In the Linux kernel, the following vulnerability has been resolved:

net/sched: cls_api: Don't replay RTM_GETCHAIN in tc_ctl_chain().

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>