netdev - Re: net/l2tp: use-after-free write in l2tp_ip6

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Date:   Mon, 7 Nov 2016 15:02:54 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Shmulik Ladkani <shmulik.ladkani@...il.com>,
        Wei Wang <weiwan@...gle.com>,
        Haishuang Yan <yanhaishuang@...s.chinamobile.com>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: net/l2tp: use-after-free write in l2tp_ip6_close

On Mon, Nov 7, 2016 at 2:35 PM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
> Hi,
>
> I've got the following error report while running the syzkaller fuzzer:
>
> ==================================================================
> BUG: KASAN: use-after-free in l2tp_ip6_close+0x239/0x2a0 at addr
> ffff8800677276d8
> Write of size 8 by task a.out/8668
> CPU: 0 PID: 8668 Comm: a.out Not tainted 4.9.0-rc4+ #354
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>  ffff8800694d7b00 ffffffff81b46a64 ffff88006adb5780 ffff8800677276c0
>  ffff880067727c68 ffff8800677276c0 ffff8800694d7b28 ffffffff8150a86c
>  ffff8800694d7bb8 ffff88006adb5780 ffff8800e77276d8 ffff8800694d7ba8
> Call Trace:
>  [<     inline     >] __dump_stack lib/dump_stack.c:15
>  [<ffffffff81b46a64>] dump_stack+0xb3/0x10f lib/dump_stack.c:51
>  [<ffffffff8150a86c>] kasan_object_err+0x1c/0x70 mm/kasan/report.c:156
>  [<     inline     >] print_address_description mm/kasan/report.c:194
>  [<ffffffff8150ab07>] kasan_report_error+0x1f7/0x4d0 mm/kasan/report.c:283
>  [<     inline     >] kasan_report mm/kasan/report.c:303
>  [<ffffffff8150b01e>] __asan_report_store8_noabort+0x3e/0x40
> mm/kasan/report.c:329
>  [<     inline     >] __write_once_size ./include/linux/compiler.h:272
>  [<     inline     >] __hlist_del ./include/linux/list.h:622
>  [<     inline     >] hlist_del_init ./include/linux/list.h:637
>  [<ffffffff83825f49>] l2tp_ip6_close+0x239/0x2a0 net/l2tp/l2tp_ip6.c:239
>  [<ffffffff8316b31f>] inet_release+0xef/0x1c0 net/ipv4/af_inet.c:415
>  [<ffffffff832cd4d0>] inet6_release+0x50/0x70 net/ipv6/af_inet6.c:422
>  [<ffffffff82b6d89e>] sock_release+0x8e/0x1d0 net/socket.c:570
>  [<ffffffff82b6d9f6>] sock_close+0x16/0x20 net/socket.c:1017
>  [<ffffffff81524bdd>] __fput+0x29d/0x720 fs/file_table.c:208
>  [<ffffffff815250e5>] ____fput+0x15/0x20 fs/file_table.c:244
>  [<ffffffff81172928>] task_work_run+0xf8/0x170 kernel/task_work.c:116
>  [<     inline     >] exit_task_work ./include/linux/task_work.h:21
>  [<ffffffff8111bda3>] do_exit+0x883/0x2ac0 kernel/exit.c:828
>  [<ffffffff8112234e>] do_group_exit+0x10e/0x340 kernel/exit.c:931
>  [<     inline     >] SYSC_exit_group kernel/exit.c:942
>  [<ffffffff8112259d>] SyS_exit_group+0x1d/0x20 kernel/exit.c:940
>  [<ffffffff83fc1501>] entry_SYSCALL_64_fastpath+0x1f/0xc2
> arch/x86/entry/entry_64.S:209

I guess we need to lock the sock for l2tp_ip6_disconnect() too.

diff --git a/net/l2tp/l2tp_ip6.c b/net/l2tp/l2tp_ip6.c
index ad3468c..ea2ae66 100644
--- a/net/l2tp/l2tp_ip6.c
+++ b/net/l2tp/l2tp_ip6.c
@@ -410,7 +410,7 @@ static int l2tp_ip6_disconnect(struct sock *sk, int flags)
        if (sock_flag(sk, SOCK_ZAPPED))
                return 0;

-       return __udp_disconnect(sk, flags);
+       return udp_disconnect(sk, flags);
 }

 static int l2tp_ip6_getname(struct socket *sock, struct sockaddr *uaddr,