[PATCH net-next] kcm: do not attach sockets if sk_user_data is already used

[James Chapman <jchapman@katalix.com>]

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@syzkaller.appspotmail.com
Fixes: ab7ac4eb9832 ("kcm: Kernel Connection Multiplexor module")
Signed-off-by: James Chapman <jchapman@katalix.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;
+
 	psock = kmem_cache_zalloc(kcm_psockp, GFP_KERNEL);
 	if (!psock)
 		return -ENOMEM;
-- 
1.9.1