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

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Message-ID: <CAJwJo6bsZg-arM6GAQM8Lv3DivWUERu0VyFQgi4DA+SxRrZypw@mail.gmail.com>
Date: Fri, 12 Sep 2025 00:26:59 +0100
From: Dmitry Safonov <0x7f454c46@...il.com>
To: Anderson Nascimento <anderson@...elesecurity.com>
Cc: Eric Dumazet <edumazet@...gle.com>, Neal Cardwell <ncardwell@...gle.com>, 
	Kuniyuki Iwashima <kuniyu@...gle.com>, "David S. Miller" <davem@...emloft.net>, 
	David Ahern <dsahern@...nel.org>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Simon Horman <horms@...nel.org>, Salam Noureddine <noureddine@...sta.com>, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH v3] net/tcp: Fix a NULL pointer dereference when using
 TCP-AO with TCP_REPAIR

On Fri, 12 Sept 2025 at 00:23, Anderson Nascimento
<anderson@...elesecurity.com> wrote:
>
> 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@...elesecurity.com>

LGTM, thanks for your fix!

Reviewed-by: Dmitry Safonov <0x7f454c46@...il.com>

Thanks,
             Dmitry