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

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Message-ID: <20250911013052.2233-1-anderson@allelesecurity.com>
Date: Wed, 10 Sep 2025 22:30:52 -0300
From: Anderson Nascimento <anderson@...elesecurity.com>
To: edumazet@...gle.com,
	ncardwell@...gle.com,
	kuniyu@...gle.com,
	davem@...emloft.net,
	dsahern@...nel.org,
	kuba@...nel.org,
	pabeni@...hat.com,
	horms@...nel.org,
	netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: Anderson Nascimento <anderson@...elesecurity.com>
Subject: [PATCH] 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. If skb is not NULL, the ao->risn is set to tcp_hdr(skb)->seq to keep compatibility with the call made from tcp_rcv_synsent_state_process(). If skb is NULL, ao->risn is set to 0.

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

[   72.414850] BUG: kernel NULL pointer dereference, address: 00000000000000b6
[   72.414863] #PF: supervisor read access in kernel mode
[   72.414869] #PF: error_code(0x0000) - not-present page
[   72.414873] PGD 116af4067 P4D 116af4067 PUD 117043067 PMD 0
[   72.414880] Oops: Oops: 0000 [#1] SMP NOPTI
[   72.414887] CPU: 2 UID: 1000 PID: 1558 Comm: tcp-ao-nullptr Not tainted 6.16.3-200.fc42.x86_64 #1 PREEMPT(lazy)
[   72.414896] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 11/12/2020
[   72.414905] RIP: 0010:tcp_ao_finish_connect+0x19/0x60

Fixes: 7c2ffaf ("net/tcp: Calculate TCP-AO traffic keys")
Signed-off-by: Anderson Nascimento <anderson@...elesecurity.com>
---
 net/ipv4/tcp_ao.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_ao.c b/net/ipv4/tcp_ao.c
index bbb8d5f0eae7..abe913de8652 100644
--- a/net/ipv4/tcp_ao.c
+++ b/net/ipv4/tcp_ao.c
@@ -1178,7 +1178,11 @@ 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);
+	else
+		WRITE_ONCE(ao->risn, 0);
 	ao->rcv_sne = 0;
 
 	hlist_for_each_entry_rcu(key, &ao->head, node, lockdep_sock_is_held(sk))
-- 
2.51.0