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

Fuad Tabba
2024-12-13T16:48:42Z

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 for
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);
+
+ 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)) {
+ 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