On Fri, Oct 23, 2026 at 11:09:17AM -0400, Michael S. Tsirkin wrote: > > 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.
Ok, I looked at netdev bot review, and there's another issue. 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 checksum will overwrite that. > ------ > > As reported first by Paulos Yibelo <[email protected]> (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" > > 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 >
