Re: [PATCH v3 20/21] KVM: arm64: Restrict EL2 stage-1 changes in protected mode

Quentin Perret
2021-08-03T10:43:40Z

On Tuesday 03 Aug 2021 at 10:22:03 (+0200), Fuad Tabba wrote:
> Hi Quentin,
>
> > diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > index 0ccea58df7e0..1b67f562b6fc 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > @@ -338,6 +338,95 @@ static int host_stage2_idmap(u64 addr)
> > return ret;
> > }
> >
> > +static inline bool check_prot(enum kvm_pgtable_prot prot,
> > + enum kvm_pgtable_prot required,
> > + enum kvm_pgtable_prot denied)
> > +{
> > + return (prot & (required | denied)) == required;
> > +}
> > +
> > +int __pkvm_host_share_hyp(u64 pfn)
> > +{
> > + phys_addr_t addr = hyp_pfn_to_phys(pfn);
> > + enum kvm_pgtable_prot prot, cur;
> > + void *virt = __hyp_va(addr);
> > + enum pkvm_page_state state;
> > + kvm_pte_t pte;
> > + u32 level;
> > + int ret;
> > +
> > + if (!range_is_memory(addr, addr + PAGE_SIZE))
> > + return -EINVAL;
> > +
> > + hyp_spin_lock(&host_kvm.lock);
> > + hyp_spin_lock(&pkvm_pgd_lock);
> > +
> > + ret = kvm_pgtable_get_leaf(&host_kvm.pgt, addr, &pte, &level);
> > + if (ret)
> > + goto unlock;
> > + if (!pte)
> > + goto map_shared;
>
> Should this check whether kvm_pte_valid as well, is that guaranteed to
> always be the case, or implicitly handled later?

Yep, this is implicitly handled by kvm_pgtable_stage2_pte_prot() which
is guaranteed not to return KVM_PGTABLE_PROT_RWX for an invalid mapping.

> > +
> > + /*
> > + * Check attributes in the host stage-2 PTE. We need the page to be:
> > + * - mapped RWX as we're sharing memory;
> > + * - not borrowed, as that implies absence of ownership.
> > + * Otherwise, we can't let it got through
> > + */
> > + cur = kvm_pgtable_stage2_pte_prot(pte);
> > + prot = pkvm_mkstate(0, PKVM_PAGE_SHARED_BORROWED);
> > + if (!check_prot(cur, KVM_PGTABLE_PROT_RWX, prot)) {
> > + ret = -EPERM;
> > + goto unlock;
> > + }
> > +
> > + state = pkvm_getstate(cur);
> > + if (state == PKVM_PAGE_OWNED)
> > + goto map_shared;
> > +
> > + /*
> > + * Tolerate double-sharing the same page, but this requires
> > + * cross-checking the hypervisor stage-1.
> > + */
> > + if (state != PKVM_PAGE_SHARED_OWNED) {
> > + ret = -EPERM;
> > + goto unlock;
> > + }
> > +
> > + ret = kvm_pgtable_get_leaf(&pkvm_pgtable, (u64)virt, &pte, &level);
> > + if (ret)
> > + goto unlock;
> > +
> > + /*
> > + * If the page has been shared with the hypervisor, it must be
> > + * SHARED_BORROWED already.
> > + */
>
> This comment confused me at first, but then I realized it's referring
> to the page from the hyp's point of view. Could you add something to
> the comment to that effect?

Sure thing.

> It might also make it easier to follow if the variables could be
> annotated to specify whether cur, state, and prot are the host's or
> hyps (and not reuse the same one for both).
>
> > + cur = kvm_pgtable_hyp_pte_prot(pte);
> > + prot = pkvm_mkstate(PAGE_HYP, PKVM_PAGE_SHARED_BORROWED);
> > + if (!check_prot(cur, prot, ~prot))
> > + ret = EPERM;
> > + goto unlock;
> > +
> > +map_shared:
> > + /*
> > + * If the page is not yet shared, adjust mappings in both page-tables
> > + * while both locks are held.
> > + */
> > + prot = pkvm_mkstate(PAGE_HYP, PKVM_PAGE_SHARED_BORROWED);
> > + ret = pkvm_create_mappings_locked(virt, virt + PAGE_SIZE, prot);
> > + BUG_ON(ret);
> > +
> > + prot = pkvm_mkstate(KVM_PGTABLE_PROT_RWX, PKVM_PAGE_SHARED_OWNED);
> > + ret = host_stage2_idmap_locked(addr, addr + PAGE_SIZE, prot);
> > + BUG_ON(ret);
> > +
> > +unlock:
> > + hyp_spin_unlock(&pkvm_pgd_lock);
> > + hyp_spin_unlock(&host_kvm.lock);
> > +
> > + return ret;
> > +}
> > +
> > void handle_host_mem_abort(struct kvm_cpu_context *host_ctxt)
> > {
> > struct kvm_vcpu_fault_info fault;
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index 0625bf2353c2..cbab146cda6a 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -259,10 +259,8 @@ static int __create_hyp_mappings(unsigned long start, unsigned long size,
> > {
> > int err;
> >
> > - if (!kvm_host_owns_hyp_mappings()) {
> > - return kvm_call_hyp_nvhe(__pkvm_create_mappings,
> > - start, size, phys, prot);
> > - }
> > + if (WARN_ON(!kvm_host_owns_hyp_mappings()))
> > + return -EINVAL;
> >
> > mutex_lock(&kvm_hyp_pgd_mutex);
> > err = kvm_pgtable_hyp_map(hyp_pgtable, start, size, phys, prot);
> > @@ -282,6 +280,21 @@ static phys_addr_t kvm_kaddr_to_phys(void *kaddr)
> > }
> > }
> >
> > +static int pkvm_share_hyp(phys_addr_t start, phys_addr_t end)
> > +{
> > + phys_addr_t addr;
> > + int ret;
> > +
> > + for (addr = ALIGN_DOWN(start, PAGE_SIZE); addr < end; addr += PAGE_SIZE) {
> > + ret = kvm_call_hyp_nvhe(__pkvm_host_share_hyp,
> > + __phys_to_pfn(addr));
>
> I guess we don't expect this to happen often, but I wonder if it would
> be better to have the looping in the hyp call rather than here, to
> reduce the number of hyp calls when sharing.

Yes, I was wondering the same thing, but ended up doing the looping here
to avoid spending long periods of time in a non-preemptible state at
EL2. Probably doesn't make a big difference for now, but it might if we
ever need to share large memory regions.

Cheers,
Quentin

>
> Thanks,
> /fuad
>
> > + if (ret)
> > + return ret;
> > + }
> > +
> > + return 0;
> > +}
> > +
> > /**
> > * create_hyp_mappings - duplicate a kernel virtual address range in Hyp mode
> > * @from: The virtual kernel start address of the range
> > @@ -302,6 +315,13 @@ int create_hyp_mappings(void *from, void *to, enum kvm_pgtable_prot prot)
> > if (is_kernel_in_hyp_mode())
> > return 0;
> >
> > + if (!kvm_host_owns_hyp_mappings()) {
> > + if (WARN_ON(prot != PAGE_HYP))
> > + return -EPERM;
> > + return pkvm_share_hyp(kvm_kaddr_to_phys(from),
> > + kvm_kaddr_to_phys(to));
> > + }
> > +
> > start = start & PAGE_MASK;
> > end = PAGE_ALIGN(end);
> >
> > --
> > 2.32.0.432.gabb21c7263-goog
> >