[PATCH net] net/sched: act_ct: don't WARN on benign flow_offload_alloc() failure

[Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>]

flow_offload_alloc() returns NULL when the conntrack entry is dying
(e.g. raced with a conntrack flush) or when the GFP_ATOMIC allocation
fails; both are expected under load and neither is a kernel bug. This
path runs from softirq on every committed packet, so with
panic_on_warn=1 an unprivileged user can panic the box just by racing
a conntrack flush against a `tc ... action ct commit` classifier.

Reproduced with a custom repro under QEMU: a small, fixed set of UDP
flows through `tc filter ... action ct commit` on lo, raced against
threads flooding bare ctnetlink CT_DELETE (flush) requests. Hits
WARNING: net/sched/act_ct.c:437 (tcf_ct_flow_table_add(), inlined
into tcf_ct_act() in this build) within ~15s on the unpatched kernel;
same setup is clean on the patched kernel. The fix itself is
behavior-preserving: both branches already did `goto err_alloc`
before and after, only the WARN is removed.

Fixes: 64ff70b80fd4 ("net/sched: act_ct: Offload established connections to flow table")
Reported-by: syzbot+6cc37aba98dac721c415@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6cc37aba98dac721c415
Signed-off-by: Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>
---
 net/sched/act_ct.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/net/sched/act_ct.c b/net/sched/act_ct.c
index 9080cb386c16..55f3521edb4c 100644
--- a/net/sched/act_ct.c
+++ b/net/sched/act_ct.c
@@ -432,11 +432,10 @@ static void tcf_ct_flow_table_add(struct tcf_ct_flow_table *ct_ft,
 	if (test_and_set_bit(IPS_OFFLOAD_BIT, &ct->status))
 		return;
 
+	/* NULL if ct is dying (raced flush) or the atomic alloc failed. */
 	entry = flow_offload_alloc(ct);
-	if (!entry) {
-		WARN_ON_ONCE(1);
+	if (!entry)
 		goto err_alloc;
-	}
 
 	if (tcp) {
 		ct->proto.tcp.seen[0].flags |= IP_CT_TCP_FLAG_BE_LIBERAL;
-- 
2.43.0