Re: [PATCH v1 bpf] tcp_bpf: Call sk_msg_free() when tcp_bpf_send_verdict() fails to allocate psock->cork.

[Martin KaFai Lau <martin.lau@linux.dev>]

On 9/9/25 11:56 PM, Kuniyuki Iwashima wrote:
> On Tue, Sep 9, 2025 at 10:15 PM Martin KaFai Lau <martin.lau@linux.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@syzkaller.appspotmail.com
>>> Closes: https://lore.kernel.org/netdev/68c0b6b5.050a0220.3c6139.0013.GAE@google.com/
>>> Signed-off-by: Kuniyuki Iwashima <kuniyu@google.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 ?

Make sense. I made the change and updated the commit message for this fix also. 
Applied. Thanks.