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

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

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

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: 7c2ffaf21bd6 ("net/tcp: Calculate TCP-AO traffic keys")
Signed-off-by: Anderson Nascimento <anderson@allelesecurity.com>
Reviewed-by: Dmitry Safonov <0x7f454c46@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250911230743.2551-3-anderson@allelesecurity.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>