cifs: do not depend on release_iface for maintaining iface_list

parse_server_interfaces should be in complete charge of maintaining
the iface_list linked list. Today, iface entries are removed
from the list only when the last refcount is dropped.
i.e. in release_iface. However, this can result in undercounting
of refcount if the server stops advertising interfaces (which
Azure SMB server does).

This change puts parse_server_interfaces in full charge of
maintaining the iface_list. So if an empty list is returned
by the server, the entries in the list will immediately be
removed. This way, a following call to the same function will
not find entries in the list.

Fixes: aa45dadd ("cifs: change iface_list from array to sorted linked list")
Cc: stable@vger.kernel.org
Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/client/cifsglob.h

@@ -994,7 +994,6 @@ release_iface(struct kref *ref)
 	struct cifs_server_iface *iface = container_of(ref,
 						       struct cifs_server_iface,
 						       refcount);
-	list_del_init(&iface->iface_head);
 	kfree(iface);
 }
 

fs/smb/client/smb2ops.c

@@ -595,16 +595,12 @@ parse_server_interfaces(struct network_interface_info_ioctl_rsp *buf,
 	}
 
 	/*
-	 * Go through iface_list and do kref_put to remove
-	 * any unused ifaces. ifaces in use will be removed
-	 * when the last user calls a kref_put on it
+	 * Go through iface_list and mark them as inactive
 	 */
 	list_for_each_entry_safe(iface, niface, &ses->iface_list,
-				 iface_head) {
+				 iface_head)
 		iface->is_active = 0;
-		kref_put(&iface->refcount, release_iface);
-		ses->iface_count--;
-	}
+
 	spin_unlock(&ses->iface_lock);
 
 	/*
@@ -678,10 +674,7 @@ parse_server_interfaces(struct network_interface_info_ioctl_rsp *buf,
 					 iface_head) {
 			ret = iface_cmp(iface, &tmp_iface);
 			if (!ret) {
-				/* just get a ref so that it doesn't get picked/freed */
 				iface->is_active = 1;
-				kref_get(&iface->refcount);
-				ses->iface_count++;
 				spin_unlock(&ses->iface_lock);
 				goto next_iface;
 			} else if (ret < 0) {
@@ -748,6 +741,20 @@ parse_server_interfaces(struct network_interface_info_ioctl_rsp *buf,
 	}
 
 out:
+	/*
+	 * Go through the list again and put the inactive entries
+	 */
+	spin_lock(&ses->iface_lock);
+	list_for_each_entry_safe(iface, niface, &ses->iface_list,
+				 iface_head) {
+		if (!iface->is_active) {
+			list_del(&iface->iface_head);
+			kref_put(&iface->refcount, release_iface);
+			ses->iface_count--;
+		}
+	}
+	spin_unlock(&ses->iface_lock);
+
 	return rc;
 }