[Patch net-next] net/sched: cls_api: fix possible infinite loop in tcf_idr_check_alloc()

[David Ruth <druth@chromium.org>]

syzbot found hanging tasks waiting on rtnl_lock [1]

When a request to add multiple actions with the same index is sent, the
second request will block forever on the first request. This results in an
infinite loop that holds rtnl_lock, 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/hung_task_timeout_secs" disables this message.
task:kworker/1:0 state:D stack:23744 pid:5088 tgid:5088 ppid:2 flags:0x00004000
Workqueue: events_power_efficient reg_check_chans_work
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5409 [inline]
__schedule+0xf15/0x5d00 kernel/sched/core.c:6746
__schedule_loop kernel/sched/core.c:6823 [inline]
schedule+0xe7/0x350 kernel/sched/core.c:6838
schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:6895
__mutex_lock_common kernel/locking/mutex.c:684 [inline]
__mutex_lock+0x5b8/0x9c0 kernel/locking/mutex.c:752
wiphy_lock include/net/cfg80211.h:5953 [inline]
reg_leave_invalid_chans net/wireless/reg.c:2466 [inline]
reg_check_chans_work+0x10a/0x10e0 net/wireless/reg.c:2481

Reported-by: syzbot+b87c222546179f4513a7@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=b87c222546179f4513a7
Signed-off-by: David Ruth <druth@chromium.org>
---
 net/sched/act_api.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/net/sched/act_api.c b/net/sched/act_api.c
index 7458b3154426..2714c4ed928e 100644
--- a/net/sched/act_api.c
+++ b/net/sched/act_api.c
@@ -830,7 +830,6 @@ int tcf_idr_check_alloc(struct tc_action_net *tn, u32 *index,
 	u32 max;
 
 	if (*index) {
-again:
 		rcu_read_lock();
 		p = idr_find(&idrinfo->action_idr, *index);
 
@@ -839,7 +838,7 @@ int tcf_idr_check_alloc(struct tc_action_net *tn, u32 *index,
 			 * index but did not assign the pointer yet.
 			 */
 			rcu_read_unlock();
-			goto again;
+			return -EAGAIN;
 		}
 
 		if (!p) {
-- 
2.45.2.627.g7a2c4fd464-goog