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Wed, 04 Nov 2026 03:59:51 -0800 (PST) Received: from redhat.com ([2a0d:6fc0:1506:fd00:9e19:9994:44b1:e6f2]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4b43c17ecdasm74195465e9.5.2026.11.04.03.59.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 04 Nov 2026 03:59:50 -0800 (PST) Date: Wed, 4 Nov 2026 06:59:48 -0500 From: "Michael S. Tsirkin" To: linux-kernel@vger.kernel.org Cc: Willem de Bruijn , Chris Arges , =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= , Phil Sutter , Jason Wang , Eugenio =?utf-8?B?UMOpcmV6?= , Xuan Zhuo , Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , netdev@vger.kernel.org, virtualization@lists.linux.dev, Max Krasnyansky , Rusty Russell Subject: [PATCH RFC v7 0/5] more virtio-net csum hardening Message-ID: X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 X-Mailer: git-send-email 2.51.2.2891.g4157995a80.dirty X-Mutt-Fcc: =sent X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: OQGTsxLflQLpAYF-zGdrO1bRTKFtjtFenaMri_xttr0_1793793593 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit Sending this one as an RFC: two issues are still open. I'm going to be working on other things in the next couple of weeks, but I thought I'd post this in case anyone wants to take this up. Issues: First, the general issue is not addressed 100%: if a tunnel inner packet happens to have a valid header at inner_nh_offset, it will pass all checks, but the transport header can in fact overlap e.g. the real TCP header and the checksum will overwrite that. I do not have a good idea how to address this without either a heavy protocol-specific anchor derivation or dropping well-formed offload. Second, completing the checksum at input breaks nf/BPF programs that might assume that the packet carries CHECKSUM_PARTIAL, then tweak the payload of a packet in flight and rely on the checksum being fixed up later. The sysctl added in the last patch could serve as a work-around, but it is a very blunt tool. The issues were mostly found by code inspection, but all reproduced by me in a VM. ------ As reported first by Paulos Yibelo (after which others, credited below, found more ways the bug can affect users): allowing untrusted hardware to specify csum start/offset for CHECKSUM_PARTIAL was not a good idea, because, if it edits the packet outside the actual checksum, and depending on the setup, the networking stack could be looking at the packet both before and after the edit, and seeing different results. Previously, commit 6785011f8b16 ("ipv4: validate checksum_start before completing checksum") validated all known instances where this causes the most grief, but the real fix is to make sure that either only actual checksum fields are ever edited, or the checksum is completed before the net stack looks at it, so it is always consistent. This basically goes back to what we did before f43798c27684 ("tun: Allow GSO using virtio_net_hdr"), except in a narrow list of specified known good cases. Summary of the patchset: These are probabably net material (and seem to be too big for stable, or I would recommend backporting there, too): net: account for ingress-resolved virtio net hdr state - accounting, not the fix itself, dependency of the fix um: vector: trim skbs before verifying the header - fixes the um receive path: without it the checksum would be completed over the unreceived tail of the buffer virtio_net: parse tunnel csum_start little-endian - bugfix, a dependency of validating the header net: resolve uncommon partial checksums at ingress - fix itself net: add a sysctl to opt out of virtio net hdr checksum validation - reducing risk of the fix by adding a sysctl to disable it These are cleanups, can be deferred to net-next, too: net: add ip4_parse() Fixes: f43798c27684 ("tun: Allow GSO using virtio_net_hdr") Changes since v1: - reorder patches to fix bisectability, addressing a comment by Björn Töpel. - code refactoring, different patch splitup, a sysctl knob to disable as suggested by Willem de Bruijn. - fix a bunch of minor issues spotted by LLMs Changes since v2: virtio_net: parse tunnel csum_start little-endian - new patch. correctly parsing the header is a dependency of validating it, otherwise i do not know what to validate net: account for ingress-resolved virtio net hdr state - make rx_csum_sw counter 64 bit so it won't wrap (spotted by LLM review) net: resolve uncommon partial checksums at ingress - tunnel fixes to bugs noticed by Willem de Bruijn and Chris Arges - restore the csum_offset checks as suggested by Willem net: add a sysctl to opt out of virtio net hdr checksum validation - the knob is no longer writable by root inside a user namespace: changing it now requires capable(CAP_NET_ADMIN) suggested by Willem de Bruijn net: add ip4_parse() - comment converted to kernel-doc format (Willem de Bruijn) um: vector: trim skbs before verifying the header - new patch, found by code review Changes since v3: net: account for ingress-resolved virtio net hdr state - um: vector: honor virtio net hdr conversion failures folded in (Björn Töpel) - count on tap and virtio_net, too (Willem de Bruijn) net: resolve uncommon partial checksums at ingress - walk the frame on loopback, too: loopback_xmit() runs eth_type_trans() on it (Willem de Bruijn) - return the mismatch handling directly, GSO is implied in the tunnel path so drop there outright, and fold the helper into its one remaining caller (Willem de Bruijn) - read the IP version through struct iphdr (Willem de Bruijn) - document the ipv6_find_hdr() flags choice net: add a sysctl to opt out of virtio net hdr checksum validation - renamed to virtio_net_hdr_validate, in case more validation is gated by the same knob later (Willem de Bruijn) - point .data at the value like the neighboring entries, the per-netns registration rebases it, and the handler reads the value directly - no ctl_table copy (Björn Töpel, Willem de Bruijn) - check the opt-out before parsing: with the knob off, packets the parser cannot handle were still dropped (Willem de Bruijn) - include virtio_net.h in net/core/dev.c so the static key definition sees its declaration (build warning reported by Jakub Kicinski) net: add ip4_parse() - add the missing kernel-doc Return: section (Jakub Kicinski) Changes since v4: net: account for ingress-resolved virtio net hdr state - count on tun only: dropped the tap, macvtap, ipvtap and virtio_net counters (Willem de Bruijn) - the driver side branches are in their final form here now, moved from "net: resolve uncommon partial checksums at ingress" (Willem de Bruijn) net: resolve uncommon partial checksums at ingress - no longer touches the drivers (Willem de Bruijn) - a packet too small to be GSO: ignore the tunnel GSO request, same as plain GSO ones, and complete wild metadata in place (sashiko) - document the zero copy interaction (sashiko) - honor a preset skb->protocol on ethernet: the stack uses it, except on loopback which re-parses the frame (sashiko) net: add a sysctl to opt out of virtio net hdr checksum validation - document the netns lifetime requirement (sashiko) net: add ip4_parse() - pull the whole header, options included, so that the transport header offset is within the linear data (Willem de Bruijn) Changes since v5: net: resolve uncommon partial checksums at ingress - read both skb->protocol and the frame ethertype: do not validate when they disagree, instead of preferring the preset protocol; same check after the IP version inference where there is no L2 header (Willem de Bruijn) - pull the IPv4 options in the helper as introduced, with the transport header offset computed before the pull (Willem de Bruijn) - fold the trivial protocol calculation cleanup, formerly a separate patch, in (Willem de Bruijn) net: add ip4_parse() - the options pull moved to the helper introduction in "net: resolve uncommon partial checksums at ingress" Changes since v6: all patches - rebased on the current net tree: v6 no longer applied (Jakub Kicinski) net: resolve uncommon partial checksums at ingress - validate header before the tunnel metadata is installed (Willem de Bruijn) - some cosmetic tweaks Tested: full suite on the final tree, tunnel included. um part compiled only. Michael S. Tsirkin (5): net: account for ingress-resolved virtio net hdr state um: vector: trim skbs before verifying the header virtio_net: parse tunnel csum_start little-endian net: resolve uncommon partial checksums at ingress net: add a sysctl to opt out of virtio net hdr checksum validation Documentation/admin-guide/sysctl/net.rst | 19 ++ arch/um/drivers/vector_kern.c | 6 +- arch/um/drivers/vector_transports.c | 4 +- drivers/net/tap.c | 4 +- drivers/net/tun.c | 41 ++- drivers/net/virtio_net.c | 2 +- include/linux/virtio_net.h | 319 ++++++++++++++++++++++- include/net/netns/core.h | 1 + net/core/dev.c | 7 + net/core/net_namespace.c | 1 + net/core/sysctl_net_core.c | 50 ++++ net/packet/af_packet.c | 4 +- 12 files changed, 441 insertions(+), 17 deletions(-) -- MST