[PATCH RFC v3 52/88] KVM: guest_memfd: Support mmap provider operation

[Ackerley Tng via B4 Relay]

From: Ackerley Tng <ackerleytng@xxxxxxxxxx>

Allow memory providers to customize VMA setup when userspace creates a
shared mapping on a guest_memfd file.

Add an optional .mmap callback to struct guest_memfd_provider_operations
and invoke it via gmem_provider_mmap() during mmap handling on
guest_memfd descriptors. If the provider does not define .mmap,
gmem_provider_mmap() simply returns 0.

Invoke the provider callback before setting VM_PFNMAP and guest_memfd
VMA operations so that PFN mode mappings always have VM_PFNMAP set and
remain mediated by guest_memfd fault handling even if the provider omits
the callback or configures its own VMA flags and operations.

Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
---
 include/linux/guest_memfd.h |  4 ++++
 virt/kvm/guest_memfd.c      | 13 +++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/include/linux/guest_memfd.h b/include/linux/guest_memfd.h
index 8cdc334d1c9aa..a2af279cd3c34 100644
--- a/include/linux/guest_memfd.h
+++ b/include/linux/guest_memfd.h
@@ -85,6 +85,9 @@ static inline bool kvm_gmem_pfn_is_cacheable(u64 pfn_attrs)
  *                 available, the provider should skip the unbacked range. Not
  *                 called on kvm_gmem_revoke(). Do not take locks held across
  *                 kvm_gmem_revoke().
+ * @mmap: Optional callback to set up user mappings. Called when userspace calls
+ *        mmap() on guest_memfd. The provider can customize VMA flags, or reject
+ *        the mapping.
  *
  * Used by filesystems and device drivers that provide memory for guest_memfd.
  */
@@ -99,6 +102,7 @@ struct guest_memfd_provider_operations {
 	int (*lookup_pfn_range)(void *provider, pgoff_t index,
 				unsigned long *pfn, size_t *nr_pages);
 	void (*put_pfn_range)(void *provider, pgoff_t index, size_t nr_pages);
+	int (*mmap)(void *provider, struct vm_area_struct *vma);
 };
 
 #ifdef CONFIG_KVM_GUEST_MEMFD
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 1fbb719f0e31f..ec908419e5faa 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -202,6 +202,14 @@ static void gmem_provider_release(struct gmem_inode *gi)
 		gi->provider_ops->release(gi->provider);
 }
 
+static int gmem_provider_mmap(struct gmem_inode *gi, struct vm_area_struct *vma)
+{
+	if (gi->provider_ops && gi->provider_ops->mmap)
+		return gi->provider_ops->mmap(gi->provider, vma);
+
+	return 0;
+}
+
 #define kvm_gmem_for_each_file(f, inode) \
 	list_for_each_entry(f, &GMEM_I(inode)->gmem_file_list, entry)
 
@@ -860,6 +868,7 @@ static const struct vm_operations_struct kvm_gmem_pfn_vm_ops = {
 static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
 {
 	struct gmem_inode *gi = GMEM_I(file_inode(file));
+	int ret;
 
 	if (!kvm_gmem_supports_mmap(file_inode(file)))
 		return -ENODEV;
@@ -869,6 +878,10 @@ static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
 		return -EINVAL;
 	}
 
+	ret = gmem_provider_mmap(gi, vma);
+	if (ret)
+		return ret;
+
 	if (kvm_gmem_is_pfn_mode(gi))
 		vm_flags_set(vma, VM_PFNMAP);
 

-- 
2.56.0.455.g6c5a503b38-goog