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Mon, 02 Nov 2026 14:47:00 -0800 (PST) Received: from localhost ([153.61.198.247]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3ae8805ebc8sm9308679a91.3.2026.11.02.14.46.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 02 Nov 2026 14:46:59 -0800 (PST) Content-Type: text/plain; charset=UTF-8 Date: Mon, 02 Nov 2026 22:46:59 +0000 Message-Id: Cc: Subject: Re: [PATCH net] net: reject a negative TCP header offset in qdisc_pkt_len_segs_init() From: "Alexei Starovoitov" To: In-Reply-To: References: <20261101194433.2637113-1-alexei.starovoitov@gmail.com> Content-Transfer-Encoding: 8bit X-Mailing-List: bpf@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 On Mon, Nov 02, 2026 at 11:44 AM sashiko-bot@kernel.org wrote: >> + /* A header in front of skb->data gives a negative offset. */ >> + if ((int)hdr_len < 0 || >> + !pskb_may_pull(skb, hdr_len + sizeof(struct tcphdr))) >> return SKB_DROP_REASON_SKB_BAD_GSO; > > [Severity: High] > Should the (int)hdr_len < 0 check be placed immediately after hdr_len is > computed rather than only inside the SKB_GSO_TCPV4 | SKB_GSO_TCPV6 branch? No. It's already explained in the commit log. > When (int)hdr_len < -8, hdr_len + sizeof(struct udphdr) wraps to an > unsigned int in [0x80000008, 0xffffffff] (> INT_MAX), which triggers > DEBUG_NET_WARN_ON_ONCE(len > INT_MAX) in > include/linux/skbuff.h:pskb_may_pull_reason() before returning > SKB_DROP_REASON_SKB_BAD_GSO. Not a bug. That's CONFIG_DEBUG_NET only and the warn is right: the header is stale. > Can this write uh->len (and uh->check in bnxt_sw_udp_gso_xmit() and > enetc_tso_hdr_csum()) before the start of the header buffer when > skb_transport_offset(skb) < -4? No. The driver doesn't see this offset. > Third, for other GSO types (such as SKB_GSO_FRAGLIST, SKB_GSO_UDP, > SKB_GSO_SCTP, or SKB_GSO_ESP), hdr_len remains 2^32 - |offset| at the end > of qdisc_pkt_len_segs_init(): > > When (gso_segs - 1) * |offset| > skb->len, can qdisc_skb_cb(skb)->pkt_len > underflow in 32-bit unsigned arithmetic to ~4.29 GiB (exceeding > QDISC_PKT_LEN_MAX), causing deficit-based schedulers such as drr_dequeue() > and ets_dequeue() to spin in their deficit-refill loops? No.