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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Shuah Khan , Tao Liu Cc: netdev@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, Hyunjung Ko , stable@vger.kernel.org Subject: [PATCH net v2 1/2] net/sched: act_ct: fix sk_buff leak when the header checks reject a packet Date: Thu, 6 Aug 2026 19:12:34 +0900 Message-Id: <20260806101235.809370-1-hj351016@gmail.com> X-Mailer: git-send-email 2.34.1 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit tcf_ct_handle_fragments() runs its header sanity checks before handing anything to the defragmentation engine: if (family == NFPROTO_IPV4) err = tcf_ct_ipv4_is_fragment(skb, &frag); else err = tcf_ct_ipv6_is_fragment(skb, &frag); if (err || !frag) return err; tcf_ct_ipv4_is_fragment() returns -EINVAL or -ENOMEM; tcf_ct_ipv6_is_fragment() adds -EPROTO when ipv6_find_hdr() fails. None of them frees or queues the skb, so on that path the caller still owns it. tcf_ct_act() however funnels every non-zero return into the ownership-transfer exit: err = tcf_ct_handle_fragments(net, skb, family, p->zone, &defrag); if (err) goto out_frag; ... out_frag: if (err != -EINPROGRESS) tcf_action_inc_drop_qstats(&c->common); return TC_ACT_CONSUMED; TC_ACT_CONSUMED means the action took ownership of the skb, so no caller frees it - sch_handle_ingress(), sch_handle_egress() and tcf_qevent_handle() all deliberately skip the free for that verdict. The skb is therefore orphaned: one sk_buff plus its data buffer is leaked per malformed packet, unbounded. Note the drop counter is already incremented for these errors, so the statistics claim a drop that never happens. Three different ownership states reach out_frag: today - the skb may be queued by the defrag engine (-EINPROGRESS), already freed by nf_ct_handle_fragments(), or still owned by us. Tell the caller which of those it is, and free the packet ourselves in the last case, which restores the TC_ACT_SHOT behaviour that predated the Fixes: commit. Reproduced on v7.2-rc6 with a 54-byte frame carrying a 40-byte IPv6 header with nexthdr = 0 (hop-by-hop) and nothing after it, on a clsact ingress chain with "action ct". kmemleak reports one leaked 232-byte skbuff_head_cache object plus its 704-byte data buffer per packet; with this patch it reports none. Fixes: 3f14b377d01d ("net/sched: act_ct: fix skb leak and crash on ooo frags") Cc: stable@vger.kernel.org # v6.8+ Assisted-by: Anthropic-Claude-Code:Claude-Opus-5 Signed-off-by: Hyunjung Ko --- net/sched/act_ct.c | 29 +++++++++++++++++++++++++---- 1 file changed, 25 insertions(+), 4 deletions(-) v2: - add a tdc selftest (patch 2/2), as requested by Jamal - add Assisted-by: tag - no functional change to the fix itself v1: https://lore.kernel.org/netdev/20260805095539.204356-1-hj351016@gmail.com/ Reproducer needs CONFIG_NET_ACT_CT, plus CONFIG_DEBUG_KMEMLEAK and kmemleak=on to observe it: ip link add veth0 type veth peer name veth1 ip link set veth0 up; ip link set veth1 up tc qdisc add dev veth0 clsact tc filter add dev veth0 ingress matchall action ct then inject at veth1 a 54-byte frame: ethertype 0x86DD, a 40-byte IPv6 header with nexthdr = 0 (hop-by-hop) and nothing after it, so ipv6_find_hdr() fails with -EBADMSG and tcf_ct_ipv6_is_fragment() returns -EPROTO. Before, one sk_buff plus its data buffer per packet: kmemleak: 50 new suspected memory leaks unreferenced object 0xffff888103ed13c0 (size 232): kmem_cache_alloc_node_noprof+0x2f1/0x3e0 __alloc_skb+0xe5/0x860 alloc_skb_with_frags+0x82/0x750 sock_alloc_send_pskb+0x658/0x7e0 packet_sendmsg+0x1833/0x4860 __x64_sys_sendto+0xe0/0x1c0 do_syscall_64+0x102/0x5a0 After: kmemleak reports no unreferenced objects. Note /proc/slabinfo is not a usable check here on a KASAN build - skbuff_head_cache active_objs still grows because the quarantine holds the freed objects. kmemleak is the reliable signal. Patch 2/2 turns the same case into a tdc test, using the clsact drop counter as the discriminator: before the fix act_ct returns TC_ACT_CONSUMED, so tc_run() never reaches its TC_ACT_SHOT arm and the counter stays at zero while the skbs leak. diff --git a/net/sched/act_ct.c b/net/sched/act_ct.c index be535a261fa0..e250969c84ac 100644 --- a/net/sched/act_ct.c +++ b/net/sched/act_ct.c @@ -840,8 +840,15 @@ static int tcf_ct_ipv6_is_fragment(struct sk_buff *skb, bool *frag) return 0; } +/* On error, tells the caller whether it still owns @skb and must free it + * itself. @skb is ours only when the header checks below reject the packet + * before it is handed to the defragmentation engine; once nf_ct_handle_ + * fragments() has been called the skb is either queued (-EINPROGRESS) or has + * already been freed by it. + */ static int tcf_ct_handle_fragments(struct net *net, struct sk_buff *skb, - u8 family, u16 zone, bool *defrag) + u8 family, u16 zone, bool *defrag, + bool *skb_is_ours) { enum ip_conntrack_info ctinfo; struct tc_skb_cb cb; @@ -859,8 +866,12 @@ static int tcf_ct_handle_fragments(struct net *net, struct sk_buff *skb, err = tcf_ct_ipv4_is_fragment(skb, &frag); else err = tcf_ct_ipv6_is_fragment(skb, &frag); - if (err || !frag) + if (err) { + *skb_is_ours = true; return err; + } + if (!frag) + return 0; cb = *tc_skb_cb(skb); err = nf_ct_handle_fragments(net, skb, zone, family, &proto, &cb.mru); @@ -977,6 +988,7 @@ TC_INDIRECT_SCOPE int tcf_ct_act(struct sk_buff *skb, const struct tc_action *a, int nh_ofs, err, retval; struct tcf_ct_params *p; bool add_helper = false; + bool skb_is_ours = false; bool skip_add = false; bool defrag = false; struct nf_conn *ct; @@ -1012,9 +1024,18 @@ TC_INDIRECT_SCOPE int tcf_ct_act(struct sk_buff *skb, const struct tc_action *a, */ nh_ofs = skb_network_offset(skb); skb_pull_rcsum(skb, nh_ofs); - err = tcf_ct_handle_fragments(net, skb, family, p->zone, &defrag); - if (err) + err = tcf_ct_handle_fragments(net, skb, family, p->zone, &defrag, + &skb_is_ours); + if (err) { + /* The skb is still ours only when the header checks rejected + * it; returning TC_ACT_CONSUMED for such a packet would leak + * it, since no caller frees an skb it was told it no longer + * owns. + */ + if (skb_is_ours) + goto drop; goto out_frag; + } err = nf_ct_skb_network_trim(skb, family); if (err) -- 2.43.0