On Thu, Oct 06, 2022 at 09:55:31AM +0100, Fuad Tabba wrote:

Hi,

On Thu, Sep 15, 2022 at 3:37 PM Chao Peng [off-list ref] wrote:

quoted

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 <redacted>
Signed-off-by: Yu Zhang <redacted>
Signed-off-by: Chao Peng <redacted>
---
 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);
+

quoted

From the discussion I had with Kirill in the first patch *, should

this check that the private_fd is inaccessible?

Yes I can add a check in KVM code, see below for where I want to add it.

[*] https://lore.kernel.org/all/20221003110129.bbee7kawhw5ed745@box.shutemov.name/ (local)

Cheers,
/fuad

quoted

        /*
         * 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;

The check will go here.

quoted

+                       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