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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Cong Wang , wenxu Cc: netdev@vger.kernel.org, Fourie Zhang , stable@kernel.org, TencentOS Corvus AI Subject: [PATCH net] net/sched: sch_frag: reject a negative L2 length in sch_fragment() Date: Thu, 27 Aug 2026 17:23:27 +0800 Message-ID: <20260827092345.2301937-1-fouriezhang@tencent.com> X-Mailer: git-send-email 2.43.7 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit sch_fragment() bounds the L2 header length only from above: if (skb_network_offset(skb) > VLAN_ETH_HLEN) skb_network_offset() returns int and the comparison is signed, so a negative offset passes. sch_frag_prepare_frag() then stores it in an unsigned int and uses it as a memcpy() length into the 18-byte per-CPU l2_data buffer: unsigned int hlen = skb_network_offset(skb); memcpy(&data->l2_data, skb->data, hlen); The offset is negative whenever the network header sits behind skb->data. ipv6_srh_rcv() runs in that state, because ip6_protocol_deliver_rcu() pulls each extension header as it walks the chain and the segments_left > 0 branch pushes back only sizeof(struct ipv6hdr). act_ct enables the static key that gates sch_frag_xmit_hook() and sets tc_skb_cb(skb)->mru, which survives receive and forward, so an act_mirred redirect from a root qdisc can reach sch_fragment() with the offset still negative. A negative L2 header length is invalid, so reject it through the existing "L2 header too long to fragment" path. A non-negative offset behaves exactly as before. Reproduced on v7.2 by an unprivileged user with only unshare(CLONE_NEWUSER|CLONE_NEWNET). RDX is the truncated length, (unsigned int)(-42): RIP: 0010:memcpy+0x8/0x20 RDX: 00000000ffffffd6 sch_frag_prepare_frag+0x364/0x440 net/sched/sch_frag.c:74 sch_fragment+0x1c7/0x7a0 net/sched/sch_frag.c:122 tcf_mirred_to_dev+0x7a9/0xf10 ip6_forward+0x114b/0x31b0 ipv6_rthdr_rcv+0x40d7/0x5e10 With the patch applied the drop path fires and the packet is discarded. Fixes: c129412f74e9 ("net/sched: sch_frag: add generic packet fragment support.") Cc: stable@kernel.org Reported-by: TencentOS Corvus AI Assisted-by: tencentos-corvus-ai:kimi-k3 Signed-off-by: Fourie Zhang --- net/sched/sch_frag.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/net/sched/sch_frag.c b/net/sched/sch_frag.c index 75ee52750919..b68a198b1646 100644 --- a/net/sched/sch_frag.c +++ b/net/sched/sch_frag.c @@ -89,9 +89,10 @@ static struct dst_ops sch_frag_dst_ops = { static int sch_fragment(struct net *net, struct sk_buff *skb, u16 mru, int (*xmit)(struct sk_buff *skb)) { + int hlen = skb_network_offset(skb); int ret = -1; - if (skb_network_offset(skb) > VLAN_ETH_HLEN) { + if (hlen < 0 || hlen > VLAN_ETH_HLEN) { net_warn_ratelimited("L2 header too long to fragment\n"); goto err; } -- 2.43.7