5.4-stable review patch. If anyone has any objections, please let me know.
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From: Yafang Shao laoar.shao@gmail.com
[ Upstream commit 66048f8b3cc7e462953c04285183cdee43a1cb89 ]
During recent testing with the netem qdisc to inject delays into TCP traffic, we observed that our CLS BPF program failed to function correctly due to incorrect classid retrieval from task_get_classid(). The issue manifests in the following call stack:
bpf_get_cgroup_classid+5 cls_bpf_classify+507 __tcf_classify+90 tcf_classify+217 __dev_queue_xmit+798 bond_dev_queue_xmit+43 __bond_start_xmit+211 bond_start_xmit+70 dev_hard_start_xmit+142 sch_direct_xmit+161 __qdisc_run+102 <<<<< Issue location __dev_xmit_skb+1015 __dev_queue_xmit+637 neigh_hh_output+159 ip_finish_output2+461 __ip_finish_output+183 ip_finish_output+41 ip_output+120 ip_local_out+94 __ip_queue_xmit+394 ip_queue_xmit+21 __tcp_transmit_skb+2169 tcp_write_xmit+959 __tcp_push_pending_frames+55 tcp_push+264 tcp_sendmsg_locked+661 tcp_sendmsg+45 inet_sendmsg+67 sock_sendmsg+98 sock_write_iter+147 vfs_write+786 ksys_write+181 __x64_sys_write+25 do_syscall_64+56 entry_SYSCALL_64_after_hwframe+100
The problem occurs when multiple tasks share a single qdisc. In such cases, __qdisc_run() may transmit skbs created by different tasks. Consequently, task_get_classid() retrieves an incorrect classid since it references the current task's context rather than the skb's originating task.
Given that dev_queue_xmit() always executes with bh disabled, we can use softirq_count() instead to obtain the correct classid.
The simple steps to reproduce this issue: 1. Add network delay to the network interface: such as: tc qdisc add dev bond0 root netem delay 1.5ms 2. Build two distinct net_cls cgroups, each with a network-intensive task 3. Initiate parallel TCP streams from both tasks to external servers.
Under this specific condition, the issue reliably occurs. The kernel eventually dequeues an SKB that originated from Task-A while executing in the context of Task-B.
It is worth noting that it will change the established behavior for a slightly different scenario:
<sock S is created by task A> <class ID for task A is changed> <skb is created by sock S xmit and classified>
prior to this patch the skb will be classified with the 'new' task A classid, now with the old/original one. The bpf_get_cgroup_classid_curr() function is a more appropriate choice for this case.
Signed-off-by: Yafang Shao laoar.shao@gmail.com Cc: Daniel Borkmann daniel@iogearbox.net Cc: Thomas Graf tgraf@suug.ch Cc: Sebastian Andrzej Siewior bigeasy@linutronix.de Cc: Nikolay Aleksandrov razor@blackwall.org Link: https://patch.msgid.link/20250902062933.30087-1-laoar.shao@gmail.com Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- include/net/cls_cgroup.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/include/net/cls_cgroup.h b/include/net/cls_cgroup.h index 4295de3e6a4ba..7c2ead6cdc538 100644 --- a/include/net/cls_cgroup.h +++ b/include/net/cls_cgroup.h @@ -58,7 +58,7 @@ static inline u32 task_get_classid(const struct sk_buff *skb) * calls by looking at the number of nested bh disable calls because * softirqs always disables bh. */ - if (in_serving_softirq()) { + if (softirq_count()) { struct sock *sk = skb_to_full_sk(skb);
/* If there is an sock_cgroup_classid we'll use that. */