[PATCH v1 net] net/sched: cls_api: Don't replay RTM_GETCHAIN in tc_ctl_chain().

[PATCH v1 net] net/sched: cls_api: Don't replay RTM_GETCHAIN in tc_ctl_chain().

[Kuniyuki Iwashima]

If a netlink socket sends RTM_GETCHAIN requests repeatedly
without recv()ing the responses, tc_ctl_chain() hogs CPU and
triggers Hung Task splat. [0]

As caught in the stack trace, netlink_attachskb() could confuse
tc_ctl_chain() by returning -EAGAIN when the userspace netlink
socket's receive buffer is full.

The replay: label exists since commit 32a4f5ecd738 ("net: sched:
introduce chain object to uapi") but was not used initially.

Since commit 9f407f1768d3 ("net: sched: introduce chain templates"),
the label is needed for RTM_NEWCHAIN because tcf_proto_lookup_ops()
may release RTNL to call request_module().

However, the replay logic is unnecessary for RTM_GETCHAIN.

Let's apply the replay logic only for RTM_NEWCHAIN.

[0]:
INFO: task repro:1018 is blocked on a mutex likely owned by task repro:1022.
task:repro           state:R  running task     stack:14096 pid:1022  tgid:1014  ppid:961    task_flags:0x400040 flags:0x00080000
Call Trace:
 <TASK>
 ? clockevents_program_event (kernel/time/clockevents.c:372)
 ? pskb_expand_head (net/core/skbuff.c:615)
 ? skb_release_data (net/core/skbuff.c:1122)
 ? netlink_attachskb (./include/linux/skbuff.h:1323 ./include/linux/skbuff.h:1332 net/netlink/af_netlink.c:1232)
 ? __netlink_lookup (./include/linux/rcupdate.h:882 ./include/linux/rhashtable.h:711 net/netlink/af_netlink.c:499)
 ? tc_chain_notify (net/sched/cls_api.c:3045)
 ? tc_chain_notify (./include/linux/skbuff.h:1384 net/sched/cls_api.c:3041)
 ? netlink_unicast (net/netlink/af_netlink.c:1335)
 ? rtnl_unicast (./include/net/netlink.h:1198 net/core/rtnetlink.c:985)
 ? tc_ctl_chain (net/sched/cls_api.c:3242)
 ? rtnetlink_rcv_msg (net/core/rtnetlink.c:7146)
 ? netlink_unicast (net/netlink/af_netlink.c:1354)
 ? __pfx_rtnetlink_rcv_msg (net/core/rtnetlink.c:7177)
 ? netlink_rcv_skb (net/netlink/af_netlink.c:2556)
 ? netlink_unicast (net/netlink/af_netlink.c:1319)
 ? netlink_sendmsg (net/netlink/af_netlink.c:1900)
 ? __sock_sendmsg (net/socket.c:800)
 ? __sys_sendto (net/socket.c:2281)
 ? __x64_sys_sendto (net/socket.c:2288 net/socket.c:2284 net/socket.c:2284)
 ? do_syscall_64 (arch/x86/entry/syscall_64.c:61 arch/x86/entry/syscall_64.c:84)
 ? entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
 </TASK>

Fixes: 2ed9db3074fc ("net: sched: cls_api: fix dead code in switch")
Reported-by: Taras Madan <tarasmadan@google.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 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 9966766661d5..c47d2ee13641 100644
--- a/net/sched/cls_api.c
+++ b/net/sched/cls_api.c
@@ -3254,7 +3254,7 @@ static int tc_ctl_chain(struct sk_buff *skb, struct nlmsghdr *n,
 	tcf_chain_put(chain);
 errout_block:
 	tcf_block_release(q, block, true);
-	if (err == -EAGAIN)
+	if (err == -EAGAIN && n->nlmsg_type == RTM_NEWCHAIN)
 		/* Replay the request. */
 		goto replay;
 	return err;
-- 
2.55.0.979.g7e5102b832-goog

Re: [PATCH v1 net] net/sched: cls_api: Don't replay RTM_GETCHAIN in tc_ctl_chain().

[Jamal Hadi Salim]

On Tue, Sep 8, 2026 at 4:55 PM Kuniyuki Iwashima <kuniyu@google.com> wrote:
>
> If a netlink socket sends RTM_GETCHAIN requests repeatedly
> without recv()ing the responses, tc_ctl_chain() hogs CPU and
> triggers Hung Task splat. [0]
>
> As caught in the stack trace, netlink_attachskb() could confuse
> tc_ctl_chain() by returning -EAGAIN when the userspace netlink
> socket's receive buffer is full.
>
> The replay: label exists since commit 32a4f5ecd738 ("net: sched:
> introduce chain object to uapi") but was not used initially.
>
> Since commit 9f407f1768d3 ("net: sched: introduce chain templates"),
> the label is needed for RTM_NEWCHAIN because tcf_proto_lookup_ops()
> may release RTNL to call request_module().
>
> However, the replay logic is unnecessary for RTM_GETCHAIN.
>
> Let's apply the replay logic only for RTM_NEWCHAIN.
>

The patch looks sane.
Can you send me a reproducer please? We are automating and testing of
all tc submissions.

Reviewed-by: Jamal Hadi Salim

cheers,
jamal
> [0]:
> INFO: task repro:1018 is blocked on a mutex likely owned by task repro:1022.
> task:repro           state:R  running task     stack:14096 pid:1022  tgid:1014  ppid:961    task_flags:0x400040 flags:0x00080000
> Call Trace:
>  <TASK>
>  ? clockevents_program_event (kernel/time/clockevents.c:372)
>  ? pskb_expand_head (net/core/skbuff.c:615)
>  ? skb_release_data (net/core/skbuff.c:1122)
>  ? netlink_attachskb (./include/linux/skbuff.h:1323 ./include/linux/skbuff.h:1332 net/netlink/af_netlink.c:1232)
>  ? __netlink_lookup (./include/linux/rcupdate.h:882 ./include/linux/rhashtable.h:711 net/netlink/af_netlink.c:499)
>  ? tc_chain_notify (net/sched/cls_api.c:3045)
>  ? tc_chain_notify (./include/linux/skbuff.h:1384 net/sched/cls_api.c:3041)
>  ? netlink_unicast (net/netlink/af_netlink.c:1335)
>  ? rtnl_unicast (./include/net/netlink.h:1198 net/core/rtnetlink.c:985)
>  ? tc_ctl_chain (net/sched/cls_api.c:3242)
>  ? rtnetlink_rcv_msg (net/core/rtnetlink.c:7146)
>  ? netlink_unicast (net/netlink/af_netlink.c:1354)
>  ? __pfx_rtnetlink_rcv_msg (net/core/rtnetlink.c:7177)
>  ? netlink_rcv_skb (net/netlink/af_netlink.c:2556)
>  ? netlink_unicast (net/netlink/af_netlink.c:1319)
>  ? netlink_sendmsg (net/netlink/af_netlink.c:1900)
>  ? __sock_sendmsg (net/socket.c:800)
>  ? __sys_sendto (net/socket.c:2281)
>  ? __x64_sys_sendto (net/socket.c:2288 net/socket.c:2284 net/socket.c:2284)
>  ? do_syscall_64 (arch/x86/entry/syscall_64.c:61 arch/x86/entry/syscall_64.c:84)
>  ? entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
>  </TASK>
>
> Fixes: 2ed9db3074fc ("net: sched: cls_api: fix dead code in switch")
> Reported-by: Taras Madan <tarasmadan@google.com>
> Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
> ---
>  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 9966766661d5..c47d2ee13641 100644
> --- a/net/sched/cls_api.c
> +++ b/net/sched/cls_api.c
> @@ -3254,7 +3254,7 @@ static int tc_ctl_chain(struct sk_buff *skb, struct nlmsghdr *n,
>         tcf_chain_put(chain);
>  errout_block:
>         tcf_block_release(q, block, true);
> -       if (err == -EAGAIN)
> +       if (err == -EAGAIN && n->nlmsg_type == RTM_NEWCHAIN)
>                 /* Replay the request. */
>                 goto replay;
>         return err;
> --
> 2.55.0.979.g7e5102b832-goog
>

Re: [PATCH v1 net] net/sched: cls_api: Don't replay RTM_GETCHAIN in tc_ctl_chain().

[Kuniyuki Iwashima]

On Wed, Sep 9, 2026 at 3:01 AM Jamal Hadi Salim <jhs@mojatatu.com> wrote:
>
> On Tue, Sep 8, 2026 at 4:55 PM Kuniyuki Iwashima <kuniyu@google.com> wrote:
> >
> > If a netlink socket sends RTM_GETCHAIN requests repeatedly
> > without recv()ing the responses, tc_ctl_chain() hogs CPU and
> > triggers Hung Task splat. [0]
> >
> > As caught in the stack trace, netlink_attachskb() could confuse
> > tc_ctl_chain() by returning -EAGAIN when the userspace netlink
> > socket's receive buffer is full.
> >
> > The replay: label exists since commit 32a4f5ecd738 ("net: sched:
> > introduce chain object to uapi") but was not used initially.
> >
> > Since commit 9f407f1768d3 ("net: sched: introduce chain templates"),
> > the label is needed for RTM_NEWCHAIN because tcf_proto_lookup_ops()
> > may release RTNL to call request_module().
> >
> > However, the replay logic is unnecessary for RTM_GETCHAIN.
> >
> > Let's apply the replay logic only for RTM_NEWCHAIN.
> >
>
> The patch looks sane.
> Can you send me a reproducer please? We are automating and testing of
> all tc submissions.
>
> Reviewed-by: Jamal Hadi Salim

FTR, sent the repro offlist and asked to resend the tag with
the email address.

Thanks !

Re: [PATCH v1 net] net/sched: cls_api: Don't replay RTM_GETCHAIN in tc_ctl_chain().

[Jamal Hadi Salim]

On Wed, Sep 9, 2026 at 12:47 PM Kuniyuki Iwashima <kuniyu@google.com> wrote:
>
> On Wed, Sep 9, 2026 at 3:01 AM Jamal Hadi Salim <jhs@mojatatu.com> wrote:
> >
> > On Tue, Sep 8, 2026 at 4:55 PM Kuniyuki Iwashima <kuniyu@google.com> wrote:
> > >
> > > If a netlink socket sends RTM_GETCHAIN requests repeatedly
> > > without recv()ing the responses, tc_ctl_chain() hogs CPU and
> > > triggers Hung Task splat. [0]
> > >
> > > As caught in the stack trace, netlink_attachskb() could confuse
> > > tc_ctl_chain() by returning -EAGAIN when the userspace netlink
> > > socket's receive buffer is full.
> > >
> > > The replay: label exists since commit 32a4f5ecd738 ("net: sched:
> > > introduce chain object to uapi") but was not used initially.
> > >
> > > Since commit 9f407f1768d3 ("net: sched: introduce chain templates"),
> > > the label is needed for RTM_NEWCHAIN because tcf_proto_lookup_ops()
> > > may release RTNL to call request_module().
> > >
> > > However, the replay logic is unnecessary for RTM_GETCHAIN.
> > >
> > > Let's apply the replay logic only for RTM_NEWCHAIN.
> > >
> >
> > The patch looks sane.
> > Can you send me a reproducer please? We are automating and testing of
> > all tc submissions.
> >
> > Reviewed-by: Jamal Hadi Salim
>
> FTR, sent the repro offlist and asked to resend the tag with
> the email address.

Thanks. Our system has tested this before and after the patch and
including relevant tdc tests and it is fine.
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Tested-by: hybris@mojatatu.ai

cheers,
jamal

> Thanks !

Re: [PATCH v1 net] net/sched: cls_api: Don't replay RTM_GETCHAIN in tc_ctl_chain().

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Paolo Abeni <pabeni@redhat.com>:

On Tue,  8 Sep 2026 20:55:25 +0000 you wrote:
> If a netlink socket sends RTM_GETCHAIN requests repeatedly
> without recv()ing the responses, tc_ctl_chain() hogs CPU and
> triggers Hung Task splat. [0]
> 
> As caught in the stack trace, netlink_attachskb() could confuse
> tc_ctl_chain() by returning -EAGAIN when the userspace netlink
> socket's receive buffer is full.
> 
> [...]

Here is the summary with links:
  - [v1,net] net/sched: cls_api: Don't replay RTM_GETCHAIN in tc_ctl_chain().
    https://git.kernel.org/netdev/net/c/dff39930ad5e

You are awesome, thank you!
-- 
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html