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

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Message-ID: <CAPhRvky96rzK1-fwtyv-59ao2YtNOBWmH2+tozcm052Q8e-nOA@mail.gmail.com>
Date: Thu, 11 Sep 2025 00:50:50 -0300
From: Anderson Nascimento <anderson@...elesecurity.com>
To: Kuniyuki Iwashima <kuniyu@...gle.com>
Cc: edumazet@...gle.com, ncardwell@...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
Subject: Re: [PATCH] net/tcp: Fix a NULL pointer dereference when using TCP-AO
 with TCP_REPAIR.

On Wed, Sep 10, 2025 at 11:44 PM Kuniyuki Iwashima <kuniyu@...gle.com> wrote:
>
> On Wed, Sep 10, 2025 at 6:32 PM 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.
>
> Thanks for the patch, the change looks good.
>
> Could you wrap the description at 75 columns ?
>
> See this doc for other guidelines:
> https://www.kernel.org/doc/html/latest/process/submitting-patches.html#the-canonical-patch-format
>
>

Thank you, Kuniyuki. I have just sent a v2 with the changes you
suggested. I hope it's fine now.

> >
> > 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
>
> Full decoded stack trace without timestamps would be nicer.
>
> How to decode stack trace:
> cat trace.txt | ./scripts/decode_stacktrace.sh vmlinux
>
> >
> > 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
> >



-- 
Anderson Nascimento
Allele Security Intelligence
https://www.allelesecurity.com