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Fri, 16 Oct 2026 10:51:26 -0700 (PDT) Date: Fri, 16 Oct 2026 13:51:21 -0400 From: "Michael S. Tsirkin" To: Willem de Bruijn Cc: linux-kernel@vger.kernel.org, Jason Wang , Eugenio =?iso-8859-1?Q?P=E9rez?= , 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 , Jonathan Corbet , Shuah Khan , Randy Dunlap , Simon Horman , linux-doc@vger.kernel.org Subject: Re: [PATCH net-next v2 5/7] net: add a sysctl to opt out of virtio net hdr checksum validation Message-ID: <20261016134823-mutt-send-email-mst@kernel.org> References: <20261016051827-mutt-send-email-mst@kernel.org> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Fri, Oct 16, 2026 at 11:30:17AM -0400, Willem de Bruijn wrote: > Michael S. Tsirkin wrote: > > On Thu, Oct 15, 2026 at 01:50:54PM -0400, Willem de Bruijn wrote: > > > Michael S. Tsirkin wrote: > > > > The ingress validation as implemented by the previous > > > > patch is stricter than what it replaced: GSO > > > > packets whose checksum metadata matches no transport header are > > > > dropped, and non-GSO ones get their checksum completed in place. > > > > Such packets used to pass with the metadata unvalidated, > > > > and with no overhead. > > > > > > > > I judge that this should not affect any deployments, but there is still > > > > some risk that it does. In case a deployment depends on them anyway, > > > > provide a per-netns switch to get the old behavior back: > > > > > > > > echo 0 > /proc/sys/net/core/virtio_net_hdr_csum_check > > > > > > > > The default is validating. > > > > > > > > Assisted-by: LLM > > > > Signed-off-by: Michael S. Tsirkin > > > > --- > > > > Documentation/admin-guide/sysctl/net.rst | 15 +++++++ > > > > include/linux/virtio_net.h | 23 ++++++++++ > > > > include/net/netns/core.h | 1 + > > > > net/core/dev.c | 6 +++ > > > > net/core/net_namespace.c | 1 + > > > > net/core/sysctl_net_core.c | 54 ++++++++++++++++++++++++ > > > > 6 files changed, 100 insertions(+) > > > > > > > > diff --git a/Documentation/admin-guide/sysctl/net.rst b/Documentation/admin-guide/sysctl/net.rst > > > > index e586e17fc7a5..7f230929c37e 100644 > > > > --- a/Documentation/admin-guide/sysctl/net.rst > > > > +++ b/Documentation/admin-guide/sysctl/net.rst > > > > @@ -246,6 +246,21 @@ processes are dropped unless socket option SOF_TIMESTAMPING_OPT_TSONLY is set. > > > > Default: 1 (on) > > > > > > > > > > > > +virtio_net_hdr_csum_check > > > > +------------------------- > > > > > > Make more generic, virtio_net_hdr_validate? > > > > > > In case we add more validation in the future (as we have in the past), > > > without having to introduce multiple such knobs. > > > > But it's uapi, are you sure we'll be able to change the meaning > > of an existing knob, even if we keep the name vague? > > > > > > + > > > > +When processing packets with a virtio net header (through tun, tap, a > > > > +packet socket or a virtio network device) that request checksum offload > > > > +(VIRTIO_NET_HDR_F_NEEDS_CSUM), validate the packet: complete the checksum > > > > +in software, or drop the packet where the header does not match it. > > > > + > > > > +When disabled, the header is trusted not to corrupt the packet, and the > > > > +checksum is offloaded to hardware (or not verified if it never reaches > > > > +hardware). This is only safe if no protocol is processing the packets. > > > > + > > > > +Default: 1 (on) > > > > + > > > > + > > > > wmem_default > > > > ------------ > > > > > > > > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h > > > > index 357115ff0ec2..aa2f8ac4657c 100644 > > > > --- a/include/linux/virtio_net.h > > > > +++ b/include/linux/virtio_net.h > > > > @@ -5,6 +5,7 @@ > > > > #include > > > > #include > > > > #include > > > > +#include > > > > #include > > > > #include > > > > #include > > > > @@ -78,6 +79,19 @@ static inline struct iphdr *virtio_net_ip4_parse(struct sk_buff *skb, > > > > *thoff = nhoff + iph->ihl * sizeof(u32); > > > > return iph; > > > > } > > > > +/* Defined in net/core/dev.c, enabled while any network namespace > > > > + * has net.core.virtio_net_hdr_csum_check set to 0. > > > > + */ > > > > > > Should this be a global sysctl, rather than per netns? > > > > > > For customers I work with, it would be much less useful. > > Third option: make it an integer rather than a boolean. > > 0: no validation > 1: current level of validation > 2 and beyond: if we have to extend it later > > Not pretty, I'll admit. So fine to keep as is too if you prefer. > But I do foresee more changes in this area, if history is a guide. > > > > > > Even if per-netns, most likely do not want it writable by netns root > > > with user namespaces. > > > > > > That is the equivalent of ns_capable(CAP_NET_RAW), which is the cause > > > of many crashes due to bad packets ingested by, e.g., a packet socket. > > > > Hmm, yes i get that. Two knobs? a. verify b. the per netns value ro? Both > > on by default? > > A single system wide field is simplest. > > Is there a benefit to making this configurable on a per netns basis? As I tried to explain: a specific container breaks, you decide to trust it, you do not need to trust all. > If so, that's fine too. As long as root-in-namespace cannot modify it, > so readonly indeed. Right. So I envision along the lines of: net.core.virtio_net_hdr_validate_all - validate packets from any netns, netns root can not change it, only host root net.core.virtio_net_hdr_validate - per netns both set by default, validation runs if either is set. -- MST