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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni Cc: Simon Horman , =?UTF-8?q?Toke=20H=C3=B8iland-J=C3=B8rgensen?= , Jamal Hadi Salim , Jiri Pirko , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, zjamg , stable@vger.kernel.org Subject: [PATCH 1/2] net: fix OOB read in qdisc_pkt_len_segs_init() on negative transport offset Date: Thu, 17 Sep 2026 20:21:52 +0800 Message-ID: <20260917122153.62722-2-ndaugoing@gmail.com> X-Mailer: git-send-email 2.50.1 In-Reply-To: <20260917122153.62722-1-ndaugoing@gmail.com> References: <20260917122153.62722-1-ndaugoing@gmail.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit In qdisc_pkt_len_segs_init(), the header length up to the transport layer is computed via skb_transport_offset() or skb_inner_transport_offset(): /* mac layer + network layer */ if (!skb->encapsulation) { if (unlikely(!skb_transport_header_was_set(skb))) return SKB_NOT_DROPPED_YET; hdr_len = skb_transport_offset(skb); } else { hdr_len = skb_inner_transport_offset(skb); } Both offset accessors return a signed int, but hdr_len is declared as unsigned int. If a packet undergoes header operations (such as skb_vlan_untag() pulling stacked VLAN tags without updating inner_transport_header, or other header stripping that advances skb->data past the transport header), the offset becomes negative. When stored into unsigned int hdr_len, negative values are converted to large positive numbers near UINT_MAX (e.g. (unsigned int)-16 is 0xFFFFFFF0). Subsequently: 1. pskb_may_pull(skb, hdr_len + sizeof(struct tcphdr)) evaluates 0xFFFFFFF0 + 20, which overflows 32-bit unsigned arithmetic to 4. Since the packet length exceeds 4 bytes, pskb_may_pull() returns true, bypassing the bounds check. 2. th = (const struct tcphdr *)(skb->data + hdr_len) zero-extends hdr_len on 64-bit architectures, creating a wild pointer pointing ~4 GiB past skb->data into unmapped memory. 3. __tcp_hdrlen(th) dereferences th->doff, triggering an immediate kernel crash (page fault / KASAN wild-memory-access panic). A similar unsigned overflow occurs for UDP (SKB_GSO_UDP_L4). Fix this by declaring hdr_len as int to match the return types of skb_transport_offset() and skb_inner_transport_offset(), and explicitly rejecting negative offsets by returning SKB_DROP_REASON_SKB_BAD_GSO. Fixes: 7fb4c1967011 ("net: pull headers in qdisc_pkt_len_segs_init()") Cc: stable@vger.kernel.org Signed-off-by: zjamg --- net/core/dev.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/net/core/dev.c b/net/core/dev.c index ecfbd72d5d1a..f2dbf99181ec 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -4159,8 +4159,9 @@ EXPORT_SYMBOL_GPL(validate_xmit_skb_list); static enum skb_drop_reason qdisc_pkt_len_segs_init(struct sk_buff *skb) { struct skb_shared_info *shinfo = skb_shinfo(skb); - unsigned int hdr_len, tlen; + unsigned int tlen; u16 gso_segs; + int hdr_len; qdisc_skb_cb(skb)->pkt_len = skb->len; if (!shinfo->gso_size) { @@ -4182,6 +4183,8 @@ static enum skb_drop_reason qdisc_pkt_len_segs_init(struct sk_buff *skb) } else { hdr_len = skb_inner_transport_offset(skb); } + if (unlikely(hdr_len < 0)) + return SKB_DROP_REASON_SKB_BAD_GSO; /* + transport layer */ if (likely(shinfo->gso_type & (SKB_GSO_TCPV4 | SKB_GSO_TCPV6))) { const struct tcphdr *th; -- 2.53.0