netdev - Re: [PATCH v1 net-next] net: sched: Don't use WARN_ON_ONCE() for -ENOMEM in tcf

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Message-ID: <CAM0EoM=3O3Nyy7rb47WxxWz-KN9xxabc5LpQz3ZhhWp7V-9c+w@mail.gmail.com>
Date: Tue, 28 Oct 2025 08:08:40 -0400
From: Jamal Hadi Salim <jhs@...atatu.com>
To: Kuniyuki Iwashima <kuniyu@...gle.com>
Cc: Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org, 
	syzbot+87e1289a044fcd0c5f62@...kaller.appspotmail.com
Subject: Re: [PATCH v1 net-next] net: sched: Don't use WARN_ON_ONCE() for
 -ENOMEM in tcf_classify().

On Mon, Oct 27, 2025 at 11:59 PM Kuniyuki Iwashima <kuniyu@...gle.com> wrote:
>
> As demonstrated by syzbot, WARN_ON_ONCE() in tcf_classify() can
> be easily triggered by fault injection. [0]
>
> We should not use WARN_ON_ONCE() for the simple -ENOMEM case.
>
> Also, we provide SKB_DROP_REASON_NOMEM for the same error.
>
> Let's remove WARN_ON_ONCE() there.
>
> [0]:
> FAULT_INJECTION: forcing a failure.
> name failslab, interval 1, probability 0, space 0, times 0
> CPU: 0 UID: 0 PID: 31392 Comm: syz.8.7081 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0x189/0x250
>  should_fail_ex+0x414/0x560
>  should_failslab+0xa8/0x100
>  kmem_cache_alloc_noprof+0x74/0x6e0
>  skb_ext_add+0x148/0x8f0
>  tcf_classify+0xeba/0x1140
>  multiq_enqueue+0xfd/0x4c0 net/sched/sch_multiq.c:66
> ...
> WARNING: CPU: 0 PID: 31392 at net/sched/cls_api.c:1869 tcf_classify+0xfd7/0x1140
> Modules linked in:
> CPU: 0 UID: 0 PID: 31392 Comm: syz.8.7081 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
> RIP: 0010:tcf_classify+0xfd7/0x1140
> Code: e8 03 42 0f b6 04 30 84 c0 0f 85 41 01 00 00 66 41 89 1f eb 05 e8 89 26 75 f8 bb ff ff ff ff e9 04 f9 ff ff e8 7a 26 75 f8 90 <0f> 0b 90 49 83 c5 44 4c 89 eb 49 c1 ed 03 43 0f b6 44 35 00 84 c0
> RSP: 0018:ffffc9000b7671f0 EFLAGS: 00010293
> RAX: ffffffff894addf6 RBX: 0000000000000002 RCX: ffff888025029e40
> RDX: 0000000000000000 RSI: ffffffff8bbf05c0 RDI: ffffffff8bbf0580
> RBP: 0000000000000000 R08: 00000000ffffffff R09: 1ffffffff1c0bfd6
> R10: dffffc0000000000 R11: fffffbfff1c0bfd7 R12: ffff88805a90de5c
> R13: ffff88805a90ddc0 R14: dffffc0000000000 R15: ffffc9000b7672c0
> FS:  00007f20739f66c0(0000) GS:ffff88812613e000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000000110c2d2a80 CR3: 0000000024e36000 CR4: 00000000003526f0
> Call Trace:
>  <TASK>
>  multiq_classify net/sched/sch_multiq.c:39 [inline]
>  multiq_enqueue+0xfd/0x4c0 net/sched/sch_multiq.c:66
>  dev_qdisc_enqueue+0x4e/0x260 net/core/dev.c:4118
>  __dev_xmit_skb net/core/dev.c:4214 [inline]
>  __dev_queue_xmit+0xe83/0x3b50 net/core/dev.c:4729
>  packet_snd net/packet/af_packet.c:3076 [inline]
>  packet_sendmsg+0x3e33/0x5080 net/packet/af_packet.c:3108
>  sock_sendmsg_nosec net/socket.c:727 [inline]
>  __sock_sendmsg+0x21c/0x270 net/socket.c:742
>  ____sys_sendmsg+0x505/0x830 net/socket.c:2630
>  ___sys_sendmsg+0x21f/0x2a0 net/socket.c:2684
>  __sys_sendmsg net/socket.c:2716 [inline]
>  __do_sys_sendmsg net/socket.c:2721 [inline]
>  __se_sys_sendmsg net/socket.c:2719 [inline]
>  __x64_sys_sendmsg+0x19b/0x260 net/socket.c:2719
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f207578efc9
> Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007f20739f6038 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
> RAX: ffffffffffffffda RBX: 00007f20759e5fa0 RCX: 00007f207578efc9
> RDX: 0000000000000004 RSI: 00002000000000c0 RDI: 0000000000000008
> RBP: 00007f20739f6090 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
> R13: 00007f20759e6038 R14: 00007f20759e5fa0 R15: 00007f2075b0fa28
>  </TASK>
>
> Reported-by: syzbot+87e1289a044fcd0c5f62@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/netdev/69003e33.050a0220.32483.00e8.GAE@google.com/
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...gle.com>

Reviewed-by: Jamal Hadi Salim <jhs@...atatu.com>

cheers,
jamal
> ---
> targetting net-next since there's no real bug.
> ---
>  net/sched/cls_api.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
> index ecec0a1e1c1a..f751cd5eeac8 100644
> --- a/net/sched/cls_api.c
> +++ b/net/sched/cls_api.c
> @@ -1866,7 +1866,7 @@ int tcf_classify(struct sk_buff *skb,
>                         struct tc_skb_cb *cb = tc_skb_cb(skb);
>
>                         ext = tc_skb_ext_alloc(skb);
> -                       if (WARN_ON_ONCE(!ext)) {
> +                       if (!ext) {
>                                 tcf_set_drop_reason(skb, SKB_DROP_REASON_NOMEM);
>                                 return TC_ACT_SHOT;
>                         }
> --
> 2.51.1.838.g19442a804e-goog
>