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

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Message-ID: <a29689e0-cabc-4fdb-a030-443f0ccfb468@linux.dev>
Date: Tue, 9 Sep 2025 22:15:37 -0700
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Kuniyuki Iwashima <kuniyu@...gle.com>
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 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);


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