[PATCH v3] net/tcp: Fix a NULL pointer dereference when using TCP-AO with TCP_REPAIR

[Anderson Nascimento <anderson@xxxxxxxxxxxxxxxxxx>]

A NULL pointer dereference can occur in tcp_ao_finish_connect() during a
connect() system call on a socket with a TCP-AO key added and TCP_REPAIR
enabled.

The function is called with skb being NULL and attempts to dereference it
on tcp_hdr(skb)->seq without a prior skb validation.

Fix this by checking if skb is NULL before dereferencing it.

The commentary is taken from bpf_skops_established(), which is also called
in the same flow. Unlike the function being patched,
bpf_skops_established() validates the skb before dereferencing it.

int main(void){
	struct sockaddr_in sockaddr;
	struct tcp_ao_add tcp_ao;
	int sk;
	int one = 1;

	memset(&sockaddr,'\0',sizeof(sockaddr));
	memset(&tcp_ao,'\0',sizeof(tcp_ao));

	sk = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

	sockaddr.sin_family = AF_INET;

	memcpy(tcp_ao.alg_name,"cmac(aes128)",12);
	memcpy(tcp_ao.key,"ABCDEFGHABCDEFGH",16);
	tcp_ao.keylen = 16;

	memcpy(&tcp_ao.addr,&sockaddr,sizeof(sockaddr));

	setsockopt(sk, IPPROTO_TCP, TCP_AO_ADD_KEY, &tcp_ao,
	sizeof(tcp_ao));
	setsockopt(sk, IPPROTO_TCP, TCP_REPAIR, &one, sizeof(one));

	sockaddr.sin_family = AF_INET;
	sockaddr.sin_port = htobe16(123);

	inet_aton("127.0.0.1", &sockaddr.sin_addr);

	connect(sk,(struct sockaddr *)&sockaddr,sizeof(sockaddr));

return 0;
}

$ gcc tcp-ao-nullptr.c -o tcp-ao-nullptr -Wall
$ unshare -Urn
# ip addr add 127.0.0.1 dev lo
# ./tcp-ao-nullptr

BUG: kernel NULL pointer dereference, address: 00000000000000b6
PGD 1f648d067 P4D 1f648d067 PUD 1982e8067 PMD 0
Oops: Oops: 0000 [#1] SMP NOPTI
Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop
Reference Platform, BIOS 6.00 11/12/2020
RIP: 0010:tcp_ao_finish_connect (net/ipv4/tcp_ao.c:1182)

Fixes: 7c2ffaf ("net/tcp: Calculate TCP-AO traffic keys")
Signed-off-by: Anderson Nascimento <anderson@xxxxxxxxxxxxxxxxxx>
---
Changes in v3
- Remove the attribution of 'ao->risn' to '0' in the else case.
- Do not add the full decoded stack trace
- Link to v2: https://lore.kernel.org/all/20250911034337.43331-2-anderson@xxxxxxxxxxxxxxxxxx/
Changes in v2:
- Wrap the description at 75 columns
- Add full decoded stack trace
- Link to v1: https://lore.kernel.org/all/20250911013052.2233-1-anderson@xxxxxxxxxxxxxxxxxx/

 net/ipv4/tcp_ao.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_ao.c b/net/ipv4/tcp_ao.c
index bbb8d5f0eae7..3338b6cc85c4 100644
--- a/net/ipv4/tcp_ao.c
+++ b/net/ipv4/tcp_ao.c
@@ -1178,7 +1178,9 @@ void tcp_ao_finish_connect(struct sock *sk, struct sk_buff *skb)
 	if (!ao)
 		return;
 
-	WRITE_ONCE(ao->risn, tcp_hdr(skb)->seq);
+	/* sk with TCP_REPAIR_ON does not have skb in tcp_finish_connect */
+	if (skb)
+		WRITE_ONCE(ao->risn, tcp_hdr(skb)->seq);
 	ao->rcv_sne = 0;
 
 	hlist_for_each_entry_rcu(key, &ao->head, node, lockdep_sock_is_held(sk))
-- 
2.51.0