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Thu, 22 Oct 2026 05:57:56 -0700 (PDT) Date: Thu, 22 Oct 2026 08:57:54 -0400 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 net v5 0/7] 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: dGjvkng-zQSF7FzIWREHnPerJWQfvlwKA-h05jYRzhI_1792673879 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit Okay this seems to be converging.... But sashiko did find some issues I thought are best fixed. Specifically I thought of a better way to handle short gso in a tunnel - just clear tunnel gso! As a side effect we now fix up the checksum for these dodgy packets like we did before, instead of a drop. Good. And more false positives that prompted me to write a bit more comments. So here goes. Apologies for reposting so frequently, I know netdev bot will be unhappy with that. But Paulos's report made a lot of publicity, there is a lot of people looking into this, so I feel there's value in defending against all this class of issues sooner rather than later. 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() virtio_net: cleanup protocol calculation - cleanup resulting code duplication (kept separate to simplify backporting) 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) Tested: full suite on the final tree, tunnel included. um part compiled only. Michael S. Tsirkin (7): 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 net: add ip4_parse() virtio_net: cleanup protocol calculation 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/ip.h | 37 +++ include/linux/virtio_net.h | 287 ++++++++++++++++++++++- include/net/netns/core.h | 1 + net/bridge/br_netfilter_hooks.c | 15 +- net/core/dev.c | 7 + net/core/net_namespace.c | 1 + net/core/sysctl_net_core.c | 50 ++++ net/ipv4/ip_input.c | 13 +- net/ipv4/netfilter/nf_reject_ipv4.c | 13 +- net/packet/af_packet.c | 4 +- 16 files changed, 461 insertions(+), 43 deletions(-) -- MST