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

[Will Deacon]

On Thu, Sep 23, 2021 at 01:56:21PM +0100, Marc Zyngier wrote:
> 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.

Yeah, I ended up addresing that one in the previous patch. The problem is
that we need to allow pkvm_prot_finalize to be called on each CPU, so I
think we'd end up having an extra "really finalize damnit!" call to be
issued _once_ after each CPU is done with the finalisation if we want
to lock it down.

The approach I took instead is to make pkvm_prot_finalize return -EBUSY
if it's called on a CPU where it's already been called.

Will