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Mon, 19 Oct 2026 05:03:38 -0700 (PDT) X-Received: by 2002:a05:600c:6207:b0:4a1:e3e4:c438 with SMTP id 5b1f17b1804b1-4a1e3e4c4efmr78429605e9.26.1792411417380; Mon, 19 Oct 2026 05:03:37 -0700 (PDT) Received: from redhat.com ([2a0d:6fc0:3fd7:5300:3d6b:52a4:a23f:9d0b]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4a1bdbb4a30sm354173035e9.15.2026.10.19.05.03.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 19 Oct 2026 05:03:34 -0700 (PDT) Date: Mon, 19 Oct 2026 08:03:31 -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 , Jonathan Corbet , Shuah Khan , Randy Dunlap , Simon Horman , linux-doc@vger.kernel.org Subject: [PATCH net v3 6/8] net: add a sysctl to opt out of virtio net hdr checksum validation Message-ID: <6f23c55c6eba3e3ec6b2c7e3a80728a326ed58af.1792411304.git.mst@redhat.com> References: Precedence: bulk X-Mailing-List: linux-kernel@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: X-Mailer: git-send-email 2.51.2.2891.g4157995a80.dirty X-Mutt-Fcc: =sent 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 | 19 ++++++++ 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 | 60 ++++++++++++++++++++++++ 6 files changed, 110 insertions(+) diff --git a/Documentation/admin-guide/sysctl/net.rst b/Documentation/admin-guide/sysctl/net.rst index e586e17fc7a5..b1a1c4714b22 100644 --- a/Documentation/admin-guide/sysctl/net.rst +++ b/Documentation/admin-guide/sysctl/net.rst @@ -246,6 +246,25 @@ processes are dropped unless socket option SOF_TIMESTAMPING_OPT_TSONLY is set. Default: 1 (on) +virtio_net_hdr_csum_check +------------------------- + +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. + +The setting is per network namespace, but only writable by the host +root: root inside a network namespace created by an unprivileged user +cannot disable the validation. + +Default: 1 (on) + + wmem_default ------------ diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h index 97b554ef7123..25aa95c11e31 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. + */ +DECLARE_STATIC_KEY_FALSE(virtio_net_hdr_csum_check_disabled_key); + +/* Failsafe: a namespace can disable the check with + * net.core.virtio_net_hdr_csum_check. + */ +static inline bool virtio_net_hdr_csum_check_disabled(struct sk_buff *skb) +{ + return static_branch_unlikely(&virtio_net_hdr_csum_check_disabled_key) && + !READ_ONCE(dev_net(skb->dev)->core.sysctl_virtio_net_hdr_csum_check); +} /* Where does the L3 header start, and what protocol is it? * Handles Ethernet and VLAN headers; other devices use the @@ -179,6 +193,12 @@ 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. + */ + if (virtio_net_hdr_csum_check_disabled(skb)) + return true; + if (!virtio_net_l4_start(skb, nhoff, proto, &thoff, &ip_proto)) return false; @@ -516,6 +536,9 @@ 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))) diff --git a/include/net/netns/core.h b/include/net/netns/core.h index 9ef3d70e5e9c..ae8110772c97 100644 --- a/include/net/netns/core.h +++ b/include/net/netns/core.h @@ -18,6 +18,7 @@ struct netns_core { u8 sysctl_txrehash; u8 sysctl_tstamp_allow_data; u8 sysctl_bypass_prot_mem; + u8 sysctl_virtio_net_hdr_csum_check; #ifdef CONFIG_PROC_FS struct prot_inuse __percpu *prot_inuse; diff --git a/net/core/dev.c b/net/core/dev.c index e76762e29360..d14950b1af3a 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -2389,6 +2389,12 @@ EXPORT_SYMBOL(tcf_sw_enabled_key); DEFINE_STATIC_KEY_FALSE(netstamp_needed_key); EXPORT_SYMBOL(netstamp_needed_key); + +/* Enabled while any network namespace has net.core.virtio_net_hdr_csum_check + * set to 0. + */ +DEFINE_STATIC_KEY_FALSE(virtio_net_hdr_csum_check_disabled_key); +EXPORT_SYMBOL(virtio_net_hdr_csum_check_disabled_key); #ifdef CONFIG_JUMP_LABEL static atomic_t netstamp_needed_deferred; static atomic_t netstamp_wanted; diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c index da5f881fbd3b..2d340fafba85 100644 --- a/net/core/net_namespace.c +++ b/net/core/net_namespace.c @@ -396,6 +396,7 @@ static __net_init void preinit_net_sysctl(struct net *net) net->core.sysctl_optmem_max = 128 * 1024; net->core.sysctl_txrehash = SOCK_TXREHASH_ENABLED; net->core.sysctl_tstamp_allow_data = 1; + net->core.sysctl_virtio_net_hdr_csum_check = 1; net->core.sysctl_txq_reselection = msecs_to_jiffies(1000); } diff --git a/net/core/sysctl_net_core.c b/net/core/sysctl_net_core.c index eb35da3556f4..c65ff86cc5f9 100644 --- a/net/core/sysctl_net_core.c +++ b/net/core/sysctl_net_core.c @@ -18,6 +18,7 @@ #include #include #include +#include #include #include @@ -678,6 +679,54 @@ static struct ctl_table net_core_table[] = { }, }; +/* 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; + + /* Root in a user namespace must not be able to weaken the + * validation for the network namespace it controls. + */ + if (write && !capable(CAP_NET_ADMIN)) + return -EPERM; + + /* + * 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 +786,15 @@ 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 +899,8 @@ 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