netfilter: nf_tables: fix NULL pointer dereference on nft_ct_helper_obj_dump()

In the nft_ct_helper_obj_dump(), always priv->helper4 is dereferenced.
But if family is ipv6, priv->helper6 should be dereferenced.

Steps to reproduces:

   #test.nft
   table ip6 filter {
	   ct helper ftp {
		   type "ftp" protocol tcp
	   }
	   chain input {
		   type filter hook input priority 4;
		   ct helper set "ftp"
	   }
   }

   %nft -f test.nft
   %nft list ruleset

we can see the below messages:

[  916.286233] kasan: GPF could be caused by NULL-ptr deref or user memory access
[  916.294777] general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN PTI
[  916.302613] Modules linked in: nft_objref nf_conntrack_sip nf_conntrack_snmp nf_conntrack_broadcast nf_conntrack_ftp nft_ct nf_conntrack nf_tables nfnetlink [last unloaded: nfnetlink]
[  916.318758] CPU: 1 PID: 2093 Comm: nft Not tainted 4.17.0-rc4+ #181
[  916.326772] Hardware name: To be filled by O.E.M. To be filled by O.E.M./Aptio CRB, BIOS 5.6.5 07/08/2015
[  916.338773] RIP: 0010:strlen+0x1a/0x90
[  916.342781] RSP: 0018:ffff88010ff0f2f8 EFLAGS: 00010292
[  916.346773] RAX: dffffc0000000000 RBX: ffff880119b26ee8 RCX: ffff88010c150038
[  916.354777] RDX: 0000000000000002 RSI: ffff880119b26ee8 RDI: 0000000000000010
[  916.362773] RBP: 0000000000000010 R08: 0000000000007e88 R09: ffff88010c15003c
[  916.370773] R10: ffff88010c150037 R11: ffffed002182a007 R12: ffff88010ff04040
[  916.378779] R13: 0000000000000010 R14: ffff880119b26f30 R15: ffff88010ff04110
[  916.387265] FS:  00007f57a1997700(0000) GS:ffff88011b800000(0000) knlGS:0000000000000000
[  916.394785] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  916.402778] CR2: 00007f57a0ac80f0 CR3: 000000010ff02000 CR4: 00000000001006e0
[  916.410772] Call Trace:
[  916.414787]  nft_ct_helper_obj_dump+0x94/0x200 [nft_ct]
[  916.418779]  ? nft_ct_set_eval+0x560/0x560 [nft_ct]
[  916.426771]  ? memset+0x1f/0x40
[  916.426771]  ? __nla_reserve+0x92/0xb0
[  916.434774]  ? memcpy+0x34/0x50
[  916.434774]  nf_tables_fill_obj_info+0x484/0x860 [nf_tables]
[  916.442773]  ? __nft_release_basechain+0x600/0x600 [nf_tables]
[  916.450779]  ? lock_acquire+0x193/0x380
[  916.454771]  ? lock_acquire+0x193/0x380
[  916.458789]  ? nf_tables_dump_obj+0x148/0xcb0 [nf_tables]
[  916.462777]  nf_tables_dump_obj+0x5f0/0xcb0 [nf_tables]
[  916.470769]  ? __alloc_skb+0x30b/0x500
[  916.474779]  netlink_dump+0x752/0xb50
[  916.478775]  __netlink_dump_start+0x4d3/0x750
[  916.482784]  nf_tables_getobj+0x27a/0x930 [nf_tables]
[  916.490774]  ? nft_obj_notify+0x100/0x100 [nf_tables]
[  916.494772]  ? nf_tables_getobj+0x930/0x930 [nf_tables]
[  916.502579]  ? nf_tables_dump_flowtable_done+0x70/0x70 [nf_tables]
[  916.506774]  ? nft_obj_notify+0x100/0x100 [nf_tables]
[  916.514808]  nfnetlink_rcv_msg+0x8ab/0xa86 [nfnetlink]
[  916.518771]  ? nfnetlink_rcv_msg+0x550/0xa86 [nfnetlink]
[  916.526782]  netlink_rcv_skb+0x23e/0x360
[  916.530773]  ? nfnetlink_bind+0x200/0x200 [nfnetlink]
[  916.534778]  ? debug_check_no_locks_freed+0x280/0x280
[  916.542770]  ? netlink_ack+0x870/0x870
[  916.546786]  ? ns_capable_common+0xf4/0x130
[  916.550765]  nfnetlink_rcv+0x172/0x16c0 [nfnetlink]
[  916.554771]  ? sched_clock_local+0xe2/0x150
[  916.558774]  ? sched_clock_cpu+0x144/0x180
[  916.566575]  ? lock_acquire+0x380/0x380
[  916.570775]  ? sched_clock_local+0xe2/0x150
[  916.574765]  ? nfnetlink_net_init+0x130/0x130 [nfnetlink]
[  916.578763]  ? sched_clock_cpu+0x144/0x180
[  916.582770]  ? lock_acquire+0x193/0x380
[  916.590771]  ? lock_acquire+0x193/0x380
[  916.594766]  ? lock_acquire+0x380/0x380
[  916.598760]  ? netlink_deliver_tap+0x262/0xa60
[  916.602766]  ? lock_acquire+0x193/0x380
[  916.606766]  netlink_unicast+0x3ef/0x5a0
[  916.610771]  ? netlink_attachskb+0x630/0x630
[  916.614763]  netlink_sendmsg+0x72a/0xb00
[  916.618769]  ? netlink_unicast+0x5a0/0x5a0
[  916.626766]  ? _copy_from_user+0x92/0xc0
[  916.630773]  __sys_sendto+0x202/0x300
[  916.634772]  ? __ia32_sys_getpeername+0xb0/0xb0
[  916.638759]  ? lock_acquire+0x380/0x380
[  916.642769]  ? lock_acquire+0x193/0x380
[  916.646761]  ? finish_task_switch+0xf4/0x560
[  916.650763]  ? __schedule+0x582/0x19a0
[  916.655301]  ? __sched_text_start+0x8/0x8
[  916.655301]  ? up_read+0x1c/0x110
[  916.655301]  ? __do_page_fault+0x48b/0xaa0
[  916.655301]  ? entry_SYSCALL_64_after_hwframe+0x59/0xbe
[  916.655301]  __x64_sys_sendto+0xdd/0x1b0
[  916.655301]  do_syscall_64+0x96/0x3d0
[  916.655301]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[  916.655301] RIP: 0033:0x7f57a0ff5e03
[  916.655301] RSP: 002b:00007fff6367e0a8 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
[  916.655301] RAX: ffffffffffffffda RBX: 00007fff6367f1e0 RCX: 00007f57a0ff5e03
[  916.655301] RDX: 0000000000000020 RSI: 00007fff6367e110 RDI: 0000000000000003
[  916.655301] RBP: 00007fff6367e100 R08: 00007f57a0ce9160 R09: 000000000000000c
[  916.655301] R10: 0000000000000000 R11: 0000000000000246 R12: 00007fff6367e110
[  916.655301] R13: 0000000000000020 R14: 00007f57a153c610 R15: 0000562417258de0
[  916.655301] Code: ff ff ff 0f 1f 40 00 66 2e 0f 1f 84 00 00 00 00 00 55 48 89 fa 53 48 c1 ea 03 48 b8 00 00 00 00 00 fc ff df 48 89 fd 48 83 ec 08 <0f> b6 04 02 48 89 fa 83 e2 07 38 d0 7f
[  916.655301] RIP: strlen+0x1a/0x90 RSP: ffff88010ff0f2f8
[  916.771929] ---[ end trace 1065e048e72479fe ]---
[  916.777204] Kernel panic - not syncing: Fatal exception
[  916.778158] Kernel Offset: 0x14000000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_ct.c

@@ -880,22 +880,26 @@ static int nft_ct_helper_obj_dump(struct sk_buff *skb,
 				  struct nft_object *obj, bool reset)
 {
 	const struct nft_ct_helper_obj *priv = nft_obj_data(obj);
-	const struct nf_conntrack_helper *helper = priv->helper4;
+	const struct nf_conntrack_helper *helper;
 	u16 family;
 
+	if (priv->helper4 && priv->helper6) {
+		family = NFPROTO_INET;
+		helper = priv->helper4;
+	} else if (priv->helper6) {
+		family = NFPROTO_IPV6;
+		helper = priv->helper6;
+	} else {
+		family = NFPROTO_IPV4;
+		helper = priv->helper4;
+	}
+
 	if (nla_put_string(skb, NFTA_CT_HELPER_NAME, helper->name))
 		return -1;
 
 	if (nla_put_u8(skb, NFTA_CT_HELPER_L4PROTO, priv->l4proto))
 		return -1;
 
-	if (priv->helper4 && priv->helper6)
-		family = NFPROTO_INET;
-	else if (priv->helper6)
-		family = NFPROTO_IPV6;
-	else
-		family = NFPROTO_IPV4;
-
 	if (nla_put_be16(skb, NFTA_CT_HELPER_L3PROTO, htons(family)))
 		return -1;