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Wed, 04 Nov 2026 08:52:23 -0800 (PST) Received: from localhost ([153.61.198.252]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3b09445a7f6sm4685568a91.6.2026.11.04.08.52.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 04 Nov 2026 08:52:22 -0800 (PST) X-Mailing-List: netdev@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Wed, 04 Nov 2026 16:52:22 +0000 Message-Id: Cc: , "network dev" Subject: Re: [PATCH net] net: reject a negative TCP header offset in qdisc_pkt_len_segs_init() From: "Alexei Starovoitov" To: "Paolo Abeni" , X-Mailer: aerc 0.20.1-355-gf870624f6c37 References: <20261101194433.2637113-1-alexei.starovoitov@gmail.com> In-Reply-To: On Wed Nov 4, 2026 at 8:41 AM UTC, Paolo Abeni wrote: > On 11/2/26 23:46, Alexei Starovoitov wrote: >> 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 i= s >>> computed rather than only inside the SKB_GSO_TCPV4 | SKB_GSO_TCPV6 bran= ch? >>=20 >> No. It's already explained in the commit log. >>=20 >>> 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. >>=20 >> Not a bug. That's CONFIG_DEBUG_NET only and the warn is right: >> the header is stale. >>=20 >>> 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? >>=20 >> No. The driver doesn't see this offset. >>=20 >>> 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 e= nd >>> of qdisc_pkt_len_segs_init(): >>> >>> When (gso_segs - 1) * |offset| > skb->len, can qdisc_skb_cb(skb)->pkt_l= en >>> underflow in 32-bit unsigned arithmetic to ~4.29 GiB (exceeding >>> QDISC_PKT_LEN_MAX), causing deficit-based schedulers such as drr_dequeu= e() >>> and ets_dequeue() to spin in their deficit-refill loops? >>=20 >> No. > > FTR the nipa sashiko instance dismissed the above concerns autonomously: > > https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261101194433.263711= 3-1-alexei.starovoitov@gmail.com > > you can reasonably safely refer only to such instance when submitting > patches to net/net-next. Nice. I'll borrow your mechanism.