netdev - Re: [PATCH v1 bpf] tcp_bpf: Call sk_msg_free() when tcp_bpf_send

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Message-ID: <CAAVpQUDeaiGUdxGQHSMRU3=zwJy7a0hMWXjoRkfdYPqaZLU09Q@mail.gmail.com>
Date: Tue, 9 Sep 2025 23:56:53 -0700
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: John Fastabend <john.fastabend@...il.com>, Jakub Sitnicki <jakub@...udflare.com>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org, bpf@...r.kernel.org, 
	syzbot+4cabd1d2fa917a456db8@...kaller.appspotmail.com
Subject: Re: [PATCH v1 bpf] tcp_bpf: Call sk_msg_free() when
 tcp_bpf_send_verdict() fails to allocate psock->cork.

On Tue, Sep 9, 2025 at 10:15 PM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
>
> On 9/9/25 4:26 PM, Kuniyuki Iwashima wrote:
> > syzbot reported the splat below. [0]
> >
> > The repro does the following:
> >
> >    1. Load a sk_msg prog that calls bpf_msg_cork_bytes(msg, cork_bytes)
> >    2. Attach the prog to a SOCKMAP
> >    3. Add a socket to the SOCKMAP
> >    4. Activate fault injection
> >    5. Send data less than cork_bytes
> >
> > At 5., the data is carried over to the next sendmsg() as it is
> > smaller than the cork_bytes specified by bpf_msg_cork_bytes().
> >
> > Then, tcp_bpf_send_verdict() tries to allocate psock->cork to hold
> > the data, but this fails silently due to fault injection + __GFP_NOWARN.
> >
> > If the allocation fails, we need to revert the sk->sk_forward_alloc
> > change done by sk_msg_alloc().
> >
> > Let's call sk_msg_free() when tcp_bpf_send_verdict fails to allocate
> > psock->cork.
> >
> > [0]:
> > WARNING: net/ipv4/af_inet.c:156 at inet_sock_destruct+0x623/0x730 net/ipv4/af_inet.c:156, CPU#1: syz-executor/5983
> > Modules linked in:
> > CPU: 1 UID: 0 PID: 5983 Comm: syz-executor Not tainted syzkaller #0 PREEMPT(full)
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
> > RIP: 0010:inet_sock_destruct+0x623/0x730 net/ipv4/af_inet.c:156
> > Code: 0f 0b 90 e9 62 fe ff ff e8 7a db b5 f7 90 0f 0b 90 e9 95 fe ff ff e8 6c db b5 f7 90 0f 0b 90 e9 bb fe ff ff e8 5e db b5 f7 90 <0f> 0b 90 e9 e1 fe ff ff 89 f9 80 e1 07 80 c1 03 38 c1 0f 8c 9f fc
> > RSP: 0018:ffffc90000a08b48 EFLAGS: 00010246
> > RAX: ffffffff8a09d0b2 RBX: dffffc0000000000 RCX: ffff888024a23c80
> > RDX: 0000000000000100 RSI: 0000000000000fff RDI: 0000000000000000
> > RBP: 0000000000000fff R08: ffff88807e07c627 R09: 1ffff1100fc0f8c4
> > R10: dffffc0000000000 R11: ffffed100fc0f8c5 R12: ffff88807e07c380
> > R13: dffffc0000000000 R14: ffff88807e07c60c R15: 1ffff1100fc0f872
> > FS:  00005555604c4500(0000) GS:ffff888125af1000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00005555604df5c8 CR3: 0000000032b06000 CR4: 00000000003526f0
> > Call Trace:
> >   <IRQ>
> >   __sk_destruct+0x86/0x660 net/core/sock.c:2339
> >   rcu_do_batch kernel/rcu/tree.c:2605 [inline]
> >   rcu_core+0xca8/0x1770 kernel/rcu/tree.c:2861
> >   handle_softirqs+0x286/0x870 kernel/softirq.c:579
> >   __do_softirq kernel/softirq.c:613 [inline]
> >   invoke_softirq kernel/softirq.c:453 [inline]
> >   __irq_exit_rcu+0xca/0x1f0 kernel/softirq.c:680
> >   irq_exit_rcu+0x9/0x30 kernel/softirq.c:696
> >   instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1052 [inline]
> >   sysvec_apic_timer_interrupt+0xa6/0xc0 arch/x86/kernel/apic/apic.c:1052
> >   </IRQ>
> >
> > Fixes: 4f738adba30a ("bpf: create tcp_bpf_ulp allowing BPF to monitor socket TX/RX data")
> > Reported-by: syzbot+4cabd1d2fa917a456db8@...kaller.appspotmail.com
> > Closes: https://lore.kernel.org/netdev/68c0b6b5.050a0220.3c6139.0013.GAE@google.com/
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@...gle.com>
> > ---
> >   net/ipv4/tcp_bpf.c | 4 +++-
> >   1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
> > index ba581785adb4..ee6a371e65a4 100644
> > --- a/net/ipv4/tcp_bpf.c
> > +++ b/net/ipv4/tcp_bpf.c
> > @@ -408,8 +408,10 @@ static int tcp_bpf_send_verdict(struct sock *sk, struct sk_psock *psock,
> >               if (!psock->cork) {
> >                       psock->cork = kzalloc(sizeof(*psock->cork),
> >                                             GFP_ATOMIC | __GFP_NOWARN);
> > -                     if (!psock->cork)
> > +                     if (!psock->cork) {
> > +                             sk_msg_free(sk, msg);
>
> Nothing has been corked yet, does it need to update the "*copied":
>
>                                 *copied -= sk_msg_free(sk, msg);

Oh exactly, or simply *copied = 0 ?


>
>
> >                               return -ENOMEM;
> > +                     }
> >               }
> >               memcpy(psock->cork, msg, sizeof(*msg));
> >               return 0;
>
>