CVE-2024-40995: net/sched: act_api: fix possible infinite loop in tcf_idr_check_alloc()

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

net/sched: actapi: fix possible infinite loop in tcfidrcheckalloc()

syzbot found hanging tasks waiting on rtnllock [1]

A reproducer is available in the syzbot bug.

When a request to add multiple actions with the same index is sent, the second request will block forever on the first request. This holds rtnllock, and causes tasks to hang.

Return -EAGAIN to prevent infinite looping, while keeping documented behavior.

[1]

INFO: task kworker/1:0:5088 blocked for more than 143 seconds. Not tainted 6.9.0-rc4-syzkaller-00173-g3cdb45594619 #0 "echo 0 > /proc/sys/kernel/hungtasktimeoutsecs" disables this message. task:kworker/1:0 state:D stack:23744 pid:5088 tgid:5088 ppid:2 flags:0x00004000 Workqueue: eventspowerefficient regcheckchanswork Call Trace: <TASK> contextswitch kernel/sched/core.c:5409 [inline] schedule+0xf15/0x5d00 kernel/sched/core.c:6746 scheduleloop kernel/sched/core.c:6823 [inline] schedule+0xe7/0x350 kernel/sched/core.c:6838 schedulepreemptdisabled+0x13/0x30 kernel/sched/core.c:6895 mutexlockcommon kernel/locking/mutex.c:684 [inline] mutexlock+0x5b8/0x9c0 kernel/locking/mutex.c:752 wiphylock include/net/cfg80211.h:5953 [inline] regleaveinvalidchans net/wireless/reg.c:2466 [inline] regcheckchanswork+0x10a/0x10e0 net/wireless/reg.c:2481