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

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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: 64ff70b80f ("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>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260915150816.36487-1-ngocthang2710.1999@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Nguyen Ngoc Thang 2026-09-15 22:08:16 +07:00 committed by Jakub Kicinski
parent 99cc2a62e0
commit 47abe7a5c4
1 changed files with 2 additions and 3 deletions
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5
net/sched/act_ct.c
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@ -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;