[PATCH nf] netfilter: ip6t_SYNPROXY: check TCP header before verifying checksum

[Palla Raghunath <raghunathpalla.0209@gmail.com>]

synproxy_tg6() passes par->thoff to nf_ip6_checksum() before it has
checked that a TCP header is actually present at that offset.

par->thoff comes from ipv6_find_hdr(), called by ip6_packet_match()
with target -1. ipv6_find_hdr() only checks that each extension header's
first two bytes are in the skb. It then adds that header's declared
length to the offset and stops at the first non-extension header, so it
can return an offset past the end of the packet. With
CHECKSUM_NONE, nf_ip6_checksum() calls skb_checksum(skb, 0, thoff, 0).
When thoff is larger than skb->len, the BUG_ON(len) in skb_checksum()
fires:

  kernel BUG at net/core/skbuff.c:3606!
  RIP: 0010:skb_checksum+0x8b2/0x8c0
  Call Trace:
   nf_ip6_checksum+0x1bd/0x320 net/netfilter/utils.c:89
   synproxy_tg6+0x1a4/0x6e0 net/ipv6/netfilter/ip6t_SYNPROXY.c:21
   ip6t_do_table+0xd37/0x15b0 net/ipv6/netfilter/ip6_tables.c:366
   ...

In the syzbot reproducer, a 60-byte packet carries an AH header, then two
hop-by-hop headers. The last hop-by-hop header has nexthdr TCP and
hdrlen 167, so thoff is 1400.

Fetch the TCP header with skb_header_pointer() first, as
nf_reject_ip6_tcphdr_get() already does. This drops packets that do not
contain a full TCP header at thoff before the checksum is computed.

Fixes: 4ad362282cb4 ("netfilter: add IPv6 SYNPROXY target")
Reported-by: syzbot+5a8667f002726fc59f88@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=5a8667f002726fc59f88
Signed-off-by: Palla Raghunath <raghunathpalla.0209@gmail.com>
---
 net/ipv6/netfilter/ip6t_SYNPROXY.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/net/ipv6/netfilter/ip6t_SYNPROXY.c b/net/ipv6/netfilter/ip6t_SYNPROXY.c
index d51d0c3e5fe9..04db1f82420b 100644
--- a/net/ipv6/netfilter/ip6t_SYNPROXY.c
+++ b/net/ipv6/netfilter/ip6t_SYNPROXY.c
@@ -18,13 +18,16 @@ synproxy_tg6(struct sk_buff *skb, const struct xt_action_param *par)
 	struct synproxy_options opts = {};
 	struct tcphdr *th, _th;
 
-	if (nf_ip6_checksum(skb, xt_hooknum(par), par->thoff, IPPROTO_TCP))
-		return NF_DROP;
-
+	/* par->thoff may point past the end of the packet; make sure a
+	 * full TCP header is present before checksumming up to it.
+	 */
 	th = skb_header_pointer(skb, par->thoff, sizeof(_th), &_th);
 	if (th == NULL)
 		return NF_DROP;
 
+	if (nf_ip6_checksum(skb, xt_hooknum(par), par->thoff, IPPROTO_TCP))
+		return NF_DROP;
+
 	if (!synproxy_parse_options(skb, par->thoff, th, &opts))
 		return NF_DROP;
 
-- 
2.34.1