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

[Simon Horman <horms@kernel.org>]

On Tue, Sep 15, 2026 at 10:08:16PM +0700, Nguyen Ngoc Thang wrote:
> 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>

Reviewed-by: Simon Horman <horms@kernel.org>