Commit 851901d3 authored Jan 28, 2022 by Duoming Zhou Committed by Greg Kroah-Hartman Feb 23, 2022

ax25: improve the incomplete fix to avoid UAF and NPD bugs



[ Upstream commit 4e0f718daf97d47cf7dec122da1be970f145c809 ]

The previous commit 1ade48d0c27d ("ax25: NPD bug when detaching
AX25 device") introduce lock_sock() into ax25_kill_by_device to
prevent NPD bug. But the concurrency NPD or UAF bug will occur,
when lock_sock() or release_sock() dereferences the ax25_cb->sock.

The NULL pointer dereference bug can be shown as below:

ax25_kill_by_device()        | ax25_release()
                             |   ax25_destroy_socket()
                             |     ax25_cb_del()
  ...                        |     ...
                             |     ax25->sk=NULL;
  lock_sock(s->sk); //(1)    |
  s->ax25_dev = NULL;        |     ...
  release_sock(s->sk); //(2) |
  ...                        |

The root cause is that the sock is set to null before dereference
site (1) or (2). Therefore, this patch extracts the ax25_cb->sock
in advance, and uses ax25_list_lock to protect it, which can synchronize
with ax25_cb_del() and ensure the value of sock is not null before
dereference sites.

The concurrency UAF bug can be shown as below:

ax25_kill_by_device()        | ax25_release()
                             |   ax25_destroy_socket()
  ...                        |   ...
                             |   sock_put(sk); //FREE
  lock_sock(s->sk); //(1)    |
  s->ax25_dev = NULL;        |   ...
  release_sock(s->sk); //(2) |
  ...                        |

The root cause is that the sock is released before dereference
site (1) or (2). Therefore, this patch uses sock_hold() to increase
the refcount of sock and uses ax25_list_lock to protect it, which
can synchronize with ax25_cb_del() in ax25_destroy_socket() and
ensure the sock wil not be released before dereference sites.

Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>

parent a1974b39

net/ax25/af_ax25.c

+6 −3

Original line number	Original line	Diff line number	Diff line
	@@ -80,6 +80,7 @@ static void ax25_kill_by_device(struct net_device *dev)
	{		{
	ax25_dev *ax25_dev;		ax25_dev *ax25_dev;
	ax25_cb *s;		ax25_cb *s;
			struct sock *sk;

	if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL)		if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL)
	return;		return;
	@@ -88,13 +89,15 @@ static void ax25_kill_by_device(struct net_device *dev)
	again:		again:
	ax25_for_each(s, &ax25_list) {		ax25_for_each(s, &ax25_list) {
	if (s->ax25_dev == ax25_dev) {		if (s->ax25_dev == ax25_dev) {
			sk = s->sk;
			sock_hold(sk);
	spin_unlock_bh(&ax25_list_lock);		spin_unlock_bh(&ax25_list_lock);
	lock_sock(s->sk);		lock_sock(sk);
	s->ax25_dev = NULL;		s->ax25_dev = NULL;
	release_sock(s->sk);		release_sock(sk);
	ax25_disconnect(s, ENETUNREACH);		ax25_disconnect(s, ENETUNREACH);
	spin_lock_bh(&ax25_list_lock);		spin_lock_bh(&ax25_list_lock);
			sock_put(sk);
	/* The entry could have been deleted from the		/* The entry could have been deleted from the
	* list meanwhile and thus the next pointer is		* list meanwhile and thus the next pointer is
	* no longer valid. Play it safe and restart		* no longer valid. Play it safe and restart