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

Tom Parkin
2024-07-03T16:37:12Z

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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

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From 6bc347af14eba3b37f9e8f0831a53ef1aae4c203 Mon Sep 17 00:00:00 2001
From: Tom Parkin <tparkin@katalix.com>
Date: Wed, 3 Jul 2024 17:32:00 +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 | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index be4bcbf291a1..afa180b7b428 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1290,13 +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) {
- session = list_entry(pos, struct l2tp_session, list);
+ for (;;) {
+ session = list_first_entry_or_null(&tunnel->session_list,
+ struct l2tp_session, list);
+ if (!session)
+ break;
list_del_init(&session->list);
spin_unlock_bh(&tunnel->list_lock);
l2tp_session_delete(session);
--
2.34.1

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