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[35.245.46.111]) by smtp.gmail.com with ESMTPSA id 71dfb90a1353d-5ea994fcf47sm222175e0c.15.2026.10.15.10.50.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 15 Oct 2026 10:50:55 -0700 (PDT) Date: Thu, 15 Oct 2026 13:50:54 -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 , Jonathan Corbet , Shuah Khan , Randy Dunlap , Simon Horman , linux-doc@vger.kernel.org Message-ID: In-Reply-To: References: Subject: Re: [PATCH net-next v2 5/7] net: add a sysctl to opt out of virtio net hdr checksum validation Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev 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: > 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. > + > +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? 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. A global sysctl also allows using proc_do_static_key and avoids the custom logic in virtio_net_hdr_csum_check_sysctl. > /* Where does the L3 header start, and what protocol is it? > * Handles Ethernet and VLAN headers; other devices use the > @@ -152,6 +166,12 @@ virtio_net_csum_partial_ok_at_offset(struct sk_buff *skb, u8 hdr_gso_type, > unsigned int thoff; > int ip_proto; > > + /* Failsafe: a namespace opted out of the validation, > + * trust the checksum metadata as before. > + */ nit: as before, imho the code here is clear and comment unnecessary. > + if (virtio_net_hdr_csum_check_disabled(skb)) > + return true; > + > if (!virtio_net_l4_start(skb, nhoff, proto, &thoff, &ip_proto)) > return false; > > @@ -483,6 +503,9 @@ virtio_net_tnl_offsets_ok(struct sk_buff *skb, bool outer_isv6, > unsigned int thoff; > int ip_proto; > > + if (virtio_net_hdr_csum_check_disabled(skb)) > + return true; > + > if (!virtio_net_l3_start(skb, &nhoff, &proto)) > return false; > if (outer_isv6 != (proto == htons(ETH_P_IPV6))) > +/* Defined in net/core/dev.c, declared for the gate in > + * include/linux/virtio_net.h. > + */ > +DECLARE_STATIC_KEY_FALSE(virtio_net_hdr_csum_check_disabled_key); > + > +/* > + * Protects changes to virtio_net_hdr_csum_check_disabled_key and each > + * net->core.sysctl_virtio_net_hdr_csum_check. Readers don't use a lock. > + */ > +static DEFINE_MUTEX(virtio_net_hdr_csum_check_mutex); > + > +static int virtio_net_hdr_csum_check_sysctl(const struct ctl_table *table, > + int write, void *buffer, > + size_t *lenp, loff_t *ppos) > +{ > + struct net *net = (struct net *)table->data; > + /* checkpatch warns but this can't be const: tmp.data is set below */ > + struct ctl_table tmp = *table; > + u8 old; > + int ret; > + > + /* > + * Make sure virtio_net_hdr_csum_check_disabled_key counts zero values of > + * net->core.sysctl_virtio_net_hdr_csum_check. > + */ > + mutex_lock(&virtio_net_hdr_csum_check_mutex); > + > + old = net->core.sysctl_virtio_net_hdr_csum_check; > + tmp.data = &net->core.sysctl_virtio_net_hdr_csum_check; > + ret = proc_dou8vec_minmax(&tmp, write, buffer, lenp, ppos); > + if (write && !ret && old != net->core.sysctl_virtio_net_hdr_csum_check) { > + if (net->core.sysctl_virtio_net_hdr_csum_check) > + static_branch_dec(&virtio_net_hdr_csum_check_disabled_key); > + else > + static_branch_inc(&virtio_net_hdr_csum_check_disabled_key); > + } > + > + mutex_unlock(&virtio_net_hdr_csum_check_mutex); > + > + return ret; > +} > + > static const struct ctl_table netns_core_table[] = { > #if IS_ENABLED(CONFIG_RPS) > { > @@ -737,6 +780,15 @@ static const struct ctl_table netns_core_table[] = { > .extra1 = SYSCTL_ZERO, > .extra2 = SYSCTL_ONE > }, > + { > + .procname = "virtio_net_hdr_csum_check", > + .data = &init_net, > + .maxlen = sizeof(u8), > + .mode = 0644, > + .proc_handler = virtio_net_hdr_csum_check_sysctl, > + .extra1 = SYSCTL_ZERO, > + .extra2 = SYSCTL_ONE > + }, > /* sysctl_core_net_init() will set the values after this > * to readonly in network namespaces > */ > @@ -841,6 +893,8 @@ static __net_exit void sysctl_core_net_exit(struct net *net) > tbl = net->core.sysctl_hdr->ctl_table_arg; > unregister_net_sysctl_table(net->core.sysctl_hdr); > BUG_ON(tbl == netns_core_table); > + if (!net->core.sysctl_virtio_net_hdr_csum_check) > + static_branch_dec(&virtio_net_hdr_csum_check_disabled_key); > #if IS_ENABLED(CONFIG_RPS) > kfree(net->core.rps_default_mask); > #endif > -- > MST >