM_HOST_SMCCC_FUNC___pkvm_host_share_hyp		9
> +#define __KVM_HOST_SMCCC_FUNC___kvm_adjust_pc			10
> +#define __KVM_HOST_SMCCC_FUNC___kvm_vcpu_run			11
> +#define __KVM_HOST_SMCCC_FUNC___kvm_flush_vm_context		12
> +#define __KVM_HOST_SMCCC_FUNC___kvm_tlb_flush_vmid_ipa		13
> +#define __KVM_HOST_SMCCC_FUNC___kvm_tlb_flush_vmid		14
> +#define __KVM_HOST_SMCCC_FUNC___kvm_flush_cpu_context		15
> +#define __KVM_HOST_SMCCC_FUNC___kvm_timer_set_cntvoff		16
> +#define __KVM_HOST_SMCCC_FUNC___vgic_v3_read_vmcr		17
> +#define __KVM_HOST_SMCCC_FUNC___vgic_v3_write_vmcr		18
> +#define __KVM_HOST_SMCCC_FUNC___vgic_v3_save_aprs		19
> +#define __KVM_HOST_SMCCC_FUNC___vgic_v3_restore_aprs		20
>  
>  #ifndef __ASSEMBLY__
>  
> diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> index 2da6aa8da868..4120e34288e1 100644
> --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> @@ -165,36 +165,42 @@ typedef void (*hcall_t)(struct kvm_cpu_context *);
>  #define HANDLE_FUNC(x)	[__KVM_HOST_SMCCC_FUNC_##x] = (hcall_t)handle_##x
>  
>  static const hcall_t host_hcall[] = {
> -	HANDLE_FUNC(__kvm_vcpu_run),
> +	/* ___kvm_hyp_init */
> +	HANDLE_FUNC(__kvm_get_mdcr_el2),
> +	HANDLE_FUNC(__pkvm_init),
> +	HANDLE_FUNC(__pkvm_create_private_mapping),
> +	HANDLE_FUNC(__pkvm_cpu_set_vector),
> +	HANDLE_FUNC(__kvm_enable_ssbs),
> +	HANDLE_FUNC(__vgic_v3_init_lrs),
> +	HANDLE_FUNC(__pkvm_prot_finalize),
> +
> +	HANDLE_FUNC(__pkvm_host_share_hyp),
>  	HANDLE_FUNC(__kvm_adjust_pc),
> +	HANDLE_FUNC(__kvm_vcpu_run),
>  	HANDLE_FUNC(__kvm_flush_vm_context),
>  	HANDLE_FUNC(__kvm_tlb_flush_vmid_ipa),
>  	HANDLE_FUNC(__kvm_tlb_flush_vmid),
>  	HANDLE_FUNC(__kvm_flush_cpu_context),
>  	HANDLE_FUNC(__kvm_timer_set_cntvoff),
> -	HANDLE_FUNC(__kvm_enable_ssbs),
>  	HANDLE_FUNC(__vgic_v3_get_gic_config),
>  	HANDLE_FUNC(__vgic_v3_read_vmcr),
>  	HANDLE_FUNC(__vgic_v3_write_vmcr),
> -	HANDLE_FUNC(__vgic_v3_init_lrs),
> -	HANDLE_FUNC(__kvm_get_mdcr_el2),
>  	HANDLE_FUNC(__vgic_v3_save_aprs),
>  	HANDLE_FUNC(__vgic_v3_restore_aprs),
> -	HANDLE_FUNC(__pkvm_init),
> -	HANDLE_FUNC(__pkvm_cpu_set_vector),
> -	HANDLE_FUNC(__pkvm_host_share_hyp),
> -	HANDLE_FUNC(__pkvm_create_private_mapping),
> -	HANDLE_FUNC(__pkvm_prot_finalize),
>  };
>  
>  static void handle_host_hcall(struct kvm_cpu_context *host_ctxt)
>  {
>  	DECLARE_REG(unsigned long, id, host_ctxt, 0);
> +	unsigned long hcall_min = 0;
>  	hcall_t hfn;
>  
> +	if (static_branch_unlikely(&kvm_protected_mode_initialized))
> +		hcall_min = __KVM_HOST_SMCCC_FUNC___pkvm_prot_finalize;
> +
>  	id -= KVM_HOST_SMCCC_ID(0);
>  
> -	if (unlikely(id >= ARRAY_SIZE(host_hcall)))
> +	if (unlikely(id < hcall_min || id >= ARRAY_SIZE(host_hcall)))

So I can still issue a pkvm_prot_finalize after finalisation? Seems
odd. As hcall_min has to be inclusive, you probably want it to be set
to __KVM_HOST_SMCCC_FUNC___pkvm_host_share_hyp once protected.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.