Re: [RFC PATCH v4 13/14] KVM: arm64: Handle guest_memfd()-backed guest page faults

Patrick Roy
2025-01-16T14:48:41Z

On Fri, 2024-12-13 at 16:48 +0000, Fuad Tabba wrote:
> Add arm64 support for resolving guest page faults on
> guest_memfd() backed memslots. This support is not contingent on
> pKVM, or other confidential computing support, and works in both
> VHE and nVHE modes.
>
> Without confidential computing, this support is useful forQ
> testing and debugging. In the future, it might also be useful
> should a user want to use guest_memfd() for all code, whether
> it's for a protected guest or not.
>
> For now, the fault granule is restricted to PAGE_SIZE.
>
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
> arch/arm64/kvm/mmu.c | 111 ++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 109 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 342a9bd3848f..1c4b3871967c 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1434,6 +1434,107 @@ static bool kvm_vma_mte_allowed(struct vm_area_struct *vma)
> return vma->vm_flags & VM_MTE_ALLOWED;
> }
>
> +static int guest_memfd_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> + struct kvm_memory_slot *memslot, bool fault_is_perm)
> +{
> + struct kvm_mmu_memory_cache *memcache = &vcpu->arch.mmu_page_cache;
> + bool exec_fault = kvm_vcpu_trap_is_exec_fault(vcpu);
> + bool logging_active = memslot_is_logging(memslot);
> + struct kvm_pgtable *pgt = vcpu->arch.hw_mmu->pgt;
> + enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R;
> + bool write_fault = kvm_is_write_fault(vcpu);
> + struct mm_struct *mm = current->mm;
> + gfn_t gfn = gpa_to_gfn(fault_ipa);
> + struct kvm *kvm = vcpu->kvm;
> + struct page *page;
> + kvm_pfn_t pfn;
> + int ret;
> +
> + /* For now, guest_memfd() only supports PAGE_SIZE granules. */
> + if (WARN_ON_ONCE(fault_is_perm &&
> + kvm_vcpu_trap_get_perm_fault_granule(vcpu) != PAGE_SIZE)) {
> + return -EFAULT;
> + }
> +
> + VM_BUG_ON(write_fault && exec_fault);
> +
> + if (fault_is_perm && !write_fault && !exec_fault) {
> + kvm_err("Unexpected L2 read permission error\n");
> + return -EFAULT;
> + }
> +
> + /*
> + * Permission faults just need to update the existing leaf entry,
> + * and so normally don't require allocations from the memcache. The
> + * only exception to this is when dirty logging is enabled at runtime
> + * and a write fault needs to collapse a block entry into a table.
> + */
> + if (!fault_is_perm || (logging_active && write_fault)) {
> + ret = kvm_mmu_topup_memory_cache(memcache,
> + kvm_mmu_cache_min_pages(vcpu->arch.hw_mmu));
> + if (ret)
> + return ret;
> + }
> +
> + /*
> + * Holds the folio lock until mapped in the guest and its refcount is
> + * stable, to avoid races with paths that check if the folio is mapped
> + * by the host.
> + */
> + ret = kvm_gmem_get_pfn_locked(kvm, memslot, gfn, &pfn, &page, NULL);
> + if (ret)
> + return ret;
> +
> + if (!kvm_slot_gmem_is_guest_mappable(memslot, gfn)) {
> + ret = -EAGAIN;
> + goto unlock_page;
> + }
> +
> + /*
> + * Once it's faulted in, a guest_memfd() page will stay in memory.
> + * Therefore, count it as locked.
> + */
> + if (!fault_is_perm) {
> + ret = account_locked_vm(mm, 1, true);
> + if (ret)
> + goto unlock_page;
> + }
> +
> + read_lock(&kvm->mmu_lock);
> + if (write_fault)
> + prot |= KVM_PGTABLE_PROT_W;
> +
> + if (exec_fault)
> + prot |= KVM_PGTABLE_PROT_X;
> +
> + if (cpus_have_final_cap(ARM64_HAS_CACHE_DIC))
> + prot |= KVM_PGTABLE_PROT_X;
> +
> + /*
> + * Under the premise of getting a FSC_PERM fault, we just need to relax
> + * permissions.
> + */
> + if (fault_is_perm)
> + ret = kvm_pgtable_stage2_relax_perms(pgt, fault_ipa, prot);
> + else
> + ret = kvm_pgtable_stage2_map(pgt, fault_ipa, PAGE_SIZE,
> + __pfn_to_phys(pfn), prot,
> + memcache,
> + KVM_PGTABLE_WALK_HANDLE_FAULT |
> + KVM_PGTABLE_WALK_SHARED);
> +
> + kvm_release_faultin_page(kvm, page, !!ret, write_fault);
> + read_unlock(&kvm->mmu_lock);
> +
> + if (ret && !fault_is_perm)
> + account_locked_vm(mm, 1, false);
> +unlock_page:
> + unlock_page(page);
> + put_page(page);

There's a double-free of `page` here, as kvm_release_faultin_page
already calls put_page. I fixed it up locally with

+ unlock_page(page);
kvm_release_faultin_page(kvm, page, !!ret, write_fault);
read_unlock(&kvm->mmu_lock);

if (ret && !fault_is_perm)
account_locked_vm(mm, 1, false);
+ goto out;
+
unlock_page:
unlock_page(page);
put_page(page);
-
+out:
return ret != -EAGAIN ? ret : 0;
}

which I'm admittedly not sure is correct either because now the locks
don't get released in reverse order of acquisition, but with this I
was able to boot simple VMs.

> +
> + return ret != -EAGAIN ? ret : 0;
> +}
> +
> static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> struct kvm_s2_trans *nested,
> struct kvm_memory_slot *memslot, unsigned long hva,
> @@ -1900,8 +2001,14 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu)
> goto out_unlock;
> }
>
> - ret = user_mem_abort(vcpu, fault_ipa, nested, memslot, hva,
> - esr_fsc_is_permission_fault(esr));
> + if (kvm_slot_can_be_private(memslot)) {

For my setup, I needed

if (kvm_mem_is_private(vcpu->kvm, gfn))

here instead, because I am making use of KVM_GENERIC_MEMORY_ATTRIBUTES,
and had a memslot with the `KVM_MEM_GUEST_MEMFD` flag set, but whose
gfn range wasn't actually set to KVM_MEMORY_ATTRIBUTE_PRIVATE.

If I'm reading patch 12 correctly, your memslots always set only one of
userspace_addr or guest_memfd, and the stage 2 table setup simply checks
which one is the case to decide what to fault in, so maybe to support
both cases, this check should be

if (kvm_mem_is_private(vcpu->kvm, gfn) || (kvm_slot_can_be_private(memslot) && !memslot->userspace_addr)

?

[1]: https://lore.kernel.org/all/20240801090117.3841080-1-tabba@google.com/

> + ret = guest_memfd_abort(vcpu, fault_ipa, memslot,
> + esr_fsc_is_permission_fault(esr));
> + } else {
> + ret = user_mem_abort(vcpu, fault_ipa, nested, memslot, hva,
> + esr_fsc_is_permission_fault(esr));
> + }
> +
> if (ret == 0)
> ret = 1;
> out:
> --
> 2.47.1.613.gc27f4b7a9f-goog

Best,
Patrick