KVM: guest_memfd: Remove RCU-protected attribute from slot->gmem.file

Remove the RCU-protected attribute from slot->gmem.file. No need to use RCU
primitives rcu_assign_pointer()/synchronize_rcu() to update this pointer.

- slot->gmem.file is updated in 3 places:
  kvm_gmem_bind(), kvm_gmem_unbind(), kvm_gmem_release().
  All of them are protected by kvm->slots_lock.

- slot->gmem.file is read in 2 paths:
  (1) kvm_gmem_populate
        kvm_gmem_get_file
        __kvm_gmem_get_pfn

  (2) kvm_gmem_get_pfn
         kvm_gmem_get_file
         __kvm_gmem_get_pfn

  Path (1) kvm_gmem_populate() requires holding kvm->slots_lock, so
  slot->gmem.file is protected by the kvm->slots_lock in this path.

  Path (2) kvm_gmem_get_pfn() does not require holding kvm->slots_lock.
  However, it's also not guarded by rcu_read_lock() and rcu_read_unlock().
  So synchronize_rcu() in kvm_gmem_unbind()/kvm_gmem_release() actually
  will not wait for the readers in kvm_gmem_get_pfn() due to lack of RCU
  read-side critical section.

  The path (2) kvm_gmem_get_pfn() is safe without RCU protection because:
  a) kvm_gmem_bind() is called on a new memslot, before the memslot is
     visible to kvm_gmem_get_pfn().
  b) kvm->srcu ensures that kvm_gmem_unbind() and freeing of a memslot
     occur after the memslot is no longer visible to kvm_gmem_get_pfn().
  c) get_file_active() ensures that kvm_gmem_get_pfn() will not access the
     stale file if kvm_gmem_release() sets it to NULL.  This is because if
     kvm_gmem_release() occurs before kvm_gmem_get_pfn(), get_file_active()
     will return NULL; if get_file_active() does not return NULL,
     kvm_gmem_release() should not occur until after kvm_gmem_get_pfn()
     releases the file reference.

Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Message-ID: <20241104084303.29909-1-yan.y.zhao@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/linux/kvm_host.h

@@ -596,7 +596,12 @@ struct kvm_memory_slot {
 
 #ifdef CONFIG_KVM_PRIVATE_MEM
 	struct {
-		struct file __rcu *file;
+		/*
+		 * Writes protected by kvm->slots_lock.  Acquiring a
+		 * reference via kvm_gmem_get_file() is protected by
+		 * either kvm->slots_lock or kvm->srcu.
+		 */
+		struct file *file;
 		pgoff_t pgoff;
 	} gmem;
 #endif

virt/kvm/guest_memfd.c

@@ -259,15 +259,19 @@ static int kvm_gmem_release(struct inode *inode, struct file *file)
 	 * dereferencing the slot for existing bindings needs to be protected
 	 * against memslot updates, specifically so that unbind doesn't race
 	 * and free the memslot (kvm_gmem_get_file() will return NULL).
+	 *
+	 * Since .release is called only when the reference count is zero,
+	 * after which file_ref_get() and get_file_active() fail,
+	 * kvm_gmem_get_pfn() cannot be using the file concurrently.
+	 * file_ref_put() provides a full barrier, and get_file_active() the
+	 * matching acquire barrier.
 	 */
 	mutex_lock(&kvm->slots_lock);
 
 	filemap_invalidate_lock(inode->i_mapping);
 
 	xa_for_each(&gmem->bindings, index, slot)
-		rcu_assign_pointer(slot->gmem.file, NULL);
+		WRITE_ONCE(slot->gmem.file, NULL);
-
-	synchronize_rcu();
 
 	/*
 	 * All in-flight operations are gone and new bindings can be created.
@@ -296,8 +300,7 @@ static inline struct file *kvm_gmem_get_file(struct kvm_memory_slot *slot)
 	/*
 	 * Do not return slot->gmem.file if it has already been closed;
 	 * there might be some time between the last fput() and when
-	 * kvm_gmem_release() clears slot->gmem.file, and you do not
+	 * kvm_gmem_release() clears slot->gmem.file.
-	 * want to spin in the meanwhile.
 	 */
 	return get_file_active(&slot->gmem.file);
 }
@@ -508,11 +511,11 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
 	}
 
 	/*
-	 * No synchronize_rcu() needed, any in-flight readers are guaranteed to
+	 * memslots of flag KVM_MEM_GUEST_MEMFD are immutable to change, so
-	 * be see either a NULL file or this new file, no need for them to go
+	 * kvm_gmem_bind() must occur on a new memslot.  Because the memslot
-	 * away.
+	 * is not visible yet, kvm_gmem_get_pfn() is guaranteed to see the file.
 	 */
-	rcu_assign_pointer(slot->gmem.file, file);
+	WRITE_ONCE(slot->gmem.file, file);
 	slot->gmem.pgoff = start;
 
 	xa_store_range(&gmem->bindings, start, end - 1, slot, GFP_KERNEL);
@@ -548,8 +551,12 @@ void kvm_gmem_unbind(struct kvm_memory_slot *slot)
 
 	filemap_invalidate_lock(file->f_mapping);
 	xa_store_range(&gmem->bindings, start, end - 1, NULL, GFP_KERNEL);
-	rcu_assign_pointer(slot->gmem.file, NULL);
+
-	synchronize_rcu();
+	/*
+	 * synchronize_srcu(&kvm->srcu) ensured that kvm_gmem_get_pfn()
+	 * cannot see this memslot.
+	 */
+	WRITE_ONCE(slot->gmem.file, NULL);
 	filemap_invalidate_unlock(file->f_mapping);
 
 	fput(file);
@@ -561,11 +568,12 @@ static struct folio *__kvm_gmem_get_pfn(struct file *file,
 					pgoff_t index, kvm_pfn_t *pfn,
 					bool *is_prepared, int *max_order)
 {
+	struct file *gmem_file = READ_ONCE(slot->gmem.file);
 	struct kvm_gmem *gmem = file->private_data;
 	struct folio *folio;
 
-	if (file != slot->gmem.file) {
+	if (file != gmem_file) {
-		WARN_ON_ONCE(slot->gmem.file);
+		WARN_ON_ONCE(gmem_file);
 		return ERR_PTR(-EFAULT);
 	}