Re: [PATCH v3 16/32] KVM: arm64: Elevate hypervisor mappings creation at EL2

Will Deacon
2021-03-04T19:27:43Z

On Tue, Mar 02, 2021 at 02:59:46PM +0000, Quentin Perret wrote:
> Previous commits have introduced infrastructure to enable the EL2 code
> to manage its own stage 1 mappings. However, this was preliminary work,
> and none of it is currently in use.
>
> Put all of this together by elevating the mapping creation at EL2 when
> memory protection is enabled. In this case, the host kernel running
> at EL1 still creates _temporary_ EL2 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@google.com>
> ---
> arch/arm64/include/asm/kvm_mmu.h | 2 +-
> arch/arm64/kvm/arm.c | 87 +++++++++++++++++++++++++++++---
> arch/arm64/kvm/mmu.c | 43 ++++++++++++++--
> 3 files changed, 120 insertions(+), 12 deletions(-)

[...]

> @@ -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)
> @@ -1691,18 +1700,59 @@ static void teardown_hyp_mode(void)
> }
> }
>
> +static int do_pkvm_init(u32 hyp_va_bits)
> +{
> + void *per_cpu_base = kvm_ksym_ref(kvm_arm_hyp_percpu_base);
> + int ret;
> +
> + preempt_disable();
> + hyp_install_host_vector();

It's a shame we need this both here _and_ on the reinit path, but it looks
like it's necessary.

> + ret = kvm_call_hyp_nvhe(__pkvm_init, hyp_mem_base, hyp_mem_size,
> + num_possible_cpus(), kern_hyp_va(per_cpu_base),
> + hyp_va_bits);
> + preempt_enable();
> +
> + return ret;
> +}

[...]

> /**
> * Inits Hyp-mode on all online CPUs
> */
> static int init_hyp_mode(void)
> {
> + u32 hyp_va_bits;
> 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;

This skips the error message you get on the out_err path.

> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 4d41d7838d53..9d331bf262d2 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -221,15 +221,39 @@ 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;
> +
> + WARN_ON(!hyp_pgtable);
> +
> + return true;

return !(WARN_ON(!hyp_pgtable) && is_protected_kvm_enabled());

?

Will