[PATCH net-next v2 0/7] more virtio-net csum hardening

[PATCH net-next v2 0/7] more virtio-net csum hardening

[Michael S. Tsirkin]

As reported first by Paulos Yibelo <habte.yibelo@gmail.com> (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:
  net: account for ingress-resolved virtio net hdr state
	- accounting, not the fix itself, dependency of the next one
  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
  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


Michael S. Tsirkin (7):
  net: account for ingress-resolved virtio net hdr state
  um: vector: honor virtio net hdr conversion failures
  um: vector: trim skbs before verifying the header
  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 |  15 ++
 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                       |  24 +++
 include/linux/virtio_net.h               | 249 ++++++++++++++++++++++-
 include/net/netns/core.h                 |   1 +
 net/bridge/br_netfilter_hooks.c          |  15 +-
 net/core/dev.c                           |   6 +
 net/core/net_namespace.c                 |   1 +
 net/core/sysctl_net_core.c               |  54 +++++
 net/ipv4/ip_input.c                      |  13 +-
 net/ipv4/netfilter/nf_reject_ipv4.c      |  13 +-
 net/packet/af_packet.c                   |   4 +-
 16 files changed, 404 insertions(+), 48 deletions(-)

-- 
MST

[PATCH net-next v2 1/7] net: account for ingress-resolved virtio net hdr state

[Michael S. Tsirkin]

The following patch will add ingress resolution of uncommon virtio net
hdr checksum metadata (software checksum completion at ingress) which
adds performance overhead to the packet processing.

We should make it visible, so a hypothetical workload relying on
uncommon metadata can be distinguished from none at all. This way issues
in the parsing will be reported and not quietly pay the host-CPU
checksum overhead instead of offload.

Flip every call to virtio_net_hdr_to_skb to test for negative errno
only, and have tun count these events per device as rx_csum_sw, exposed
via ethtool -S.

Assisted-by: LLM
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 drivers/net/tap.c        |  4 ++--
 drivers/net/tun.c        | 41 ++++++++++++++++++++++++++++++++++++++--
 drivers/net/virtio_net.c |  2 +-
 net/packet/af_packet.c   |  4 ++--
 4 files changed, 44 insertions(+), 7 deletions(-)

diff --git a/drivers/net/tap.c b/drivers/net/tap.c
index ff67d99deb39..73e883394446 100644
--- a/drivers/net/tap.c
+++ b/drivers/net/tap.c
@@ -649,7 +649,7 @@ static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,
 
 	if (vnet_hdr_len) {
 		err = tun_vnet_hdr_to_skb(q->flags, skb, &vnet_hdr);
-		if (err) {
+		if (err < 0) {
 			rcu_read_unlock();
 			drop_reason = SKB_DROP_REASON_DEV_HDR;
 			goto err_kfree;
@@ -1084,7 +1084,7 @@ static int tap_get_user_xdp(struct tap_queue *q, struct xdp_buff *xdp)
 
 	if (vnet_hdr_len) {
 		err = tun_vnet_hdr_to_skb(q->flags, skb, gso);
-		if (err) {
+		if (err < 0) {
 			rcu_read_unlock();
 			goto err_kfree;
 		}
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 242899f7fd07..44d95a8c75a8 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -210,6 +210,7 @@ struct tun_struct {
 	u32 flow_count;
 	u32 rx_batched;
 	atomic_long_t rx_frame_errors;
+	atomic_long_t rx_csum_sw;
 	struct bpf_prog __rcu *xdp_prog;
 	struct tun_prog __rcu *steering_prog;
 	struct tun_prog __rcu *filter_prog;
@@ -1935,11 +1936,16 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 		break;
 	}
 
-	if (tun_vnet_hdr_tnl_to_skb(tun->flags, features, skb, &hdr)) {
+	err = tun_vnet_hdr_tnl_to_skb(tun->flags, features, skb, &hdr);
+	if (err < 0) {
 		atomic_long_inc(&tun->rx_frame_errors);
 		err = -EINVAL;
 		goto free_skb;
 	}
+	if (err > 0) {
+		atomic_long_inc(&tun->rx_csum_sw);
+		err = 0;
+	}
 
 	if ((tun->flags & TUN_TYPE_MASK) == IFF_TAP)
 		skb->protocol = eth_type_trans(skb, tun->dev);
@@ -2605,12 +2611,15 @@ static int tun_xdp_one(struct tun_struct *tun,
 	tnl_hdr = (struct virtio_net_hdr_v1_hash_tunnel *)gso;
 	skb->dev = tun->dev;
 	skb_set_network_header(skb, ETH_HLEN);
-	if (tun_vnet_hdr_tnl_to_skb(tun->flags, features, skb, tnl_hdr)) {
+	ret = tun_vnet_hdr_tnl_to_skb(tun->flags, features, skb, tnl_hdr);
+	if (ret < 0) {
 		atomic_long_inc(&tun->rx_frame_errors);
 		kfree_skb(skb);
 		ret = -EINVAL;
 		goto out;
 	}
+	if (ret > 0)
+		atomic_long_inc(&tun->rx_csum_sw);
 
 	skb->protocol = eth_type_trans(skb, tun->dev);
 	skb_reset_network_header(skb);
@@ -3739,7 +3748,35 @@ static void tun_get_channels(struct net_device *dev,
 	channels->max_combined = tun->flags & IFF_MULTI_QUEUE ? MAX_TAP_QUEUES : 1;
 }
 
+#define TUN_STATS_LEN	1
+static const char tun_ethtool_strings[TUN_STATS_LEN][ETH_GSTRING_LEN] = {
+	"rx_csum_sw",
+};
+
+static int tun_get_sset_count(struct net_device *dev, int sset)
+{
+	return sset == ETH_SS_STATS ? TUN_STATS_LEN : -EOPNOTSUPP;
+}
+
+static void tun_get_strings(struct net_device *dev, u32 stringset, u8 *data)
+{
+	if (stringset == ETH_SS_STATS)
+		memcpy(data, tun_ethtool_strings,
+		       sizeof(tun_ethtool_strings));
+}
+
+static void tun_get_ethtool_stats(struct net_device *dev,
+				  struct ethtool_stats *stats, u64 *data)
+{
+	struct tun_struct *tun = netdev_priv(dev);
+
+	data[0] = (unsigned long)atomic_long_read(&tun->rx_csum_sw);
+}
+
 static const struct ethtool_ops tun_ethtool_ops = {
+	.get_sset_count		= tun_get_sset_count,
+	.get_strings		= tun_get_strings,
+	.get_ethtool_stats	= tun_get_ethtool_stats,
 	.supported_coalesce_params = ETHTOOL_COALESCE_RX_MAX_FRAMES,
 	.get_drvinfo	= tun_get_drvinfo,
 	.get_msglevel	= tun_get_msglevel,
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index daab43ac92ce..2e6fd81df9c4 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -2519,7 +2519,7 @@ static void virtnet_receive_done(struct virtnet_info *vi, struct receive_queue *
 	skb_set_network_header(skb, ETH_HLEN);
 	if (virtio_net_hdr_tnl_to_skb(skb, &hdr->tnl_hdr, vi->rx_tnl,
 				      vi->rx_tnl_csum,
-				      virtio_is_little_endian(vi->vdev))) {
+				      virtio_is_little_endian(vi->vdev)) < 0) {
 		net_warn_ratelimited("%s: bad gso: type: %x, size: %u, flags %x tunnel %d tnl csum %d\n",
 				     dev->name, hdr->hdr.gso_type,
 				     hdr->hdr.gso_size, hdr->hdr.flags,
diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 52ae3d875e97..e6aec9ecfa99 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2913,7 +2913,7 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 		}
 
 		if (has_vnet_hdr) {
-			if (virtio_net_hdr_to_skb(skb, &vnet_hdr, vio_le())) {
+			if (virtio_net_hdr_to_skb(skb, &vnet_hdr, vio_le()) < 0) {
 				tp_len = -EINVAL;
 				goto tpacket_error;
 			}
@@ -3136,7 +3136,7 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
 
 	if (vnet_hdr_sz) {
 		err = virtio_net_hdr_to_skb(skb, &vnet_hdr, vio_le());
-		if (err)
+		if (err < 0)
 			goto out_free;
 		len += vnet_hdr_sz;
 	}
-- 
MST

[PATCH net-next v2 2/7] um: vector: honor virtio net hdr conversion failures

[Michael S. Tsirkin]

virtio_net_hdr_to_skb() refuses packets whose checksum metadata
it cannot make sense of, but the vector raw transport ignored
its return value: rejected packets went on into the stack with
their metadata unvalidated.

Propagate the failure.

Assisted-by: LLM
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 arch/um/drivers/vector_transports.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/um/drivers/vector_transports.c b/arch/um/drivers/vector_transports.c
index e1fb2a76fdff..8b7548458663 100644
--- a/arch/um/drivers/vector_transports.c
+++ b/arch/um/drivers/vector_transports.c
@@ -210,7 +210,9 @@ static int raw_verify_header(
 		return 1;
 
 	skb_set_network_header(skb, ETH_HLEN);
-	virtio_net_hdr_to_skb(skb, vheader, virtio_legacy_is_little_endian());
+	if (virtio_net_hdr_to_skb(skb, vheader,
+			virtio_legacy_is_little_endian()) < 0)
+		return -1;
 	return 0;
 }
 
-- 
MST

Re: [PATCH net-next v2 2/7] um: vector: honor virtio net hdr conversion failures

[Johannes Berg]

On Thu, 2026-10-15 at 08:41 -0400, Michael S. Tsirkin wrote:
> virtio_net_hdr_to_skb() refuses packets whose checksum metadata
> it cannot make sense of, but the vector raw transport ignored
> its return value: rejected packets went on into the stack with
> their metadata unvalidated.
> 
> Propagate the failure.
> 
> Assisted-by: LLM
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  arch/um/drivers/vector_transports.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/um/drivers/vector_transports.c b/arch/um/drivers/vector_transports.c
> index e1fb2a76fdff..8b7548458663 100644
> --- a/arch/um/drivers/vector_transports.c
> +++ b/arch/um/drivers/vector_transports.c
> @@ -210,7 +210,9 @@ static int raw_verify_header(
>  		return 1;
>  
>  	skb_set_network_header(skb, ETH_HLEN);
> -	virtio_net_hdr_to_skb(skb, vheader, virtio_legacy_is_little_endian());
> +	if (virtio_net_hdr_to_skb(skb, vheader,
> +			virtio_legacy_is_little_endian()) < 0)
> +		return -1;
>  	return 0;
> 

Seems it'd be simpler to just

	return virtio_net_hdr_to_skb(...);

As far as I can tell, the function only returns -EINVAL or 0 (never a
positive value), and the caller only cares about negative (error) or
positive (no checksum needed), so that'd be equivalent?

johannes

Re: [PATCH net-next v2 2/7] um: vector: honor virtio net hdr conversion failures

[Michael S. Tsirkin]

On Thu, Oct 15, 2026 at 02:47:01PM +0200, Johannes Berg wrote:
> On Thu, 2026-10-15 at 08:41 -0400, Michael S. Tsirkin wrote:
> > virtio_net_hdr_to_skb() refuses packets whose checksum metadata
> > it cannot make sense of, but the vector raw transport ignored
> > its return value: rejected packets went on into the stack with
> > their metadata unvalidated.
> > 
> > Propagate the failure.
> > 
> > Assisted-by: LLM
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  arch/um/drivers/vector_transports.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/um/drivers/vector_transports.c b/arch/um/drivers/vector_transports.c
> > index e1fb2a76fdff..8b7548458663 100644
> > --- a/arch/um/drivers/vector_transports.c
> > +++ b/arch/um/drivers/vector_transports.c
> > @@ -210,7 +210,9 @@ static int raw_verify_header(
> >  		return 1;
> >  
> >  	skb_set_network_header(skb, ETH_HLEN);
> > -	virtio_net_hdr_to_skb(skb, vheader, virtio_legacy_is_little_endian());
> > +	if (virtio_net_hdr_to_skb(skb, vheader,
> > +			virtio_legacy_is_little_endian()) < 0)
> > +		return -1;
> >  	return 0;
> > 
> 
> Seems it'd be simpler to just
> 
> 	return virtio_net_hdr_to_skb(...);
> 
> As far as I can tell, the function only returns -EINVAL or 0 (never a
> positive value), and the caller only cares about negative (error) or
> positive (no checksum needed), so that'd be equivalent?
> 
> johannes

if raw_verify_header can safely return any errno - i did not want to
audit it.

-- 
MST

Re: [PATCH net-next v2 2/7] um: vector: honor virtio net hdr conversion failures

[Johannes Berg]

On Thu, 2026-10-15 at 08:52 -0400, Michael S. Tsirkin wrote:
> On Thu, Oct 15, 2026 at 02:47:01PM +0200, Johannes Berg wrote:
> > On Thu, 2026-10-15 at 08:41 -0400, Michael S. Tsirkin wrote:
> > > virtio_net_hdr_to_skb() refuses packets whose checksum metadata
> > > it cannot make sense of, but the vector raw transport ignored
> > > its return value: rejected packets went on into the stack with
> > > their metadata unvalidated.
> > > 
> > > Propagate the failure.
> > > 
> > > Assisted-by: LLM
> > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > ---
> > >  arch/um/drivers/vector_transports.c | 4 +++-
> > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/um/drivers/vector_transports.c b/arch/um/drivers/vector_transports.c
> > > index e1fb2a76fdff..8b7548458663 100644
> > > --- a/arch/um/drivers/vector_transports.c
> > > +++ b/arch/um/drivers/vector_transports.c
> > > @@ -210,7 +210,9 @@ static int raw_verify_header(
> > >  		return 1;
> > >  
> > >  	skb_set_network_header(skb, ETH_HLEN);
> > > -	virtio_net_hdr_to_skb(skb, vheader, virtio_legacy_is_little_endian());
> > > +	if (virtio_net_hdr_to_skb(skb, vheader,
> > > +			virtio_legacy_is_little_endian()) < 0)
> > > +		return -1;
> > >  	return 0;
> > > 
> > 
> > Seems it'd be simpler to just
> > 
> > 	return virtio_net_hdr_to_skb(...);
> > 
> > As far as I can tell, the function only returns -EINVAL or 0 (never a
> > positive value), and the caller only cares about negative (error) or
> > positive (no checksum needed), so that'd be equivalent?
> > 
> > johannes
> 
> if raw_verify_header can safely return any errno - i did not want to
> audit it.

You already used an LLM and still didn't want to? ;-)

I checked briefly (by hand) and it seems fine, it's assigned to the
function pointer, which only has two callers on the _kern.c side.

johannes

Re: [PATCH net-next v2 2/7] um: vector: honor virtio net hdr conversion failures

[Willem de Bruijn]

Michael S. Tsirkin wrote:
> virtio_net_hdr_to_skb() refuses packets whose checksum metadata
> it cannot make sense of, but the vector raw transport ignored
> its return value: rejected packets went on into the stack with
> their metadata unvalidated.
> 
> Propagate the failure.
> 
> Assisted-by: LLM
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  arch/um/drivers/vector_transports.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/um/drivers/vector_transports.c b/arch/um/drivers/vector_transports.c
> index e1fb2a76fdff..8b7548458663 100644
> --- a/arch/um/drivers/vector_transports.c
> +++ b/arch/um/drivers/vector_transports.c
> @@ -210,7 +210,9 @@ static int raw_verify_header(
>  		return 1;
>  
>  	skb_set_network_header(skb, ETH_HLEN);
> -	virtio_net_hdr_to_skb(skb, vheader, virtio_legacy_is_little_endian());
> +	if (virtio_net_hdr_to_skb(skb, vheader,
> +			virtio_legacy_is_little_endian()) < 0)
> +		return -1;

Should this return 1, like the above?

Not sure whether the verify_header return value is well defined beyond
just boolean.

[PATCH net-next v2 3/7] um: vector: trim skbs before verifying the header

[Michael S. Tsirkin]

The vector receive path over-allocates the skb and only trims it
to the received length after the transport header is verified.

A follow up patch will complete the packet checksum in software within
virtio_net_hdr_to_skb so we need the packet trimmed (otherwise the
checksum would be computed incorrectly over the unreceived tail of the
buffer).

Trim first, so the import sees the packet as it was received.

Assisted-by: LLM
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 arch/um/drivers/vector_kern.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/um/drivers/vector_kern.c b/arch/um/drivers/vector_kern.c
index 8a70b3a62553..1afbef78df72 100644
--- a/arch/um/drivers/vector_kern.c
+++ b/arch/um/drivers/vector_kern.c
@@ -865,6 +865,7 @@ static int vector_legacy_rx(struct vector_private *vp)
 
 	if (skb != NULL) {
 		if (pkt_len > vp->header_size) {
+			pskb_trim(skb, pkt_len - vp->rx_header_size);
 			if (vp->header_size > 0) {
 				header_check = vp->verify_header(
 					vp->header_rxbuffer, skb, vp);
@@ -879,7 +880,6 @@ static int vector_legacy_rx(struct vector_private *vp)
 					skb->ip_summed = CHECKSUM_UNNECESSARY;
 				}
 			}
-			pskb_trim(skb, pkt_len - vp->rx_header_size);
 			skb->protocol = eth_type_trans(skb, skb->dev);
 			vp->dev->stats.rx_bytes += skb->len;
 			vp->dev->stats.rx_packets++;
@@ -981,6 +981,8 @@ static int vector_mmsg_rx(struct vector_private *vp, int budget)
 	for (i = 0; i < packet_count; i++) {
 		skb = (*skbuff_vector);
 		if (mmsg_vector->msg_len > vp->header_size) {
+			pskb_trim(skb,
+				mmsg_vector->msg_len - vp->rx_header_size);
 			if (vp->header_size > 0) {
 				header_check = vp->verify_header(
 					mmsg_vector->msg_hdr.msg_iov->iov_base,
@@ -1005,8 +1007,6 @@ static int vector_mmsg_rx(struct vector_private *vp, int budget)
 					skb->ip_summed = CHECKSUM_UNNECESSARY;
 				}
 			}
-			pskb_trim(skb,
-				mmsg_vector->msg_len - vp->rx_header_size);
 			skb->protocol = eth_type_trans(skb, skb->dev);
 			/*
 			 * We do not need to lock on updating stats here
-- 
MST

[PATCH net-next v2 4/7] net: resolve uncommon partial checksums at ingress

[Michael S. Tsirkin]

As reported first by Paulos Yibelo <habte.yibelo@gmail.com> (after which
others, credited below, found more ways the bug can affect users) a
virtio net header can place CHECKSUM_PARTIAL at any in-packet offset.
If that state is retained, later checksum completion can modify the
packet after protocol processing has made decisions from it.

Let's fix that: let's validate csum_start and csum_offset and only keep
CHECKSUM_PARTIAL when csum_start and csum_offset match a parsed TCP or
UDP header, while handling VLAN headers, IPv4 options and IPv6 extension
headers.  Complete other non-GSO checksums immediately, before any
protocol processing runs.

We can't easily "complete the checksum" for GSO we don't recognize - GSO
assumes knowledge of the protocol. We could segment ignoring the
checksum then do whatever csum_start and csum_offset say, but that
seems like an overkill for something that shouldn't happen.

There's some risk here:
- non-gso packets which can't be parsed get checksum calculated in
  software, adding performance overhead
- gso packets which can't be parsed get dropped

To mitigate the risks, the following patch will add a sysctl knob to
bypass the resolution. This is reasonable if the setup is such that
there is no protocol processing at all.

UDP tunnel headers get the same handling on the inner header.

Packets without VIRTIO_NET_HDR_F_NEEDS_CSUM are unaffected.

Assisted-by: LLM
Reported-by: Paulos Yibelo <habte.yibelo@gmail.com>
Reported-by: Chris Arges <carges@cloudflare.com>
Tested-by: Chris Arges <carges@cloudflare.com>
Fixes: f43798c27684 ("tun: Allow GSO using virtio_net_hdr")
Suggested-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 include/linux/virtio_net.h | 251 +++++++++++++++++++++++++++++++++++--
 1 file changed, 241 insertions(+), 10 deletions(-)

diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index d6466f96cdd0..357115ff0ec2 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -5,7 +5,11 @@
 #include <linux/if_vlan.h>
 #include <linux/ip.h>
 #include <linux/ipv6.h>
+#include <linux/netdevice.h>
 #include <linux/udp.h>
+#include <net/gso.h>
+#include <net/ip.h>
+#include <net/ipv6.h>
 #include <net/tcp.h>
 #include <uapi/linux/tcp.h>
 #include <uapi/linux/virtio_net.h>
@@ -48,6 +52,171 @@ static inline int virtio_net_hdr_set_proto(struct sk_buff *skb,
 	return 0;
 }
 
+/* TODO: nothing virtio specific here, move to ip.h */
+
+/*
+ * Pull and minimally validate the IPv4 header at @nhoff.
+ * Returns the header and stores the transport header offset in
+ * @thoff, or NULL if truncated or malformed.  Only the fixed header
+ * is guaranteed linear; callers that need the options or the
+ * transport header must pull further.
+ */
+static inline struct iphdr *virtio_net_ip4_parse(struct sk_buff *skb,
+						  unsigned int nhoff,
+						  unsigned int *thoff)
+{
+	struct iphdr *iph;
+
+	if (!pskb_may_pull(skb, nhoff + sizeof(struct iphdr)))
+		return NULL;
+
+	iph = (struct iphdr *)(skb->data + nhoff);
+	if (iph->version != 4 ||
+	    iph->ihl < sizeof(struct iphdr) / sizeof(u32))
+		return NULL;
+
+	*thoff = nhoff + iph->ihl * sizeof(u32);
+	return iph;
+}
+
+/* Where does the L3 header start, and what protocol is it?
+ * Handles Ethernet and VLAN headers; other devices use the
+ * network header offset the caller set up.
+ */
+static inline bool
+virtio_net_l3_start(struct sk_buff *skb, int *nhoff, __be16 *proto)
+{
+	if (skb->dev->type == ARPHRD_ETHER) {
+		if (!pskb_may_pull(skb, ETH_HLEN))
+			return false;
+		*nhoff = ETH_HLEN;
+		*proto = eth_hdr(skb)->h_proto;
+		if (eth_type_vlan(*proto)) {
+			*proto = __vlan_get_protocol(skb, *proto, nhoff);
+			if (!*proto)
+				return false;
+		}
+	} else {
+		*nhoff = skb_network_offset(skb);
+		*proto = skb->protocol;
+	}
+	return true;
+}
+
+/* Walk the L3 header at @nhoff: where does its transport header
+ * start, and which protocol is it?  Returns false if the header
+ * is truncated, malformed or fragmented.
+ */
+static inline bool
+virtio_net_l4_start(struct sk_buff *skb, unsigned int nhoff, __be16 proto,
+		    unsigned int *thoff, int *ip_proto)
+{
+	if (proto == htons(ETH_P_IP)) {
+		const struct iphdr *iph = virtio_net_ip4_parse(skb, nhoff, thoff);
+
+		if (!iph || ip_is_fragment(iph))
+			return false;
+		*ip_proto = iph->protocol;
+		return true;
+	}
+
+	if (proto == htons(ETH_P_IPV6)) {
+		const struct ipv6hdr *iph;
+		int flags = 0;
+
+		if (!pskb_may_pull(skb, nhoff + sizeof(*iph)))
+			return false;
+		iph = (const void *)(skb->data + nhoff);
+		if (iph->version != 6)
+			return false;
+		*thoff = nhoff + sizeof(*iph);
+		*ip_proto = iph->nexthdr;
+		if (*ip_proto != IPPROTO_TCP && *ip_proto != IPPROTO_UDP) {
+			*thoff = nhoff;
+			*ip_proto = ipv6_find_hdr(skb, thoff, -1, NULL,
+						  &flags);
+			if (*ip_proto < 0 || flags & IP6_FH_F_FRAG)
+				return false;
+		}
+		return true;
+	}
+
+	return false;
+}
+
+/* Does checksum metadata match a known TCP or UDP header? */
+static inline bool
+virtio_net_csum_partial_ok_at_offset(struct sk_buff *skb, u8 hdr_gso_type,
+				     unsigned int nhoff, __be16 proto)
+{
+	unsigned int thoff;
+	int ip_proto;
+
+	if (!virtio_net_l4_start(skb, nhoff, proto, &thoff, &ip_proto))
+		return false;
+
+	if (skb_checksum_start_offset(skb) != thoff)
+		return false;
+
+	switch (ip_proto) {
+	case IPPROTO_TCP:
+		if (skb->csum_offset != offsetof(struct tcphdr, check) ||
+		    !pskb_may_pull(skb, thoff + sizeof(struct tcphdr)))
+			return false;
+		break;
+	case IPPROTO_UDP:
+		if (skb->csum_offset != offsetof(struct udphdr, check) ||
+		    !pskb_may_pull(skb, thoff + sizeof(struct udphdr)))
+			return false;
+		break;
+	default:
+		return false;
+	}
+
+	switch (hdr_gso_type & ~VIRTIO_NET_HDR_GSO_ECN) {
+	case VIRTIO_NET_HDR_GSO_NONE:
+		return true;
+	case VIRTIO_NET_HDR_GSO_TCPV4:
+		return proto == htons(ETH_P_IP) && ip_proto == IPPROTO_TCP;
+	case VIRTIO_NET_HDR_GSO_TCPV6:
+		return proto == htons(ETH_P_IPV6) && ip_proto == IPPROTO_TCP;
+	case VIRTIO_NET_HDR_GSO_UDP:
+	case VIRTIO_NET_HDR_GSO_UDP_L4:
+		return ip_proto == IPPROTO_UDP;
+	default:
+		return false;
+	}
+}
+
+static inline bool
+virtio_net_csum_partial_ok(struct sk_buff *skb, u8 hdr_gso_type)
+{
+	int nhoff;
+	__be16 proto;
+
+	if (!virtio_net_l3_start(skb, &nhoff, &proto))
+		return false;
+
+	return virtio_net_csum_partial_ok_at_offset(skb, hdr_gso_type, nhoff,
+						   proto);
+}
+
+/*
+ * Handle a packet whose checksum metadata does not match the packet:
+ * GSO packets are dropped, as segmentation needs one checksum per
+ * segment; others are completed in software.
+ *
+ * Return 1 if the checksum was completed in software, else a negative
+ * errno.
+ */
+static inline int virtio_net_resolve_csum_partial_mismatch(struct sk_buff *skb)
+{
+	if (skb_is_gso(skb))
+		return -EINVAL;
+
+	return skb_checksum_help(skb) ?: 1;
+}
+
 static inline int __virtio_net_hdr_to_skb(struct sk_buff *skb,
 					  const struct virtio_net_hdr *hdr,
 					  bool little_endian, u8 hdr_gso_type)
@@ -158,8 +327,6 @@ static inline int __virtio_net_hdr_to_skb(struct sk_buff *skb,
 		case SKB_GSO_UDP_L4:
 			if (!(hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM))
 				return -EINVAL;
-			if (skb->csum_offset != offsetof(struct udphdr, check))
-				return -EINVAL;
 			if (skb->len - p_off > gso_size * UDP_MAX_SEGMENTS)
 				return -EINVAL;
 			if (gso_type != SKB_GSO_UDP_L4)
@@ -167,10 +334,6 @@ static inline int __virtio_net_hdr_to_skb(struct sk_buff *skb,
 			break;
 		case SKB_GSO_TCPV4:
 		case SKB_GSO_TCPV6:
-			if (skb->ip_summed == CHECKSUM_PARTIAL &&
-			    skb->csum_offset != offsetof(struct tcphdr, check))
-				return -EINVAL;
-
 			BUILD_BUG_ON(TCP_MIN_GSO_SIZE * GSO_MAX_SEGS < GSO_MAX_SIZE);
 			gso_size = max(gso_size, TCP_MIN_GSO_SIZE);
 			break;
@@ -194,11 +357,36 @@ static inline int __virtio_net_hdr_to_skb(struct sk_buff *skb,
 	return 0;
 }
 
+/*
+ * Return value:
+ * 0 - success
+ * >0 - checksum was completed in software (slow path)
+ * <0 - failure
+ */
 static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
 					const struct virtio_net_hdr *hdr,
 					bool little_endian)
 {
-	return __virtio_net_hdr_to_skb(skb, hdr, little_endian, hdr->gso_type);
+	typeof(skb->transport_header) old_transport = skb->transport_header;
+	int ret = __virtio_net_hdr_to_skb(skb, hdr, little_endian,
+					  hdr->gso_type);
+
+	if (ret < 0)
+		return ret;
+
+	if (skb->ip_summed != CHECKSUM_PARTIAL ||
+	    virtio_net_csum_partial_ok(skb, hdr->gso_type))
+		return 0;
+
+	/*
+	 * Here we do not trust the checksum metadata.
+	 * __virtio_net_hdr_to_skb set the transport header to csum_start
+	 * from virtio hdr, restore it to a known good value.
+	 */
+	skb->transport_header = old_transport;
+	ret = virtio_net_resolve_csum_partial_mismatch(skb);
+
+	return ret;
 }
 
 /* This function must be called after virtio_net_hdr_from_skb(). */
@@ -282,6 +470,35 @@ static inline unsigned int virtio_l3min(bool is_ipv6)
 	return is_ipv6 ? sizeof(struct ipv6hdr) : sizeof(struct iphdr);
 }
 
+/* Do the tunnel offsets match the outer headers in the frame?
+ * The L3 header the tunnel claims must be the one the frame
+ * carries, with UDP following it at @outer_th.
+ */
+static inline bool
+virtio_net_tnl_offsets_ok(struct sk_buff *skb, bool outer_isv6,
+			  unsigned int outer_th)
+{
+	int nhoff;
+	__be16 proto;
+	unsigned int thoff;
+	int ip_proto;
+
+	if (!virtio_net_l3_start(skb, &nhoff, &proto))
+		return false;
+	if (outer_isv6 != (proto == htons(ETH_P_IPV6)))
+		return false;
+	if (!virtio_net_l4_start(skb, nhoff, proto, &thoff, &ip_proto))
+		return false;
+
+	return ip_proto == IPPROTO_UDP && outer_th == thoff;
+}
+
+/*
+ * Return value:
+ * 0 - success
+ * >0 - checksum was completed in software (slow path)
+ * <0 - failure
+ */
 static inline int
 virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
 			  const struct virtio_net_hdr_v1_hash_tunnel *vhdr,
@@ -293,6 +510,7 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
 	unsigned int inner_nh, outer_th, inner_th;
 	unsigned int inner_l3min, outer_l3min;
 	u8 gso_inner_type, gso_tunnel_type;
+	__be16 inner_proto;
 	bool outer_isv6, inner_isv6;
 	int ret;
 
@@ -332,10 +550,13 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
 	    inner_th < inner_nh + inner_l3min)
 		return -EINVAL;
 
+	if (!virtio_net_tnl_offsets_ok(skb, outer_isv6, outer_th))
+		return -EINVAL;
+
 	/* Let the basic parsing deal with plain GSO features. */
 	ret = __virtio_net_hdr_to_skb(skb, hdr, true,
 				      hdr->gso_type & ~gso_tunnel_type);
-	if (ret)
+	if (ret < 0)
 		return ret;
 
 	/* In case of USO, the inner protocol is still unknown and
@@ -353,7 +574,7 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
 	}
 
 	skb_set_inner_protocol(skb, inner_isv6 ? htons(ETH_P_IPV6) :
-						 htons(ETH_P_IP));
+			    htons(ETH_P_IP));
 	if (hdr->flags & VIRTIO_NET_HDR_F_UDP_TUNNEL_CSUM) {
 		if (!tnl_csum_negotiated)
 			return -EINVAL;
@@ -368,7 +589,17 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
 	skb->inner_mac_header = inner_nh + skb_headroom(skb);
 	skb->transport_header = outer_th + skb_headroom(skb);
 	skb->encapsulation = 1;
-	return 0;
+
+	/*
+	 * We know VIRTIO_NET_HDR_F_NEEDS_CSUM was set so __virtio_net_hdr_to_skb
+	 * set CHECKSUM_PARTIAL.
+	 */
+	inner_proto = inner_isv6 ? htons(ETH_P_IPV6) : htons(ETH_P_IP);
+	if (virtio_net_csum_partial_ok_at_offset(skb, gso_inner_type, inner_nh,
+						 inner_proto))
+		return 0;
+
+	return virtio_net_resolve_csum_partial_mismatch(skb);
 }
 
 /* Checksum-related fields validation for the driver */
-- 
MST

Re: [PATCH net-next v2 4/7] net: resolve uncommon partial checksums at ingress

[Willem de Bruijn]

Michael S. Tsirkin wrote:
> As reported first by Paulos Yibelo <habte.yibelo@gmail.com> (after which
> others, credited below, found more ways the bug can affect users) a
> virtio net header can place CHECKSUM_PARTIAL at any in-packet offset.
> If that state is retained, later checksum completion can modify the
> packet after protocol processing has made decisions from it.
> 
> Let's fix that: let's validate csum_start and csum_offset and only keep
> CHECKSUM_PARTIAL when csum_start and csum_offset match a parsed TCP or
> UDP header, while handling VLAN headers, IPv4 options and IPv6 extension
> headers.  Complete other non-GSO checksums immediately, before any
> protocol processing runs.
> 
> We can't easily "complete the checksum" for GSO we don't recognize - GSO
> assumes knowledge of the protocol. We could segment ignoring the
> checksum then do whatever csum_start and csum_offset say, but that
> seems like an overkill for something that shouldn't happen.
> 
> There's some risk here:
> - non-gso packets which can't be parsed get checksum calculated in
>   software, adding performance overhead
> - gso packets which can't be parsed get dropped
> 
> To mitigate the risks, the following patch will add a sysctl knob to
> bypass the resolution. This is reasonable if the setup is such that
> there is no protocol processing at all.
> 
> UDP tunnel headers get the same handling on the inner header.
> 
> Packets without VIRTIO_NET_HDR_F_NEEDS_CSUM are unaffected.
> 
> Assisted-by: LLM
> Reported-by: Paulos Yibelo <habte.yibelo@gmail.com>
> Reported-by: Chris Arges <carges@cloudflare.com>
> Tested-by: Chris Arges <carges@cloudflare.com>
> Fixes: f43798c27684 ("tun: Allow GSO using virtio_net_hdr")
> Suggested-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  include/linux/virtio_net.h | 251 +++++++++++++++++++++++++++++++++++--
>  1 file changed, 241 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index d6466f96cdd0..357115ff0ec2 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -5,7 +5,11 @@
>  #include <linux/if_vlan.h>
>  #include <linux/ip.h>
>  #include <linux/ipv6.h>
> +#include <linux/netdevice.h>
>  #include <linux/udp.h>
> +#include <net/gso.h>
> +#include <net/ip.h>
> +#include <net/ipv6.h>
>  #include <net/tcp.h>
>  #include <uapi/linux/tcp.h>
>  #include <uapi/linux/virtio_net.h>
> @@ -48,6 +52,171 @@ static inline int virtio_net_hdr_set_proto(struct sk_buff *skb,
>  	return 0;
>  }
>  
> +/* TODO: nothing virtio specific here, move to ip.h */
> +
> +/*
> + * Pull and minimally validate the IPv4 header at @nhoff.
> + * Returns the header and stores the transport header offset in
> + * @thoff, or NULL if truncated or malformed.  Only the fixed header
> + * is guaranteed linear; callers that need the options or the
> + * transport header must pull further.
> + */

nit: comment basically describes the code (what), which is quite
self explanatory. I'd drop. Or else consider converting to kdoc fmt.

> +static inline struct iphdr *virtio_net_ip4_parse(struct sk_buff *skb,
> +						  unsigned int nhoff,
> +						  unsigned int *thoff)
> +{
> +	struct iphdr *iph;
> +
> +	if (!pskb_may_pull(skb, nhoff + sizeof(struct iphdr)))
> +		return NULL;
> +
> +	iph = (struct iphdr *)(skb->data + nhoff);
> +	if (iph->version != 4 ||
> +	    iph->ihl < sizeof(struct iphdr) / sizeof(u32))
> +		return NULL;
> +
> +	*thoff = nhoff + iph->ihl * sizeof(u32);
> +	return iph;
> +}
> +
> +/* Where does the L3 header start, and what protocol is it?
> + * Handles Ethernet and VLAN headers; other devices use the
> + * network header offset the caller set up.
> + */
> +static inline bool
> +virtio_net_l3_start(struct sk_buff *skb, int *nhoff, __be16 *proto)
> +{
> +	if (skb->dev->type == ARPHRD_ETHER) {

Only since recent commit 44378d02c5aa ("net: always dissect GSO
packets in __virtio_net_hdr_to_skb()") can skb->dev be relied on to be
non-zero.

We'll have to be careful that this does not get backported to stable
without that.

Perhaps an explicit precondition test and error return (and
DEBUG_NET_WARN_ON_ONCE) if a NULL skb->dev is encountered.

OTOH, it results in a hard crash so will be discoverd fast without
that too.


> +		if (!pskb_may_pull(skb, ETH_HLEN))
> +			return false;
> +		*nhoff = ETH_HLEN;
> +		*proto = eth_hdr(skb)->h_proto;

When called from virtio_net_hdr_tnl_to_skb, according to my LLM mac
header may not be set yet. Have not verified yet.

skb_reset_mac_header(skb) is called inside __virtio_net_hdr_to_skb,
which is then not called yet.

> +		if (eth_type_vlan(*proto)) {
> +			*proto = __vlan_get_protocol(skb, *proto, nhoff);
> +			if (!*proto)
> +				return false;
> +		}
> +	} else {
> +		*nhoff = skb_network_offset(skb);

The same is true for skb->network_header. Since an uninitialized/zero
value there causes more subtle issues, here an explicit check may be
more valuable (even) than for skb->dev.

> +		*proto = skb->protocol;
> +	}
> +	return true;
> +}
> +

>  
>  /* This function must be called after virtio_net_hdr_from_skb(). */
> @@ -282,6 +470,35 @@ static inline unsigned int virtio_l3min(bool is_ipv6)
>  	return is_ipv6 ? sizeof(struct ipv6hdr) : sizeof(struct iphdr);
>  }
>  
> +/* Do the tunnel offsets match the outer headers in the frame?
> + * The L3 header the tunnel claims must be the one the frame
> + * carries, with UDP following it at @outer_th.
> + */
> +static inline bool
> +virtio_net_tnl_offsets_ok(struct sk_buff *skb, bool outer_isv6,
> +			  unsigned int outer_th)
> +{
> +	int nhoff;
> +	__be16 proto;
> +	unsigned int thoff;
> +	int ip_proto;
> +
> +	if (!virtio_net_l3_start(skb, &nhoff, &proto))
> +		return false;
> +	if (outer_isv6 != (proto == htons(ETH_P_IPV6)))
> +		return false;
> +	if (!virtio_net_l4_start(skb, nhoff, proto, &thoff, &ip_proto))
> +		return false;
> +
> +	return ip_proto == IPPROTO_UDP && outer_th == thoff;
> +}
> +
> +/*
> + * Return value:
> + * 0 - success
> + * >0 - checksum was completed in software (slow path)
> + * <0 - failure
> + */
>  static inline int
>  virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
>  			  const struct virtio_net_hdr_v1_hash_tunnel *vhdr,
> @@ -293,6 +510,7 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
>  	unsigned int inner_nh, outer_th, inner_th;
>  	unsigned int inner_l3min, outer_l3min;
>  	u8 gso_inner_type, gso_tunnel_type;
> +	__be16 inner_proto;
>  	bool outer_isv6, inner_isv6;
>  	int ret;
>  
> @@ -332,10 +550,13 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
>  	    inner_th < inner_nh + inner_l3min)
>  		return -EINVAL;
>  
> +	if (!virtio_net_tnl_offsets_ok(skb, outer_isv6, outer_th))
> +		return -EINVAL;
> +
>  	/* Let the basic parsing deal with plain GSO features. */
>  	ret = __virtio_net_hdr_to_skb(skb, hdr, true,
>  				      hdr->gso_type & ~gso_tunnel_type);
> -	if (ret)
> +	if (ret < 0)
>  		return ret;
>  
>  	/* In case of USO, the inner protocol is still unknown and
> @@ -353,7 +574,7 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
>  	}
>  
>  	skb_set_inner_protocol(skb, inner_isv6 ? htons(ETH_P_IPV6) :
> -						 htons(ETH_P_IP));
> +			    htons(ETH_P_IP));
>  	if (hdr->flags & VIRTIO_NET_HDR_F_UDP_TUNNEL_CSUM) {
>  		if (!tnl_csum_negotiated)
>  			return -EINVAL;
> @@ -368,7 +589,17 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
>  	skb->inner_mac_header = inner_nh + skb_headroom(skb);
>  	skb->transport_header = outer_th + skb_headroom(skb);
>  	skb->encapsulation = 1;
> -	return 0;
> +
> +	/*
> +	 * We know VIRTIO_NET_HDR_F_NEEDS_CSUM was set so __virtio_net_hdr_to_skb
> +	 * set CHECKSUM_PARTIAL.
> +	 */
> +	inner_proto = inner_isv6 ? htons(ETH_P_IPV6) : htons(ETH_P_IP);
> +	if (virtio_net_csum_partial_ok_at_offset(skb, gso_inner_type, inner_nh,
> +						 inner_proto))
> +		return 0;
> +
> +	return virtio_net_resolve_csum_partial_mismatch(skb);
>  }
>  
>  /* Checksum-related fields validation for the driver */
> -- 
> MST
> 

Re: [PATCH net-next v2 4/7] net: resolve uncommon partial checksums at ingress

[Chris Arges]

On 2026-10-15 13:43:56, Willem de Bruijn wrote:
> Michael S. Tsirkin wrote:
> > As reported first by Paulos Yibelo <habte.yibelo@gmail.com> (after which
> > others, credited below, found more ways the bug can affect users) a
> > virtio net header can place CHECKSUM_PARTIAL at any in-packet offset.
> > If that state is retained, later checksum completion can modify the
> > packet after protocol processing has made decisions from it.
> > 
> > Let's fix that: let's validate csum_start and csum_offset and only keep
> > CHECKSUM_PARTIAL when csum_start and csum_offset match a parsed TCP or
> > UDP header, while handling VLAN headers, IPv4 options and IPv6 extension
> > headers.  Complete other non-GSO checksums immediately, before any
> > protocol processing runs.
> > 
> > We can't easily "complete the checksum" for GSO we don't recognize - GSO
> > assumes knowledge of the protocol. We could segment ignoring the
> > checksum then do whatever csum_start and csum_offset say, but that
> > seems like an overkill for something that shouldn't happen.
> > 
> > There's some risk here:
> > - non-gso packets which can't be parsed get checksum calculated in
> >   software, adding performance overhead
> > - gso packets which can't be parsed get dropped
> > 
> > To mitigate the risks, the following patch will add a sysctl knob to
> > bypass the resolution. This is reasonable if the setup is such that
> > there is no protocol processing at all.
> > 
> > UDP tunnel headers get the same handling on the inner header.
> > 
> > Packets without VIRTIO_NET_HDR_F_NEEDS_CSUM are unaffected.
> > 
> > Assisted-by: LLM
> > Reported-by: Paulos Yibelo <habte.yibelo@gmail.com>
> > Reported-by: Chris Arges <carges@cloudflare.com>
> > Tested-by: Chris Arges <carges@cloudflare.com>
> > Fixes: f43798c27684 ("tun: Allow GSO using virtio_net_hdr")
> > Suggested-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  include/linux/virtio_net.h | 251 +++++++++++++++++++++++++++++++++++--
> >  1 file changed, 241 insertions(+), 10 deletions(-)
> > 
> > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> > index d6466f96cdd0..357115ff0ec2 100644
> > --- a/include/linux/virtio_net.h
> > +++ b/include/linux/virtio_net.h
> > @@ -5,7 +5,11 @@
> >  #include <linux/if_vlan.h>
> >  #include <linux/ip.h>
> >  #include <linux/ipv6.h>
> > +#include <linux/netdevice.h>
> >  #include <linux/udp.h>
> > +#include <net/gso.h>
> > +#include <net/ip.h>
> > +#include <net/ipv6.h>
> >  #include <net/tcp.h>
> >  #include <uapi/linux/tcp.h>
> >  #include <uapi/linux/virtio_net.h>
> > @@ -48,6 +52,171 @@ static inline int virtio_net_hdr_set_proto(struct sk_buff *skb,
> >  	return 0;
> >  }
> >  
> > +/* TODO: nothing virtio specific here, move to ip.h */
> > +
> > +/*
> > + * Pull and minimally validate the IPv4 header at @nhoff.
> > + * Returns the header and stores the transport header offset in
> > + * @thoff, or NULL if truncated or malformed.  Only the fixed header
> > + * is guaranteed linear; callers that need the options or the
> > + * transport header must pull further.
> > + */
> 
> nit: comment basically describes the code (what), which is quite
> self explanatory. I'd drop. Or else consider converting to kdoc fmt.
> 
> > +static inline struct iphdr *virtio_net_ip4_parse(struct sk_buff *skb,
> > +						  unsigned int nhoff,
> > +						  unsigned int *thoff)
> > +{
> > +	struct iphdr *iph;
> > +
> > +	if (!pskb_may_pull(skb, nhoff + sizeof(struct iphdr)))
> > +		return NULL;
> > +
> > +	iph = (struct iphdr *)(skb->data + nhoff);
> > +	if (iph->version != 4 ||
> > +	    iph->ihl < sizeof(struct iphdr) / sizeof(u32))
> > +		return NULL;
> > +
> > +	*thoff = nhoff + iph->ihl * sizeof(u32);
> > +	return iph;
> > +}
> > +
> > +/* Where does the L3 header start, and what protocol is it?
> > + * Handles Ethernet and VLAN headers; other devices use the
> > + * network header offset the caller set up.
> > + */
> > +static inline bool
> > +virtio_net_l3_start(struct sk_buff *skb, int *nhoff, __be16 *proto)
> > +{
> > +	if (skb->dev->type == ARPHRD_ETHER) {
> 
> Only since recent commit 44378d02c5aa ("net: always dissect GSO
> packets in __virtio_net_hdr_to_skb()") can skb->dev be relied on to be
> non-zero.
> 
> We'll have to be careful that this does not get backported to stable
> without that.
> 
> Perhaps an explicit precondition test and error return (and
> DEBUG_NET_WARN_ON_ONCE) if a NULL skb->dev is encountered.
> 
> OTOH, it results in a hard crash so will be discoverd fast without
> that too.
> 
> 
> > +		if (!pskb_may_pull(skb, ETH_HLEN))
> > +			return false;
> > +		*nhoff = ETH_HLEN;
> > +		*proto = eth_hdr(skb)->h_proto;
> 
> When called from virtio_net_hdr_tnl_to_skb, according to my LLM mac
> header may not be set yet. Have not verified yet.
> 
> skb_reset_mac_header(skb) is called inside __virtio_net_hdr_to_skb,
> which is then not called yet.
> 

I am able to confirm this in my testing. I have KASAN enabled, and it
triggered when testing leading me to this change.

Adding `skb_reset_mac_header(skb);` before calling virtio_net_tnl_offsets_ok
mitigates this issue.

--chris


> > +		if (eth_type_vlan(*proto)) {
> > +			*proto = __vlan_get_protocol(skb, *proto, nhoff);
> > +			if (!*proto)
> > +				return false;
> > +		}
> > +	} else {
> > +		*nhoff = skb_network_offset(skb);
> 
> The same is true for skb->network_header. Since an uninitialized/zero
> value there causes more subtle issues, here an explicit check may be
> more valuable (even) than for skb->dev.
> 
> > +		*proto = skb->protocol;
> > +	}
> > +	return true;
> > +}
> > +
> 
> >  
> >  /* This function must be called after virtio_net_hdr_from_skb(). */
> > @@ -282,6 +470,35 @@ static inline unsigned int virtio_l3min(bool is_ipv6)
> >  	return is_ipv6 ? sizeof(struct ipv6hdr) : sizeof(struct iphdr);
> >  }
> >  
> > +/* Do the tunnel offsets match the outer headers in the frame?
> > + * The L3 header the tunnel claims must be the one the frame
> > + * carries, with UDP following it at @outer_th.
> > + */
> > +static inline bool
> > +virtio_net_tnl_offsets_ok(struct sk_buff *skb, bool outer_isv6,
> > +			  unsigned int outer_th)
> > +{
> > +	int nhoff;
> > +	__be16 proto;
> > +	unsigned int thoff;
> > +	int ip_proto;
> > +
> > +	if (!virtio_net_l3_start(skb, &nhoff, &proto))
> > +		return false;
> > +	if (outer_isv6 != (proto == htons(ETH_P_IPV6)))
> > +		return false;
> > +	if (!virtio_net_l4_start(skb, nhoff, proto, &thoff, &ip_proto))
> > +		return false;
> > +
> > +	return ip_proto == IPPROTO_UDP && outer_th == thoff;
> > +}
> > +
> > +/*
> > + * Return value:
> > + * 0 - success
> > + * >0 - checksum was completed in software (slow path)
> > + * <0 - failure
> > + */
> >  static inline int
> >  virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
> >  			  const struct virtio_net_hdr_v1_hash_tunnel *vhdr,
> > @@ -293,6 +510,7 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
> >  	unsigned int inner_nh, outer_th, inner_th;
> >  	unsigned int inner_l3min, outer_l3min;
> >  	u8 gso_inner_type, gso_tunnel_type;
> > +	__be16 inner_proto;
> >  	bool outer_isv6, inner_isv6;
> >  	int ret;
> >  
> > @@ -332,10 +550,13 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
> >  	    inner_th < inner_nh + inner_l3min)
> >  		return -EINVAL;
> >  
> > +	if (!virtio_net_tnl_offsets_ok(skb, outer_isv6, outer_th))
> > +		return -EINVAL;
> > +
> >  	/* Let the basic parsing deal with plain GSO features. */
> >  	ret = __virtio_net_hdr_to_skb(skb, hdr, true,
> >  				      hdr->gso_type & ~gso_tunnel_type);
> > -	if (ret)
> > +	if (ret < 0)
> >  		return ret;
> >  
> >  	/* In case of USO, the inner protocol is still unknown and
> > @@ -353,7 +574,7 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
> >  	}
> >  
> >  	skb_set_inner_protocol(skb, inner_isv6 ? htons(ETH_P_IPV6) :
> > -						 htons(ETH_P_IP));
> > +			    htons(ETH_P_IP));
> >  	if (hdr->flags & VIRTIO_NET_HDR_F_UDP_TUNNEL_CSUM) {
> >  		if (!tnl_csum_negotiated)
> >  			return -EINVAL;
> > @@ -368,7 +589,17 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
> >  	skb->inner_mac_header = inner_nh + skb_headroom(skb);
> >  	skb->transport_header = outer_th + skb_headroom(skb);
> >  	skb->encapsulation = 1;
> > -	return 0;
> > +
> > +	/*
> > +	 * We know VIRTIO_NET_HDR_F_NEEDS_CSUM was set so __virtio_net_hdr_to_skb
> > +	 * set CHECKSUM_PARTIAL.
> > +	 */
> > +	inner_proto = inner_isv6 ? htons(ETH_P_IPV6) : htons(ETH_P_IP);
> > +	if (virtio_net_csum_partial_ok_at_offset(skb, gso_inner_type, inner_nh,
> > +						 inner_proto))
> > +		return 0;
> > +
> > +	return virtio_net_resolve_csum_partial_mismatch(skb);
> >  }
> >  
> >  /* Checksum-related fields validation for the driver */
> > -- 
> > MST
> > 
> 
> 

Re: [PATCH net-next v2 4/7] net: resolve uncommon partial checksums at ingress

[Willem de Bruijn]

Michael S. Tsirkin wrote:
> As reported first by Paulos Yibelo <habte.yibelo@gmail.com> (after which
> others, credited below, found more ways the bug can affect users) a
> virtio net header can place CHECKSUM_PARTIAL at any in-packet offset.
> If that state is retained, later checksum completion can modify the
> packet after protocol processing has made decisions from it.
> 
> Let's fix that: let's validate csum_start and csum_offset and only keep
> CHECKSUM_PARTIAL when csum_start and csum_offset match a parsed TCP or
> UDP header, while handling VLAN headers, IPv4 options and IPv6 extension
> headers.  Complete other non-GSO checksums immediately, before any
> protocol processing runs.
> 
> We can't easily "complete the checksum" for GSO we don't recognize - GSO
> assumes knowledge of the protocol. We could segment ignoring the
> checksum then do whatever csum_start and csum_offset say, but that
> seems like an overkill for something that shouldn't happen.
> 
> There's some risk here:
> - non-gso packets which can't be parsed get checksum calculated in
>   software, adding performance overhead
> - gso packets which can't be parsed get dropped
> 
> To mitigate the risks, the following patch will add a sysctl knob to
> bypass the resolution. This is reasonable if the setup is such that
> there is no protocol processing at all.
> 
> UDP tunnel headers get the same handling on the inner header.
> 
> Packets without VIRTIO_NET_HDR_F_NEEDS_CSUM are unaffected.
> 
> Assisted-by: LLM
> Reported-by: Paulos Yibelo <habte.yibelo@gmail.com>
> Reported-by: Chris Arges <carges@cloudflare.com>
> Tested-by: Chris Arges <carges@cloudflare.com>
> Fixes: f43798c27684 ("tun: Allow GSO using virtio_net_hdr")
> Suggested-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  include/linux/virtio_net.h | 251 +++++++++++++++++++++++++++++++++++--
>  1 file changed, 241 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index d6466f96cdd0..357115ff0ec2 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -5,7 +5,11 @@
>  #include <linux/if_vlan.h>
>  #include <linux/ip.h>
>  #include <linux/ipv6.h>
> +#include <linux/netdevice.h>
>  #include <linux/udp.h>
> +#include <net/gso.h>
> +#include <net/ip.h>
> +#include <net/ipv6.h>
>  #include <net/tcp.h>
>  #include <uapi/linux/tcp.h>
>  #include <uapi/linux/virtio_net.h>

>  static inline int __virtio_net_hdr_to_skb(struct sk_buff *skb,
>  					  const struct virtio_net_hdr *hdr,
>  					  bool little_endian, u8 hdr_gso_type)
> @@ -158,8 +327,6 @@ static inline int __virtio_net_hdr_to_skb(struct sk_buff *skb,
>  		case SKB_GSO_UDP_L4:
>  			if (!(hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM))
>  				return -EINVAL;
> -			if (skb->csum_offset != offsetof(struct udphdr, check))
> -				return -EINVAL;
>  			if (skb->len - p_off > gso_size * UDP_MAX_SEGMENTS)
>  				return -EINVAL;
>  			if (gso_type != SKB_GSO_UDP_L4)
> @@ -167,10 +334,6 @@ static inline int __virtio_net_hdr_to_skb(struct sk_buff *skb,
>  			break;
>  		case SKB_GSO_TCPV4:
>  		case SKB_GSO_TCPV6:
> -			if (skb->ip_summed == CHECKSUM_PARTIAL &&
> -			    skb->csum_offset != offsetof(struct tcphdr, check))
> -				return -EINVAL;
> -

Maybe keep this and the above check, to keep things exactly as is when the
sysctl is disabled.

Removing these checks weakens validation vs the present state.

Re: [PATCH net-next v2 4/7] net: resolve uncommon partial checksums at ingress

[Willem de Bruijn]

Michael S. Tsirkin wrote:
> As reported first by Paulos Yibelo <habte.yibelo@gmail.com> (after which
> others, credited below, found more ways the bug can affect users) a
> virtio net header can place CHECKSUM_PARTIAL at any in-packet offset.
> If that state is retained, later checksum completion can modify the
> packet after protocol processing has made decisions from it.
> 
> Let's fix that: let's validate csum_start and csum_offset and only keep
> CHECKSUM_PARTIAL when csum_start and csum_offset match a parsed TCP or
> UDP header, while handling VLAN headers, IPv4 options and IPv6 extension
> headers.  Complete other non-GSO checksums immediately, before any
> protocol processing runs.
> 
> We can't easily "complete the checksum" for GSO we don't recognize - GSO
> assumes knowledge of the protocol. We could segment ignoring the
> checksum then do whatever csum_start and csum_offset say, but that
> seems like an overkill for something that shouldn't happen.
> 
> There's some risk here:
> - non-gso packets which can't be parsed get checksum calculated in
>   software, adding performance overhead
> - gso packets which can't be parsed get dropped

The biggest risk I see of valid packets reaching this path is packet
sockets that are not bound to a specific protocol (ETH_P_ALL) over a
non Ethernet device.

Iff proto is 0 we could try to infer from the first nibble at the
network header:

+	iph = (const void *)(skb->data + nhoff);
+	if (!proto) {
+		if (iph->version == 4)
+			proto = htons(ETH_P_IP);
+		else if (iph->version == 6)
+			proto = htons(ETH_P_IPV6);
+	}

[PATCH net-next v2 5/7] net: add a sysctl to opt out of virtio net hdr checksum validation

[Michael S. Tsirkin]

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 <mst@redhat.com>
---
 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
+-------------------------
+
+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 <linux/if_vlan.h>
 #include <linux/ip.h>
 #include <linux/ipv6.h>
+#include <linux/jump_label.h>
 #include <linux/netdevice.h>
 #include <linux/udp.h>
 #include <net/gso.h>
@@ -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
@@ -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.
+	 */
+	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)))
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..c784c0143b0e 100644
--- a/net/core/sysctl_net_core.c
+++ b/net/core/sysctl_net_core.c
@@ -18,6 +18,7 @@
 #include <linux/slab.h>
 #include <linux/sched/isolation.h>
 #include <linux/hex.h>
+#include <linux/jump_label.h>
 
 #include <net/ip.h>
 #include <net/sock.h>
@@ -678,6 +679,48 @@ 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;
+
+	/*
+	 * 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

Re: [PATCH net-next v2 5/7] net: add a sysctl to opt out of virtio net hdr checksum validation

[Willem de Bruijn]

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 <mst@redhat.com>
> ---
>  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 <linux/if_vlan.h>
>  #include <linux/ip.h>
>  #include <linux/ipv6.h>
> +#include <linux/jump_label.h>
>  #include <linux/netdevice.h>
>  #include <linux/udp.h>
>  #include <net/gso.h>
> @@ -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
> 

[PATCH net-next v2 6/7] net: add ip4_parse()

[Michael S. Tsirkin]

Multiple places in the net stack open-code the same minimal IPv4 header
validation: pull the fixed header, check version 4 and IHL >= 5 (where 5
is the fixed header size in 32-bit units) - and locate the transport
header at IHL * 4 past the network header.

Add a helper for doing that: it returns the validated header and
the transport header offset, or NULL if truncated or malformed.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Assisted-by: LLM
---
 include/linux/ip.h                  | 24 ++++++++++++++++++++++++
 include/linux/virtio_net.h          | 28 +---------------------------
 net/bridge/br_netfilter_hooks.c     | 15 +++++----------
 net/ipv4/ip_input.c                 | 13 +++++--------
 net/ipv4/netfilter/nf_reject_ipv4.c | 13 +++++--------
 5 files changed, 40 insertions(+), 53 deletions(-)

diff --git a/include/linux/ip.h b/include/linux/ip.h
index d11c25f5030a..98012f6d0efb 100644
--- a/include/linux/ip.h
+++ b/include/linux/ip.h
@@ -56,4 +56,28 @@ static inline void iph_set_totlen(struct iphdr *iph, unsigned int len)
 {
 	iph->tot_len = len <= IP_MAX_MTU ? htons(len) : 0;
 }
+
+/*
+ * Pull and minimally validate the IPv4 header at @nhoff.
+ * Returns the header and stores the transport header offset in
+ * @thoff, or NULL if truncated or malformed.  Only the fixed header
+ * is guaranteed linear; callers that need the options or the
+ * transport header must pull further.
+ */
+static inline struct iphdr *ip4_parse(struct sk_buff *skb,
+				      unsigned int nhoff,
+				      unsigned int *thoff)
+{
+	struct iphdr *iph;
+
+	if (!pskb_may_pull(skb, nhoff + sizeof(struct iphdr)))
+		return NULL;
+
+	iph = (struct iphdr *)(skb->data + nhoff);
+	if (iph->version != 4 || iph->ihl < sizeof(struct iphdr) / sizeof(u32))
+		return NULL;
+
+	*thoff = nhoff + iph->ihl * sizeof(u32);
+	return iph;
+}
 #endif	/* _LINUX_IP_H */
diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index aa2f8ac4657c..85ec1039f73a 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -53,32 +53,6 @@ static inline int virtio_net_hdr_set_proto(struct sk_buff *skb,
 	return 0;
 }
 
-/* TODO: nothing virtio specific here, move to ip.h */
-
-/*
- * Pull and minimally validate the IPv4 header at @nhoff.
- * Returns the header and stores the transport header offset in
- * @thoff, or NULL if truncated or malformed.  Only the fixed header
- * is guaranteed linear; callers that need the options or the
- * transport header must pull further.
- */
-static inline struct iphdr *virtio_net_ip4_parse(struct sk_buff *skb,
-						  unsigned int nhoff,
-						  unsigned int *thoff)
-{
-	struct iphdr *iph;
-
-	if (!pskb_may_pull(skb, nhoff + sizeof(struct iphdr)))
-		return NULL;
-
-	iph = (struct iphdr *)(skb->data + nhoff);
-	if (iph->version != 4 ||
-	    iph->ihl < sizeof(struct iphdr) / sizeof(u32))
-		return NULL;
-
-	*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.
  */
@@ -126,7 +100,7 @@ virtio_net_l4_start(struct sk_buff *skb, unsigned int nhoff, __be16 proto,
 		    unsigned int *thoff, int *ip_proto)
 {
 	if (proto == htons(ETH_P_IP)) {
-		const struct iphdr *iph = virtio_net_ip4_parse(skb, nhoff, thoff);
+		const struct iphdr *iph = ip4_parse(skb, nhoff, thoff);
 
 		if (!iph || ip_is_fragment(iph))
 			return false;
diff --git a/net/bridge/br_netfilter_hooks.c b/net/bridge/br_netfilter_hooks.c
index 0a394e5f4391..e89da0e41542 100644
--- a/net/bridge/br_netfilter_hooks.c
+++ b/net/bridge/br_netfilter_hooks.c
@@ -206,18 +206,13 @@ static inline void nf_bridge_pull_encap_header_rcsum(struct sk_buff *skb)
 static int br_validate_ipv4(struct net *net, struct sk_buff *skb)
 {
 	const struct iphdr *iph;
-	u32 len;
+	u32 len, thoff;
 
-	if (!pskb_may_pull(skb, sizeof(struct iphdr)))
+	iph = ip4_parse(skb, 0, &thoff);
+	if (!iph)
 		goto inhdr_error;
 
-	iph = ip_hdr(skb);
-
-	/* Basic sanity checks */
-	if (iph->ihl < 5 || iph->version != 4)
-		goto inhdr_error;
-
-	if (!pskb_may_pull(skb, iph->ihl*4))
+	if (!pskb_may_pull(skb, thoff))
 		goto inhdr_error;
 
 	iph = ip_hdr(skb);
@@ -228,7 +223,7 @@ static int br_validate_ipv4(struct net *net, struct sk_buff *skb)
 	if (skb->len < len) {
 		__IP_INC_STATS(net, IPSTATS_MIB_INTRUNCATEDPKTS);
 		goto drop;
-	} else if (len < (iph->ihl*4))
+	} else if (len < thoff)
 		goto inhdr_error;
 
 	if (pskb_trim_rcsum(skb, len)) {
diff --git a/net/ipv4/ip_input.c b/net/ipv4/ip_input.c
index 9860178752b8..5adf01763e85 100644
--- a/net/ipv4/ip_input.c
+++ b/net/ipv4/ip_input.c
@@ -500,7 +500,7 @@ static struct sk_buff *ip_rcv_core(struct sk_buff *skb, struct net *net)
 {
 	const struct iphdr *iph;
 	int drop_reason;
-	u32 len;
+	u32 len, thoff;
 
 	/* When the interface is in promisc. mode, drop all the crap
 	 * that it receives, do not try to analyse it.
@@ -520,10 +520,6 @@ static struct sk_buff *ip_rcv_core(struct sk_buff *skb, struct net *net)
 	}
 
 	drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
-	if (!pskb_may_pull(skb, sizeof(struct iphdr)))
-		goto inhdr_error;
-
-	iph = ip_hdr(skb);
 
 	/*
 	 *	RFC1122: 3.2.1.2 MUST silently discard any IP frame that fails the checksum.
@@ -536,7 +532,8 @@ static struct sk_buff *ip_rcv_core(struct sk_buff *skb, struct net *net)
 	 *	4.	Doesn't have a bogus length
 	 */
 
-	if (iph->ihl < 5 || iph->version != 4)
+	iph = ip4_parse(skb, 0, &thoff);
+	if (!iph)
 		goto inhdr_error;
 
 	BUILD_BUG_ON(IPSTATS_MIB_ECT1PKTS != IPSTATS_MIB_NOECTPKTS + INET_ECN_ECT_1);
@@ -546,7 +543,7 @@ static struct sk_buff *ip_rcv_core(struct sk_buff *skb, struct net *net)
 		       IPSTATS_MIB_NOECTPKTS + (iph->tos & INET_ECN_MASK),
 		       max_t(unsigned short, 1, skb_shinfo(skb)->gso_segs));
 
-	if (!pskb_may_pull(skb, iph->ihl*4))
+	if (!pskb_may_pull(skb, thoff))
 		goto inhdr_error;
 
 	iph = ip_hdr(skb);
@@ -559,7 +556,7 @@ static struct sk_buff *ip_rcv_core(struct sk_buff *skb, struct net *net)
 		drop_reason = SKB_DROP_REASON_PKT_TOO_SMALL;
 		__IP_INC_STATS(net, IPSTATS_MIB_INTRUNCATEDPKTS);
 		goto drop;
-	} else if (len < (iph->ihl*4))
+	} else if (len < thoff)
 		goto inhdr_error;
 
 	/* Our transport medium may have padded the buffer out. Now we know it
diff --git a/net/ipv4/netfilter/nf_reject_ipv4.c b/net/ipv4/netfilter/nf_reject_ipv4.c
index 59ec465a9df9..42d639854ddf 100644
--- a/net/ipv4/netfilter/nf_reject_ipv4.c
+++ b/net/ipv4/netfilter/nf_reject_ipv4.c
@@ -25,22 +25,19 @@ nf_reject_ip_tcphdr_get(struct sk_buff *oldskb,
 static int nf_reject_iphdr_validate(struct sk_buff *skb)
 {
 	struct iphdr *iph;
-	u32 len;
+	u32 len, thoff;
 
-	if (!pskb_may_pull(skb, sizeof(struct iphdr)))
-		return 0;
-
-	iph = ip_hdr(skb);
-	if (iph->ihl < 5 || iph->version != 4)
+	iph = ip4_parse(skb, 0, &thoff);
+	if (!iph)
 		return 0;
 
 	len = ntohs(iph->tot_len);
 	if (skb->len < len)
 		return 0;
-	else if (len < (iph->ihl*4))
+	else if (len < thoff)
 		return 0;
 
-	if (!pskb_may_pull(skb, iph->ihl*4))
+	if (!pskb_may_pull(skb, thoff))
 		return 0;
 
 	return 1;
-- 
MST

Re: [PATCH net-next v2 6/7] net: add ip4_parse()

[Phil Sutter]

Hi Michael,

On Thu, Oct 15, 2026 at 08:41:45AM -0400, Michael S. Tsirkin wrote:
> Multiple places in the net stack open-code the same minimal IPv4 header
> validation: pull the fixed header, check version 4 and IHL >= 5 (where 5
> is the fixed header size in 32-bit units) - and locate the transport
> header at IHL * 4 past the network header.
> 
> Add a helper for doing that: it returns the validated header and
> the transport header offset, or NULL if truncated or malformed.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> Assisted-by: LLM

Reviewed-by: Phil Sutter <phil@nwl.cc>

Did you consider nft_inner_parse_l2l3 in net/netfilter/nft_inner.c and
ip_mc_check_iphdr in net/ipv4/igmp.c as well?

Cheers, Phil

[PATCH net-next v2 7/7] virtio_net: cleanup protocol calculation

[Michael S. Tsirkin]

A trivial cleanup removing a duplicated code line.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 include/linux/virtio_net.h | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index 85ec1039f73a..a9ed42013476 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -570,8 +570,8 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
 			return -EINVAL;
 	}
 
-	skb_set_inner_protocol(skb, inner_isv6 ? htons(ETH_P_IPV6) :
-			    htons(ETH_P_IP));
+	inner_proto = inner_isv6 ? htons(ETH_P_IPV6) : htons(ETH_P_IP);
+	skb_set_inner_protocol(skb, inner_proto);
 	if (hdr->flags & VIRTIO_NET_HDR_F_UDP_TUNNEL_CSUM) {
 		if (!tnl_csum_negotiated)
 			return -EINVAL;
@@ -591,7 +591,6 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
 	 * We know VIRTIO_NET_HDR_F_NEEDS_CSUM was set so __virtio_net_hdr_to_skb
 	 * set CHECKSUM_PARTIAL.
 	 */
-	inner_proto = inner_isv6 ? htons(ETH_P_IPV6) : htons(ETH_P_IP);
 	if (virtio_net_csum_partial_ok_at_offset(skb, gso_inner_type, inner_nh,
 						 inner_proto))
 		return 0;
-- 
MST

Re: [PATCH net-next v2 0/7] more virtio-net csum hardening

[Chris Arges]

On 2026-10-15 08:41:12, Michael S. Tsirkin wrote:
> 
> As reported first by Paulos Yibelo <habte.yibelo@gmail.com> (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:
>   net: account for ingress-resolved virtio net hdr state
> 	- accounting, not the fix itself, dependency of the next one
>   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
>   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
> 
> 
> Michael S. Tsirkin (7):
>   net: account for ingress-resolved virtio net hdr state
>   um: vector: honor virtio net hdr conversion failures
>   um: vector: trim skbs before verifying the header
>   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 |  15 ++
>  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                       |  24 +++
>  include/linux/virtio_net.h               | 249 ++++++++++++++++++++++-
>  include/net/netns/core.h                 |   1 +
>  net/bridge/br_netfilter_hooks.c          |  15 +-
>  net/core/dev.c                           |   6 +
>  net/core/net_namespace.c                 |   1 +
>  net/core/sysctl_net_core.c               |  54 +++++
>  net/ipv4/ip_input.c                      |  13 +-
>  net/ipv4/netfilter/nf_reject_ipv4.c      |  13 +-
>  net/packet/af_packet.c                   |   4 +-
>  16 files changed, 404 insertions(+), 48 deletions(-)
> 
> -- 
> MST
>

Aside from the KASAN issue described in the other thread, I was able to
test this series and it fixes my local testcases.

Tested-by: Chris Arges <carges@cloudflare.com>