[dpdk-dev] [PATCH] Fix `eventfd_link' module leakages and races

[Pavel Boldin <pboldin@mirantis.com>]

From: Pavel Boldin <pboldin@mirantis.com>

The `eventfd_link' module provides an API to "steal" fd from another
process had been written with a bug that leaks `struct file' because of the
extra reference counter increment and missing `fput' call.

The other bug is using another process' `task_struct' without incrementing
a reference counter.

Fix these bugs and refactor the module.
---
 examples/vhost/eventfd_link/eventfd_link.c | 236 ++++++++++++++++-------------
 1 file changed, 130 insertions(+), 106 deletions(-)

diff --git a/examples/vhost/eventfd_link/eventfd_link.c b/examples/vhost/eventfd_link/eventfd_link.c
index 69470ba..9f1f8fb 100644
--- a/examples/vhost/eventfd_link/eventfd_link.c
+++ b/examples/vhost/eventfd_link/eventfd_link.c
@@ -42,15 +42,15 @@
  * get_files_struct is copied from fs/file.c
  */
 struct files_struct *
-get_files_struct (struct task_struct *task)
+get_files_struct(struct task_struct *task)
 {
 	struct files_struct *files;
 
-	task_lock (task);
+	task_lock(task);
 	files = task->files;
 	if (files)
-		atomic_inc (&files->count);
-	task_unlock (task);
+		atomic_inc(&files->count);
+	task_unlock(task);
 
 	return files;
 }
@@ -59,117 +59,142 @@ get_files_struct (struct task_struct *task)
  * put_files_struct is extracted from fs/file.c
  */
 void
-put_files_struct (struct files_struct *files)
+put_files_struct(struct files_struct *files)
 {
-	if (atomic_dec_and_test (&files->count))
-	{
+	if (atomic_dec_and_test(&files->count))
 		BUG ();
+}
+
+static struct file *
+fget_from_files(struct files_struct *files, unsigned fd)
+{
+	struct file *file;
+
+	rcu_read_lock();
+	file = fcheck_files(files, fd);
+	if (file)
+	{
+		if (file->f_mode & FMODE_PATH
+			|| !atomic_long_inc_not_zero(&file->f_count))
+		    file = NULL;
 	}
+	rcu_read_unlock();
+
+	return file;
+}
+
+static int
+close_fd(unsigned fd)
+{
+	struct file *file;
+	struct files_struct *files = current->files;
+	struct fdtable *fdt;
+
+	spin_lock(&files->file_lock);
+	fdt = files_fdtable(files);
+	if (fd >= fdt->max_fds)
+		goto out_unlock;
+	file = fdt->fd[fd];
+	if (!file)
+		goto out_unlock;
+	rcu_assign_pointer(fdt->fd[fd], NULL);
+	__clear_bit(fd, fdt->close_on_exec);
+	spin_unlock(&files->file_lock);
+	return filp_close(file, files);
+
+out_unlock:
+	spin_unlock(&files->file_lock);
+	return -EBADF;
 }
 
 
 static long
-eventfd_link_ioctl (struct file *f, unsigned int ioctl, unsigned long arg)
+eventfd_link_ioctl_copy(unsigned long arg)
 {
-	void __user *argp = (void __user *) arg;
+	long ret = -EFAULT;
 	struct task_struct *task_target = NULL;
-	struct file *file;
-	struct files_struct *files;
-	struct fdtable *fdt;
+	struct file *target_file = NULL;
+	struct files_struct *target_files = NULL;
 	struct eventfd_copy eventfd_copy;
+	struct pid *pid;
+
+	if (copy_from_user(&eventfd_copy, (void __user*)arg,
+			    sizeof(struct eventfd_copy)))
+		goto out;
+
+	/*
+	 * Find the task struct for the target pid
+	 */
+	pid = find_vpid(eventfd_copy.target_pid);
+	if (pid == NULL) {
+		printk(KERN_INFO "Unable to find pid %d\n",
+			eventfd_copy.target_pid);
+		goto out;
+	}
+
+	task_target = get_pid_task(pid, PIDTYPE_PID);
+	if (task_target == NULL) {
+		printk(KERN_INFO "Failed to get task for pid %d\n",
+			eventfd_copy.target_pid);
+		goto out;
+	}
+
+	ret = close_fd(eventfd_copy.source_fd);
+	if (ret)
+		goto out_task;
+	ret = -EFAULT;
+
+	/*
+	 * Find the file struct associated with the target fd.
+	 */
+
+	target_files = get_files_struct(task_target);
+	if (target_files == NULL) {
+		printk (KERN_INFO "Failed to get target files struct\n");
+		goto out_task;
+	}
+
+	target_file = fget_from_files(target_files, eventfd_copy.target_fd);
+
+	if (target_file == NULL) {
+		printk (KERN_INFO "Failed to get file from target pid\n");
+		goto out_target_files;
+	}
+
+
+	/*
+	 * Install the file struct from the target process into the
+	 * file desciptor of the source process,
+	 */
+
+	fd_install(eventfd_copy.source_fd, target_file);
+
+	ret = 0;
+
+out_target_files:
+	put_files_struct(target_files);
+out_task:
+	put_task_struct(task_target);
+out:
+	return ret;
+}
+
+static long
+eventfd_link_ioctl(struct file *f, unsigned int ioctl, unsigned long arg)
+{
+	long ret;
 
 	switch (ioctl)
 	{
 		case EVENTFD_COPY:
-			if (copy_from_user (&eventfd_copy, argp, sizeof (struct eventfd_copy)))
-				return -EFAULT;
-
-			/*
-			 * Find the task struct for the target pid
-			 */
-			task_target =
-				pid_task (find_vpid (eventfd_copy.target_pid), PIDTYPE_PID);
-			if (task_target == NULL)
-			{
-				printk (KERN_DEBUG "Failed to get mem ctx for target pid\n");
-				return -EFAULT;
-			}
-
-			files = get_files_struct (current);
-			if (files == NULL)
-			{
-				printk (KERN_DEBUG "Failed to get files struct\n");
-				return -EFAULT;
-			}
-
-			rcu_read_lock ();
-			file = fcheck_files (files, eventfd_copy.source_fd);
-			if (file)
-			{
-				if (file->f_mode & FMODE_PATH
-						|| !atomic_long_inc_not_zero (&file->f_count))
-					file = NULL;
-			}
-			rcu_read_unlock ();
-			put_files_struct (files);
-
-			if (file == NULL)
-			{
-				printk (KERN_DEBUG "Failed to get file from source pid\n");
-				return 0;
-			}
-
-			/*
-			 * Release the existing eventfd in the source process
-			 */
-			spin_lock (&files->file_lock);
-			fput(file);
-			filp_close (file, files);
-			fdt = files_fdtable (files);
-			fdt->fd[eventfd_copy.source_fd] = NULL;
-			spin_unlock (&files->file_lock);
-
-			/*
-			 * Find the file struct associated with the target fd.
-			 */
-
-			files = get_files_struct (task_target);
-			if (files == NULL)
-			{
-				printk (KERN_DEBUG "Failed to get files struct\n");
-				return -EFAULT;
-			}
-
-			rcu_read_lock ();
-			file = fcheck_files (files, eventfd_copy.target_fd);
-			if (file)
-			{
-				if (file->f_mode & FMODE_PATH
-						|| !atomic_long_inc_not_zero (&file->f_count))
-					file = NULL;
-			}
-			rcu_read_unlock ();
-			put_files_struct (files);
-
-			if (file == NULL)
-			{
-				printk (KERN_DEBUG "Failed to get file from target pid\n");
-				return 0;
-			}
-
-
-			/*
-			 * Install the file struct from the target process into the
-			 * file desciptor of the source process,
-			 */
-
-			fd_install (eventfd_copy.source_fd, file);
-
-			return 0;
-
+			ret = eventfd_link_ioctl_copy(arg);
+			break;
 		default:
-			return -ENOIOCTLCMD;
+			ret = -ENOIOCTLCMD;
+			break;
 	}
+
+	return ret;
 }
 
 static const struct file_operations eventfd_link_fops = {
@@ -184,21 +209,20 @@ static struct miscdevice eventfd_link_misc = {
 };
 
 static int __init
-eventfd_link_init (void)
+eventfd_link_init(void)
 {
-	printk(KERN_INFO "eventfd_link is broken, use it at your risk\n");
-	return misc_register (&eventfd_link_misc);
+	return misc_register(&eventfd_link_misc);
 }
 
-module_init (eventfd_link_init);
+module_init(eventfd_link_init);
 
 static void __exit
-eventfd_link_exit (void)
+eventfd_link_exit(void)
 {
-	misc_deregister (&eventfd_link_misc);
+	misc_deregister(&eventfd_link_misc);
 }
 
-module_exit (eventfd_link_exit);
+module_exit(eventfd_link_exit);
 
 MODULE_VERSION ("0.0.1");
 MODULE_LICENSE ("GPL v2");
-- 
1.9.1