[PATCH net] xfrm: reschedule NAT keepalive work for new states

["Baek Kyu, Kim" <pocj8ur4in@gmail.com>]

The per-net NAT keepalive work can already be pending for a state with a
long interval when a new state with a shorter interval is added.
schedule_delayed_work(..., 0) does not move the pending deadline, so the
new state's keepalive waits for the older state's deadline.

Use mod_delayed_work() for a newly added state to run the worker
immediately and recalculate the next deadline. Keep the existing
schedule_delayed_work() behavior when a state is deleted. States without
a keepalive interval still return early.

Fixes: f531d13bdfe3 ("xfrm: support sending NAT keepalives in ESP in UDP states")
Signed-off-by: Baek Kyu, Kim <pocj8ur4in@gmail.com>
---
An arm64 QEMU/KVM test of the immediate-reschedule change showed that a
new 2-second SA sent a keepalive immediately while the original kernel
sent none for five seconds. With a virtual NAT gateway, the original
mapping expired and a 9-second inbound UDP packet was lost; the changed
kernel retained the mapping and received it. A boot test of the final
patch confirmed an immediate keepalive for the new SA and no immediate
workqueue event or later keepalive for the deleted SA. An arm64 W=1
object build passed. No physical NAT device or ESP payload was tested.

 net/xfrm/xfrm_nat_keepalive.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/xfrm/xfrm_nat_keepalive.c b/net/xfrm/xfrm_nat_keepalive.c
index 8ed30e6c263b66..6814f9063969ef 100644
--- a/net/xfrm/xfrm_nat_keepalive.c
+++ b/net/xfrm/xfrm_nat_keepalive.c
@@ -288,7 +288,10 @@ void xfrm_nat_keepalive_state_updated(struct xfrm_state *x)
 		return;
 
 	net = xs_net(x);
-	schedule_delayed_work(&net->xfrm.nat_keepalive_work, 0);
+	if (x->km.state == XFRM_STATE_DEAD)
+		schedule_delayed_work(&net->xfrm.nat_keepalive_work, 0);
+	else
+		mod_delayed_work(system_wq, &net->xfrm.nat_keepalive_work, 0);
 }
 
 int __net_init xfrm_nat_keepalive_net_init(struct net *net)