Re: [PATCH v8 8/8] KVM: Enable and expose KVM_MEM_PRIVATE

[Fuad Tabba]

Hi,

On Thu, Sep 15, 2022 at 3:37 PM Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> wrote:
>
> Expose KVM_MEM_PRIVATE and memslot fields private_fd/offset to
> userspace. KVM will register/unregister private memslot to fd-based
> memory backing store and response to invalidation event from
> inaccessible_notifier to zap the existing memory mappings in the
> secondary page table.
>
> Whether KVM_MEM_PRIVATE is actually exposed to userspace is determined
> by architecture code which can turn on it by overriding the default
> kvm_arch_has_private_mem().
>
> A 'kvm' reference is added in memslot structure since in
> inaccessible_notifier callback we can only obtain a memslot reference
> but 'kvm' is needed to do the zapping.
>
> Co-developed-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> ---
>  include/linux/kvm_host.h |   1 +
>  virt/kvm/kvm_main.c      | 116 +++++++++++++++++++++++++++++++++++++--
>  2 files changed, 111 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index b9906cdf468b..cb4eefac709c 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -589,6 +589,7 @@ struct kvm_memory_slot {
>         struct file *private_file;
>         loff_t private_offset;
>         struct inaccessible_notifier notifier;
> +       struct kvm *kvm;
>  };
>
>  static inline bool kvm_slot_can_be_private(const struct kvm_memory_slot *slot)
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 97d893f7482c..87e239d35b96 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -983,6 +983,57 @@ static int kvm_vm_ioctl_set_mem_attr(struct kvm *kvm, gpa_t gpa, gpa_t size,
>                 xa_erase(&kvm->mem_attr_array, index);
>         return r;
>  }
> +
> +static void kvm_private_notifier_invalidate(struct inaccessible_notifier *notifier,
> +                                           pgoff_t start, pgoff_t end)
> +{
> +       struct kvm_memory_slot *slot = container_of(notifier,
> +                                                   struct kvm_memory_slot,
> +                                                   notifier);
> +       unsigned long base_pgoff = slot->private_offset >> PAGE_SHIFT;
> +       gfn_t start_gfn = slot->base_gfn;
> +       gfn_t end_gfn = slot->base_gfn + slot->npages;
> +
> +
> +       if (start > base_pgoff)
> +               start_gfn = slot->base_gfn + start - base_pgoff;
> +
> +       if (end < base_pgoff + slot->npages)
> +               end_gfn = slot->base_gfn + end - base_pgoff;
> +
> +       if (start_gfn >= end_gfn)
> +               return;
> +
> +       kvm_zap_gfn_range(slot->kvm, start_gfn, end_gfn);
> +}
> +
> +static struct inaccessible_notifier_ops kvm_private_notifier_ops = {
> +       .invalidate = kvm_private_notifier_invalidate,
> +};
> +
> +static inline void kvm_private_mem_register(struct kvm_memory_slot *slot)
> +{
> +       slot->notifier.ops = &kvm_private_notifier_ops;
> +       inaccessible_register_notifier(slot->private_file, &slot->notifier);
> +}
> +
> +static inline void kvm_private_mem_unregister(struct kvm_memory_slot *slot)
> +{
> +       inaccessible_unregister_notifier(slot->private_file, &slot->notifier);
> +}
> +
> +#else /* !CONFIG_HAVE_KVM_PRIVATE_MEM */
> +
> +static inline void kvm_private_mem_register(struct kvm_memory_slot *slot)
> +{
> +       WARN_ON_ONCE(1);
> +}
> +
> +static inline void kvm_private_mem_unregister(struct kvm_memory_slot *slot)
> +{
> +       WARN_ON_ONCE(1);
> +}
> +
>  #endif /* CONFIG_HAVE_KVM_PRIVATE_MEM */
>
>  #ifdef CONFIG_HAVE_KVM_PM_NOTIFIER
> @@ -1029,6 +1080,11 @@ static void kvm_destroy_dirty_bitmap(struct kvm_memory_slot *memslot)
>  /* This does not remove the slot from struct kvm_memslots data structures */
>  static void kvm_free_memslot(struct kvm *kvm, struct kvm_memory_slot *slot)
>  {
> +       if (slot->flags & KVM_MEM_PRIVATE) {
> +               kvm_private_mem_unregister(slot);
> +               fput(slot->private_file);
> +       }
> +
>         kvm_destroy_dirty_bitmap(slot);
>
>         kvm_arch_free_memslot(kvm, slot);
> @@ -1600,10 +1656,16 @@ bool __weak kvm_arch_has_private_mem(struct kvm *kvm)
>         return false;
>  }
>
> -static int check_memory_region_flags(const struct kvm_user_mem_region *mem)
> +static int check_memory_region_flags(struct kvm *kvm,
> +                                    const struct kvm_user_mem_region *mem)
>  {
>         u32 valid_flags = KVM_MEM_LOG_DIRTY_PAGES;
>
> +#ifdef CONFIG_HAVE_KVM_PRIVATE_MEM
> +       if (kvm_arch_has_private_mem(kvm))
> +               valid_flags |= KVM_MEM_PRIVATE;
> +#endif
> +
>  #ifdef __KVM_HAVE_READONLY_MEM
>         valid_flags |= KVM_MEM_READONLY;
>  #endif
> @@ -1679,6 +1741,9 @@ static int kvm_prepare_memory_region(struct kvm *kvm,
>  {
>         int r;
>
> +       if (change == KVM_MR_CREATE && new->flags & KVM_MEM_PRIVATE)
> +               kvm_private_mem_register(new);
> +

>From the discussion I had with Kirill in the first patch *, should
this check that the private_fd is inaccessible?

[*] https://lore.kernel.org/all/20221003110129.bbee7kawhw5ed745@xxxxxxxxxxxxxxxxx/

Cheers,
/fuad

>         /*
>          * If dirty logging is disabled, nullify the bitmap; the old bitmap
>          * will be freed on "commit".  If logging is enabled in both old and
> @@ -1707,6 +1772,9 @@ static int kvm_prepare_memory_region(struct kvm *kvm,
>         if (r && new && new->dirty_bitmap && (!old || !old->dirty_bitmap))
>                 kvm_destroy_dirty_bitmap(new);
>
> +       if (r && change == KVM_MR_CREATE && new->flags & KVM_MEM_PRIVATE)
> +               kvm_private_mem_unregister(new);
> +
>         return r;
>  }
>
> @@ -2004,7 +2072,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
>         int as_id, id;
>         int r;
>
> -       r = check_memory_region_flags(mem);
> +       r = check_memory_region_flags(kvm, mem);
>         if (r)
>                 return r;
>
> @@ -2023,6 +2091,10 @@ int __kvm_set_memory_region(struct kvm *kvm,
>              !access_ok((void __user *)(unsigned long)mem->userspace_addr,
>                         mem->memory_size))
>                 return -EINVAL;
> +       if (mem->flags & KVM_MEM_PRIVATE &&
> +               (mem->private_offset & (PAGE_SIZE - 1) ||
> +                mem->private_offset > U64_MAX - mem->memory_size))
> +               return -EINVAL;
>         if (as_id >= KVM_ADDRESS_SPACE_NUM || id >= KVM_MEM_SLOTS_NUM)
>                 return -EINVAL;
>         if (mem->guest_phys_addr + mem->memory_size < mem->guest_phys_addr)
> @@ -2061,6 +2133,9 @@ int __kvm_set_memory_region(struct kvm *kvm,
>                 if ((kvm->nr_memslot_pages + npages) < kvm->nr_memslot_pages)
>                         return -EINVAL;
>         } else { /* Modify an existing slot. */
> +               /* Private memslots are immutable, they can only be deleted. */
> +               if (mem->flags & KVM_MEM_PRIVATE)
> +                       return -EINVAL;
>                 if ((mem->userspace_addr != old->userspace_addr) ||
>                     (npages != old->npages) ||
>                     ((mem->flags ^ old->flags) & KVM_MEM_READONLY))
> @@ -2089,10 +2164,27 @@ int __kvm_set_memory_region(struct kvm *kvm,
>         new->npages = npages;
>         new->flags = mem->flags;
>         new->userspace_addr = mem->userspace_addr;
> +       if (mem->flags & KVM_MEM_PRIVATE) {
> +               new->private_file = fget(mem->private_fd);
> +               if (!new->private_file) {
> +                       r = -EINVAL;
> +                       goto out;
> +               }
> +               new->private_offset = mem->private_offset;
> +       }
> +
> +       new->kvm = kvm;
>
>         r = kvm_set_memslot(kvm, old, new, change);
>         if (r)
> -               kfree(new);
> +               goto out;
> +
> +       return 0;
> +
> +out:
> +       if (new->private_file)
> +               fput(new->private_file);
> +       kfree(new);
>         return r;
>  }
>  EXPORT_SYMBOL_GPL(__kvm_set_memory_region);
> @@ -4747,16 +4839,28 @@ static long kvm_vm_ioctl(struct file *filp,
>         }
>         case KVM_SET_USER_MEMORY_REGION: {
>                 struct kvm_user_mem_region mem;
> -               unsigned long size = sizeof(struct kvm_userspace_memory_region);
> +               unsigned int flags_offset = offsetof(typeof(mem), flags);
> +               unsigned long size;
> +               u32 flags;
>
>                 kvm_sanity_check_user_mem_region_alias();
>
> +               memset(&mem, 0, sizeof(mem));
> +
>                 r = -EFAULT;
> -               if (copy_from_user(&mem, argp, size);
> +               if (get_user(flags, (u32 __user *)(argp + flags_offset)))
> +                       goto out;
> +
> +               if (flags & KVM_MEM_PRIVATE)
> +                       size = sizeof(struct kvm_userspace_memory_region_ext);
> +               else
> +                       size = sizeof(struct kvm_userspace_memory_region);
> +
> +               if (copy_from_user(&mem, argp, size))
>                         goto out;
>
>                 r = -EINVAL;
> -               if (mem.flags & KVM_MEM_PRIVATE)
> +               if ((flags ^ mem.flags) & KVM_MEM_PRIVATE)
>                         goto out;
>
>                 r = kvm_vm_ioctl_set_memory_region(kvm, &mem);
> --
> 2.25.1
>