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[35.245.46.111]) by smtp.gmail.com with ESMTPSA id 71dfb90a1353d-5ea99491db7sm304799e0c.10.2026.10.15.11.07.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 15 Oct 2026 11:07:38 -0700 (PDT) Date: Thu, 15 Oct 2026 14:07:38 -0400 From: Willem de Bruijn To: "Michael S. Tsirkin" , linux-kernel@vger.kernel.org Cc: Willem de Bruijn , Jason Wang , =?UTF-8?B?RXVnZW5pbyBQw6lyZXo=?= , Xuan Zhuo , Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , netdev@vger.kernel.org, virtualization@lists.linux.dev, Rusty Russell , Max Krasnyansky , Paulos Yibelo , Chris Arges Message-ID: In-Reply-To: References: Subject: Re: [PATCH net-next v2 4/7] net: resolve uncommon partial checksums at ingress Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Michael S. Tsirkin wrote: > As reported first by Paulos Yibelo (after which > others, credited below, found more ways the bug can affect users) a > virtio net header can place CHECKSUM_PARTIAL at any in-packet offset. > If that state is retained, later checksum completion can modify the > packet after protocol processing has made decisions from it. > > Let's fix that: let's validate csum_start and csum_offset and only keep > CHECKSUM_PARTIAL when csum_start and csum_offset match a parsed TCP or > UDP header, while handling VLAN headers, IPv4 options and IPv6 extension > headers. Complete other non-GSO checksums immediately, before any > protocol processing runs. > > We can't easily "complete the checksum" for GSO we don't recognize - GSO > assumes knowledge of the protocol. We could segment ignoring the > checksum then do whatever csum_start and csum_offset say, but that > seems like an overkill for something that shouldn't happen. > > There's some risk here: > - non-gso packets which can't be parsed get checksum calculated in > software, adding performance overhead > - gso packets which can't be parsed get dropped The biggest risk I see of valid packets reaching this path is packet sockets that are not bound to a specific protocol (ETH_P_ALL) over a non Ethernet device. Iff proto is 0 we could try to infer from the first nibble at the network header: + iph = (const void *)(skb->data + nhoff); + if (!proto) { + if (iph->version == 4) + proto = htons(ETH_P_IP); + else if (iph->version == 6) + proto = htons(ETH_P_IPV6); + }