[PATCH v4 04/11] KVM: Track weak reference to vm_file in struct kvm

[Tarun Sahu <tarunsahu@google.com>]

- Add a weak reference 'vm_file' in struct kvm under
  CONFIG_LIVEUPDATE_GUEST_MEMFD to enable reverse file resolution
  without circular lifetime dependencies.

Signed-off-by: Tarun Sahu <tarunsahu@google.com>
---
 include/linux/kvm_host.h | 12 ++++++++++++
 virt/kvm/kvm_main.c      | 26 ++++++++++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index d9908c19e42f..8da9589425ee 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -874,6 +874,18 @@ struct kvm {
 #ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
 	/* Protected by slots_lock (for writes) and RCU (for reads) */
 	struct xarray mem_attr_array;
+#endif
+#ifdef CONFIG_LIVEUPDATE_GUEST_MEMFD
+	/*
+	 * Weak reference to the VFS file backing this KVM instance. Stored
+	 * without incrementing the file refcount to prevent a circular lifetime
+	 * dependency (since file->private_data already pins this struct kvm).
+	 * Used exclusively to resolve the file pointer back from struct kvm.
+	 *
+	 * Written/cleared via rcu_assign_pointer() and read locklessly under
+	 * RCU (e.g. via get_file_active() to prevent ABA races).
+	 */
+	struct file __rcu *vm_file;
 #endif
 	char stats_id[KVM_STATS_NAME_SIZE];
 };
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 701d4596a91c..14c32541ae34 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1346,6 +1346,19 @@ static int kvm_vm_release(struct inode *inode, struct file *filp)
 {
 	struct kvm *kvm = filp->private_data;
 
+#ifdef CONFIG_LIVEUPDATE_GUEST_MEMFD
+	/*
+	 * Clear the weak reference of the vm file.
+	 * In case vm file is closed by userspace, but kvm still has
+	 * other users like vCPUs, clearing this pointer ensures
+	 * that we don't have a dangling pointer to a closed file.
+	 *
+	 * Cleared via rcu_assign_pointer() to ensure proper memory visibility
+	 * for concurrent lockless readers under RCU.
+	 */
+	rcu_assign_pointer(kvm->vm_file, NULL);
+#endif
+
 	kvm_irqfd_release(kvm);
 
 	kvm_put_kvm(kvm);
@@ -5488,6 +5501,19 @@ struct file *kvm_create_vm_file(unsigned long type, const char *fdname)
 		return file;
 	}
 
+#ifdef CONFIG_LIVEUPDATE_GUEST_MEMFD
+	/*
+	 * Weak reference to the file (without get_file()) to prevent a circular
+	 * dependency. Safe because the file's release path clears this pointer
+	 * and drops its reference to the VM.
+	 *
+	 * Written via rcu_assign_pointer() because the pointer can be read
+	 * locklessly under RCU (e.g., in kvm_gmem_luo_preserve() via
+	 * get_file_active() to prevent lockless ABA races).
+	 */
+	rcu_assign_pointer(kvm->vm_file, file);
+#endif
+
 	/*
 	 * Don't call kvm_put_kvm anymore at this point; file->f_op is
 	 * already set, with ->release() being kvm_vm_release().  In error
-- 
2.55.0.229.g6434b31f56-goog