Subject: Re: [syzbot] [net?] KASAN: slab-use-after-free Write in l2tp_session_delete

From: Tom Parkin <[email-masked]>
Date: Wed, 3 Jul 2024 13:07:10 +0100

On  Tue, Jun 25, 2024 at 06:25:23 -0700, syzbot wrote:
> syzbot found the following issue on:
> 
> HEAD commit:    185d72112b95 net: xilinx: axienet: Enable multicast by def..
> git tree:       net-next
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1062bd46980000

#syz test https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git  185d72112b95
 
From 15d6d4c290810d5b8f357b9e494c5ad420c8a2fb Mon Sep 17 00:00:00 2001
From: Tom Parkin <[email-masked]>
Date: Wed, 3 Jul 2024 13:02:51 +0100
Subject: [PATCH] l2tp: fix possible UAF when cleaning up tunnels

syzbot reported a UAF caused by a race when the L2TP work queue closes a
tunnel at the same time as a userspace thread closes a session in that
tunnel.

Tunnel cleanup is handled by a work queue which iterates through the
sessions contained within a tunnel, and closes them in turn.

Meanwhile, a userspace thread may arbitrarily close a session via
either netlink command or by closing the pppox socket in the case of
l2tp_ppp.

The race condition may occur when l2tp_tunnel_closeall walks the list
of sessions in the tunnel and deletes each one.  Currently this is
implemented using list_for_each_safe, but because the list spinlock is
dropped in the loop body it's possible for other threads to manipulate
the list during list_for_each_safe's list walk.  This can lead to the
list iterator being corrupted, leading to list_for_each_safe spinning.
One sequence of events which may lead to this is as follows:

 * A tunnel is created, containing two sessions A and B.
 * A thread closes the tunnel, triggering tunnel cleanup via the work
   queue.
 * l2tp_tunnel_closeall runs in the context of the work queue.  It
   removes session A from the tunnel session list, then drops the list
   lock.  At this point the list_for_each_safe temporary variable is
   pointing to the other session on the list, which is session B, and
   the list can be manipulated by other threads since the list lock has
   been released.
 * Userspace closes session B, which removes the session from its parent
   tunnel via l2tp_session_delete.  Since l2tp_tunnel_closeall has
   released the tunnel list lock, l2tp_session_delete is able to call
   list_del_init on the session B list node.
 * Back on the work queue, l2tp_tunnel_closeall resumes execution and
   will now spin forever on the same list entry until the underlying
   session structure is freed, at which point UAF occurs.

The solution is to iterate over the tunnel's session list using
list_first_entry_not_null to avoid the possibility of the list
iterator pointing at a list item which may be removed during the walk.
---
 net/l2tp/l2tp_core.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index be4bcbf291a1..fa75a8eb8782 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1290,12 +1290,14 @@ static void l2tp_session_unhash(struct l2tp_session *session)
 static void l2tp_tunnel_closeall(struct l2tp_tunnel *tunnel)
 {
 	struct l2tp_session *session;
-	struct list_head __rcu *pos;
-	struct list_head *tmp;
 
 	spin_lock_bh(&tunnel->list_lock);
 	tunnel->acpt_newsess = false;
-	list_for_each_safe(pos, tmp, &tunnel->session_list) {
+	for (;;) {
+		session = list_first_entry_or_null(&tunnel->session_list,
+						   struct l2tp_session, list);
+		if (!session)
+			break;
 		session = list_entry(pos, struct l2tp_session, list);
 		list_del_init(&session->list);
 		spin_unlock_bh(&tunnel->list_lock);
-- 
2.34.1