Re: [RFC PATCH v2 17/26] KVM: arm64: Elevate Hyp mappings creation at EL2

[Will Deacon]

On Fri, Jan 08, 2021 at 12:15:15PM +0000, Quentin Perret wrote:
> Previous commits have introduced infrastructure at EL2 to enable the Hyp
> code to manage its own memory, and more specifically its stage 1 page
> tables. However, this was preliminary work, and none of it is currently
> in use.
> 
> Put all of this together by elevating the hyp mappings creation at EL2
> when memory protection is enabled. In this case, the host kernel running
> at EL1 still creates _temporary_ Hyp mappings, only used while
> initializing the hypervisor, but frees them right after.
> 
> As such, all calls to create_hyp_mappings() after kvm init has finished
> turn into hypercalls, as the host now has no 'legal' way to modify the
> hypevisor page tables directly.
> 
> Signed-off-by: Quentin Perret <qperret@xxxxxxxxxx>
> ---
>  arch/arm64/include/asm/kvm_mmu.h |  1 -
>  arch/arm64/kvm/arm.c             | 62 +++++++++++++++++++++++++++++---
>  arch/arm64/kvm/mmu.c             | 34 ++++++++++++++++++
>  3 files changed, 92 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> index d7ebd73ec86f..6c8466a042a9 100644
> --- a/arch/arm64/include/asm/kvm_mmu.h
> +++ b/arch/arm64/include/asm/kvm_mmu.h
> @@ -309,6 +309,5 @@ static __always_inline void __load_guest_stage2(struct kvm_s2_mmu *mmu)
>  	 */
>  	asm(ALTERNATIVE("nop", "isb", ARM64_WORKAROUND_SPECULATIVE_AT));
>  }
> -
>  #endif /* __ASSEMBLY__ */
>  #endif /* __ARM64_KVM_MMU_H__ */
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 6af9204bcd5b..e524682c2ccf 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -1421,7 +1421,7 @@ static void cpu_prepare_hyp_mode(int cpu)
>  	kvm_flush_dcache_to_poc(params, sizeof(*params));
>  }
>  
> -static void cpu_init_hyp_mode(void)
> +static void kvm_set_hyp_vector(void)

Please do something about the naming: now we have both cpu_set_hyp_vector()
and kvm_set_hyp_vector()!

>  {
>  	struct kvm_nvhe_init_params *params;
>  	struct arm_smccc_res res;
> @@ -1439,6 +1439,11 @@ static void cpu_init_hyp_mode(void)
>  	params = this_cpu_ptr_nvhe_sym(kvm_init_params);
>  	arm_smccc_1_1_hvc(KVM_HOST_SMCCC_FUNC(__kvm_hyp_init), virt_to_phys(params), &res);
>  	WARN_ON(res.a0 != SMCCC_RET_SUCCESS);
> +}
> +
> +static void cpu_init_hyp_mode(void)
> +{
> +	kvm_set_hyp_vector();
>  
>  	/*
>  	 * Disabling SSBD on a non-VHE system requires us to enable SSBS
> @@ -1481,7 +1486,10 @@ static void cpu_set_hyp_vector(void)
>  	struct bp_hardening_data *data = this_cpu_ptr(&bp_hardening_data);
>  	void *vector = hyp_spectre_vector_selector[data->slot];
>  
> -	*this_cpu_ptr_hyp_sym(kvm_hyp_vector) = (unsigned long)vector;
> +	if (!is_protected_kvm_enabled())
> +		*this_cpu_ptr_hyp_sym(kvm_hyp_vector) = (unsigned long)vector;
> +	else
> +		kvm_call_hyp_nvhe(__pkvm_cpu_set_vector, data->slot);

*Very* minor nit, but it might be cleaner to have static inline functions
with the same prototypes as the hypercalls, just to make the code even
easier to read. e.g

	if (!is_protected_kvm_enabled())
		_cpu_set_vector(data->slot);
	else
		kvm_call_hyp_nvhe(__pkvm_cpu_set_vector, data->slot);

you could then conceivably wrap that in a macro and avoid having the
"is_protected_kvm_enabled()" checks explicit every time.

>  }
>  
>  static void cpu_hyp_reinit(void)
> @@ -1489,13 +1497,14 @@ static void cpu_hyp_reinit(void)
>  	kvm_init_host_cpu_context(&this_cpu_ptr_hyp_sym(kvm_host_data)->host_ctxt);
>  
>  	cpu_hyp_reset();
> -	cpu_set_hyp_vector();
>  
>  	if (is_kernel_in_hyp_mode())
>  		kvm_timer_init_vhe();
>  	else
>  		cpu_init_hyp_mode();
>  
> +	cpu_set_hyp_vector();
> +
>  	kvm_arm_init_debug();
>  
>  	if (vgic_present)
> @@ -1714,13 +1723,52 @@ static int copy_cpu_ftr_regs(void)
>  	return 0;
>  }
>  
> +static int kvm_hyp_enable_protection(void)
> +{
> +	void *per_cpu_base = kvm_ksym_ref(kvm_arm_hyp_percpu_base);
> +	int ret, cpu;
> +	void *addr;
> +
> +	if (!is_protected_kvm_enabled())
> +		return 0;

Maybe I'm hung up on my previous suggestion, but I feel like we shouldn't
get here if protected kvm isn't enabled.

> +	if (!hyp_mem_base)
> +		return -ENOMEM;
> +
> +	addr = phys_to_virt(hyp_mem_base);
> +	ret = create_hyp_mappings(addr, addr + hyp_mem_size - 1, PAGE_HYP);
> +	if (ret)
> +		return ret;
> +
> +	preempt_disable();
> +	kvm_set_hyp_vector();
> +	ret = kvm_call_hyp_nvhe(__pkvm_init, hyp_mem_base, hyp_mem_size,
> +				num_possible_cpus(), kern_hyp_va(per_cpu_base));

Would it make sense for the __pkvm_init() hypercall to set the vector as
well, so that we wouldn't need to disable preemption over two hypercalls?

Failing that, maybe move the whole preempt_disable/enable sequence into
another function.

> +	preempt_enable();
> +	if (ret)
> +		return ret;
> +
> +	free_hyp_pgds();
> +	for_each_possible_cpu(cpu)
> +		free_page(per_cpu(kvm_arm_hyp_stack_page, cpu));
> +
> +	return 0;
> +}
> +
>  /**
>   * Inits Hyp-mode on all online CPUs
>   */
>  static int init_hyp_mode(void)
>  {
>  	int cpu;
> -	int err = 0;
> +	int err = -ENOMEM;
> +
> +	/*
> +	 * The protected Hyp-mode cannot be initialized if the memory pool
> +	 * allocation has failed.
> +	 */
> +	if (is_protected_kvm_enabled() && !hyp_mem_base)
> +		return err;
>  
>  	/*
>  	 * Copy the required CPU feature register in their EL2 counterpart
> @@ -1854,6 +1902,12 @@ static int init_hyp_mode(void)
>  	for_each_possible_cpu(cpu)
>  		cpu_prepare_hyp_mode(cpu);
>  
> +	err = kvm_hyp_enable_protection();
> +	if (err) {
> +		kvm_err("Failed to enable hyp memory protection: %d\n", err);
> +		goto out_err;
> +	}
> +
>  	return 0;
>  
>  out_err:
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 3cf9397dabdb..9d4c9251208e 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -225,15 +225,39 @@ void free_hyp_pgds(void)
>  	if (hyp_pgtable) {
>  		kvm_pgtable_hyp_destroy(hyp_pgtable);
>  		kfree(hyp_pgtable);
> +		hyp_pgtable = NULL;
>  	}
>  	mutex_unlock(&kvm_hyp_pgd_mutex);
>  }
>  
> +static bool kvm_host_owns_hyp_mappings(void)
> +{
> +	if (static_branch_likely(&kvm_protected_mode_initialized))
> +		return false;
> +
> +	/*
> +	 * This can happen at boot time when __create_hyp_mappings() is called
> +	 * after the hyp protection has been enabled, but the static key has
> +	 * not been flipped yet.
> +	 */
> +	if (!hyp_pgtable && is_protected_kvm_enabled())
> +		return false;
> +
> +	BUG_ON(!hyp_pgtable);

Can we fail more gracefully, e.g. by continuing without KVM?

Will