apparmor_secid_to_secctx() has a bad debug statement tripping on a
condition handle by the code.  When kconfig SECURITY_APPARMOR_DEBUG is
enabled the debug WARN_ON will trip when **secdata is NULL resulting
in the following trace.

------------[ cut here ]------------
AppArmor WARN apparmor_secid_to_secctx: ((!secdata)):
WARNING: CPU: 0 PID: 14826 at security/apparmor/secid.c:82 apparmor_secid_to_secctx+0x2b5/0x2f0 security/apparmor/secid.c:82
Kernel panic - not syncing: panic_on_warn set ...

CPU: 0 PID: 14826 Comm: syz-executor1 Not tainted 4.19.0-rc1+ #193
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1c9/0x2b4 lib/dump_stack.c:113
 panic+0x238/0x4e7 kernel/panic.c:184
 __warn.cold.8+0x163/0x1ba kernel/panic.c:536
 report_bug+0x252/0x2d0 lib/bug.c:186
 fixup_bug arch/x86/kernel/traps.c:178 [inline]
 do_error_trap+0x1fc/0x4d0 arch/x86/kernel/traps.c:296
 do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:316
 invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:993
RIP: 0010:apparmor_secid_to_secctx+0x2b5/0x2f0 security/apparmor/secid.c:82
Code: c7 c7 40 66 58 87 e8 6a 6d 0f fe 0f 0b e9 6c fe ff ff e8 3e aa 44 fe 48 c7 c6 80 67 58 87 48 c7 c7 a0 65 58 87 e8 4b 6d 0f fe <0f> 0b e9 3f fe ff ff 48 89 df e8 fc a7 83 fe e9 ed fe ff ff bb f4
RSP: 0018:ffff8801ba1bed10 EFLAGS: 00010286
RAX: 0000000000000000 RBX: ffff8801ba1beed0 RCX: ffffc9000227e000
RDX: 0000000000018482 RSI: ffffffff8163ac01 RDI: 0000000000000001
RBP: ffff8801ba1bed30 R08: ffff8801b80ec080 R09: ffffed003b603eca
R10: ffffed003b603eca R11: ffff8801db01f657 R12: 0000000000000001
R13: 0000000000000000 R14: 0000000000000000 R15: ffff8801ba1beed0
 security_secid_to_secctx+0x63/0xc0 security/security.c:1314
 ctnetlink_secctx_size net/netfilter/nf_conntrack_netlink.c:621 [inline]
 ctnetlink_nlmsg_size net/netfilter/nf_conntrack_netlink.c:659 [inline]
 ctnetlink_conntrack_event+0x303/0x1470 net/netfilter/nf_conntrack_netlink.c:706
 nf_conntrack_eventmask_report+0x55f/0x930 net/netfilter/nf_conntrack_ecache.c:151
 nf_conntrack_event_report include/net/netfilter/nf_conntrack_ecache.h:112 [inline]
 nf_ct_delete+0x33c/0x5d0 net/netfilter/nf_conntrack_core.c:601
 nf_ct_iterate_cleanup+0x48c/0x5e0 net/netfilter/nf_conntrack_core.c:1892
 nf_ct_iterate_cleanup_net+0x23c/0x2d0 net/netfilter/nf_conntrack_core.c:1974
 ctnetlink_flush_conntrack net/netfilter/nf_conntrack_netlink.c:1226 [inline]
 ctnetlink_del_conntrack+0x66c/0x850 net/netfilter/nf_conntrack_netlink.c:1258
 nfnetlink_rcv_msg+0xd88/0x1070 net/netfilter/nfnetlink.c:228
 netlink_rcv_skb+0x172/0x440 net/netlink/af_netlink.c:2454
 nfnetlink_rcv+0x1c0/0x4d0 net/netfilter/nfnetlink.c:560
 netlink_unicast_kernel net/netlink/af_netlink.c:1317 [inline]
 netlink_unicast+0x5a0/0x760 net/netlink/af_netlink.c:1343
 netlink_sendmsg+0xa18/0xfc0 net/netlink/af_netlink.c:1908
 sock_sendmsg_nosec net/socket.c:621 [inline]
 sock_sendmsg+0xd5/0x120 net/socket.c:631
 ___sys_sendmsg+0x7fd/0x930 net/socket.c:2114
 __sys_sendmsg+0x11d/0x290 net/socket.c:2152
 __do_sys_sendmsg net/socket.c:2161 [inline]
 __se_sys_sendmsg net/socket.c:2159 [inline]
 __x64_sys_sendmsg+0x78/0xb0 net/socket.c:2159
 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x457089
Code: fd b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 cb b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f7bc6e03c78 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007f7bc6e046d4 RCX: 0000000000457089
RDX: 0000000000000000 RSI: 0000000020d65000 RDI: 0000000000000003
RBP: 00000000009300a0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 00000000004d4588 R14: 00000000004c8d5c R15: 0000000000000000
Dumping ftrace buffer:
   (ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..

CC: <stable@vger.kernel.org> #4.18
Fixes: c0929212 ("apparmor: add support for mapping secids and using secctxes")
Reported-by: syzbot+21016130b0580a9de3b5@syzkaller.appspotmail.com
Signed-off-by: John Johansen <john.johansen@canonical.com>