netdev - Re: [PATCH net-next] kcm: do not attach sockets if sk_user

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Message-ID: <CALx6S35gZbfY-SwLjrvdazMWAMpC+fVhO8+O7C0QzoVaTdtKjg@mail.gmail.com>
Date:   Tue, 16 Jan 2018 09:36:41 -0800
From:   Tom Herbert <tom@...bertland.com>
To:     James Chapman <jchapman@...alix.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] kcm: do not attach sockets if sk_user_data is
 already used

On Sun, Jan 14, 2018 at 3:32 AM, James Chapman <jchapman@...alix.com> wrote:
> SIOCKCMATTACH writes a connected socket's sk_user_data for its own
> use. Prevent it doing so if the socket's sk_user_data is already set
> since some sockets (e.g. encapsulated sockets) use sk_user_data
> internally.
>
> This issue was found by syzbot.
>
> kernel BUG at net/l2tp/l2tp_ppp.c:176!
> invalid opcode: 0000 [#1] SMP KASAN
> Dumping ftrace buffer:
>    (ftrace buffer empty)
> Modules linked in:
> CPU: 1 PID: 3503 Comm: syzkaller938388 Not tainted 4.15.0-rc7+ #181
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> RIP: 0010:pppol2tp_sock_to_session net/l2tp/l2tp_ppp.c:176 [inline]
> RIP: 0010:pppol2tp_sendmsg+0x512/0x670 net/l2tp/l2tp_ppp.c:304
> RSP: 0018:ffff8801d4887438 EFLAGS: 00010293
> RAX: ffff8801bfef2180 RBX: ffff8801bff88440 RCX: ffffffff84ffbca2
> RDX: 0000000000000000 RSI: ffff8801d4887598 RDI: ffff8801bff88820
> RBP: ffff8801d48874a8 R08: 0000000000000000 R09: 1ffff1003a910e17
> R10: 0000000000000003 R11: 0000000000000001 R12: ffff8801bfff9bc0
> R13: 0000000000000000 R14: 0000000000008000 R15: 0000000000000000
> FS:  0000000001194880(0000) GS:ffff8801db300000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000020ea0000 CR3: 00000001bfecf001 CR4: 00000000001606e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  sock_sendmsg_nosec net/socket.c:628 [inline]
>  sock_sendmsg+0xca/0x110 net/socket.c:638
>  kernel_sendmsg+0x47/0x60 net/socket.c:646
>  sock_no_sendpage+0x1cc/0x280 net/core/sock.c:2581
>  kernel_sendpage+0xbf/0xe0 net/socket.c:3349
>  kcm_write_msgs+0x404/0x1b80 net/kcm/kcmsock.c:646
>  kcm_sendmsg+0x148d/0x22d0 net/kcm/kcmsock.c:1035
>  sock_sendmsg_nosec net/socket.c:628 [inline]
>  sock_sendmsg+0xca/0x110 net/socket.c:638
>  ___sys_sendmsg+0x767/0x8b0 net/socket.c:2018
>  __sys_sendmsg+0xe5/0x210 net/socket.c:2052
>  SYSC_sendmsg net/socket.c:2063 [inline]
>  SyS_sendmsg+0x2d/0x50 net/socket.c:2059
>  entry_SYSCALL_64_fastpath+0x23/0x9a
> RIP: 0033:0x440159
> RSP: 002b:00007ffe74df8288 EFLAGS: 00000217 ORIG_RAX: 000000000000002e
> RAX: ffffffffffffffda RBX: ffffffffffffffff RCX: 0000000000440159
> RDX: 0000000000000000 RSI: 00000000201fcfc8 RDI: 0000000000000005
> RBP: 00000000006ca018 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000217 R12: 0000000000401ac0
> R13: 0000000000401b50 R14: 0000000000000000 R15: 0000000000000000
> Code: c5 61 70 fc 48 8b 7d d0 e8 7c c2 5b fd 84 c0 74 0d e8 b3 61 70 fc 48 89 df e8 3b 49 2f ff 41 bd f7 ff ff ff eb 86 e8 9e 61 70 fc <0f> 0b 41 bd 95 ff ff ff e9 74 ff ff ff e8 ec 32 a8 fc e9 77 fb
> RIP: pppol2tp_sock_to_session net/l2tp/l2tp_ppp.c:176 [inline] RSP: ffff8801d4887438
> RIP: pppol2tp_sendmsg+0x512/0x670 net/l2tp/l2tp_ppp.c:304 RSP: ffff8801d4887438
>
> Reported-by: syzbot+114b15f2be420a8886c3@...kaller.appspotmail.com
> Fixes: ab7ac4eb9832 ("kcm: Kernel Connection Multiplexor module")
> Signed-off-by: James Chapman <jchapman@...alix.com>
> ---
>  net/kcm/kcmsock.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
> index d4e98f20fc2a..65392ed58f4a 100644
> --- a/net/kcm/kcmsock.c
> +++ b/net/kcm/kcmsock.c
> @@ -1391,6 +1391,10 @@ static int kcm_attach(struct socket *sock, struct socket *csock,
>         if (csk->sk_family == PF_KCM)
>                 return -EOPNOTSUPP;
>
> +       /* Cannot proceed if connected socket already uses sk_user_data */
> +       if (csk->sk_user_data)
> +               return -EOPNOTSUPP;
> +

sk_user_data is set with the sk_callback lock held in code below.
Should be able to take the lock earlier can do this check under the
lock.

Tom

>         psock = kmem_cache_zalloc(kcm_psockp, GFP_KERNEL);
>         if (!psock)
>                 return -ENOMEM;
> --
> 1.9.1
>