Re: [PATCH v13 16/20] KVM: arm64: Handle guest_memfd-backed guest page faults

[Fuad Tabba <tabba@google.com>]

Hi Patrick,


On Fri, 11 Jul 2025 at 10:59, Roy, Patrick <roypat@amazon.co.uk> wrote:
>
>
> Hi Fuad,
>
> On Wed, 2025-07-09 at 11:59 +0100, Fuad Tabba wrote:> -snip-
> > +#define KVM_PGTABLE_WALK_MEMABORT_FLAGS (KVM_PGTABLE_WALK_HANDLE_FAULT | KVM_PGTABLE_WALK_SHARED)
> > +
> > +static int gmem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> > +                     struct kvm_s2_trans *nested,
> > +                     struct kvm_memory_slot *memslot, bool is_perm)
> > +{
> > +       bool write_fault, exec_fault, writable;
> > +       enum kvm_pgtable_walk_flags flags = KVM_PGTABLE_WALK_MEMABORT_FLAGS;
> > +       enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R;
> > +       struct kvm_pgtable *pgt = vcpu->arch.hw_mmu->pgt;
> > +       struct page *page;
> > +       struct kvm *kvm = vcpu->kvm;
> > +       void *memcache;
> > +       kvm_pfn_t pfn;
> > +       gfn_t gfn;
> > +       int ret;
> > +
> > +       ret = prepare_mmu_memcache(vcpu, true, &memcache);
> > +       if (ret)
> > +               return ret;
> > +
> > +       if (nested)
> > +               gfn = kvm_s2_trans_output(nested) >> PAGE_SHIFT;
> > +       else
> > +               gfn = fault_ipa >> PAGE_SHIFT;
> > +
> > +       write_fault = kvm_is_write_fault(vcpu);
> > +       exec_fault = kvm_vcpu_trap_is_exec_fault(vcpu);
> > +
> > +       if (write_fault && exec_fault) {
> > +               kvm_err("Simultaneous write and execution fault\n");
> > +               return -EFAULT;
> > +       }
> > +
> > +       if (is_perm && !write_fault && !exec_fault) {
> > +               kvm_err("Unexpected L2 read permission error\n");
> > +               return -EFAULT;
> > +       }
> > +
> > +       ret = kvm_gmem_get_pfn(kvm, memslot, gfn, &pfn, &page, NULL);
> > +       if (ret) {
> > +               kvm_prepare_memory_fault_exit(vcpu, fault_ipa, PAGE_SIZE,
> > +                                             write_fault, exec_fault, false);
> > +               return ret;
> > +       }
> > +
> > +       writable = !(memslot->flags & KVM_MEM_READONLY);
> > +
> > +       if (nested)
> > +               adjust_nested_fault_perms(nested, &prot, &writable);
> > +
> > +       if (writable)
> > +               prot |= KVM_PGTABLE_PROT_W;
> > +
> > +       if (exec_fault ||
> > +           (cpus_have_final_cap(ARM64_HAS_CACHE_DIC) &&
> > +            (!nested || kvm_s2_trans_executable(nested))))
> > +               prot |= KVM_PGTABLE_PROT_X;
> > +
> > +       kvm_fault_lock(kvm);
>
> Doesn't this race with gmem invalidations (e.g. fallocate(PUNCH_HOLE))?
> E.g. if between kvm_gmem_get_pfn() above and this kvm_fault_lock() a
> gmem invalidation occurs, don't we end up with stage-2 page tables
> refering to a stale host page? In user_mem_abort() there's the "grab
> mmu_invalidate_seq before dropping mmap_lock and check it hasnt changed
> after grabbing mmu_lock" which prevents this, but I don't really see an
> equivalent here.

You're right. I'll add a check for this.

Thanks for pointing this out,
/fuad

> > +       ret = KVM_PGT_FN(kvm_pgtable_stage2_map)(pgt, fault_ipa, PAGE_SIZE,
> > +                                                __pfn_to_phys(pfn), prot,
> > +                                                memcache, flags);
> > +       kvm_release_faultin_page(kvm, page, !!ret, writable);
> > +       kvm_fault_unlock(kvm);
> > +
> > +       if (writable && !ret)
> > +               mark_page_dirty_in_slot(kvm, memslot, gfn);
> > +
> > +       return ret != -EAGAIN ? ret : 0;
> > +}
> > +
> > -snip-
>
> Best,
> Patrick
>
>