net: prevent partial checksums from modifying IPv4 headers

[PATCH net v4 0/2] net: prevent partial checksums from modifying IPv4 headers

Posted by Paulos Yibelo 4 days, 19 hours ago

A TUN or virtio-net user can supply CHECKSUM_PARTIAL metadata whose
checksum start resolves inside the IPv4 header after link-layer headers
are removed. On the IPv4 fragmentation path, skb_checksum_help() may then
modify an IHL which was already parsed and validated. ip_do_fragment()
subsequently trusts the changed IHL and can copy beyond the skb's logical
linear head into emitted IPv4 options.

Patch 1 validates the checksum start relative to skb_network_header().
Patch 2 independently validates and retains the IPv4 header length before
checksum completion.

The issue and this series were reviewed privately. The source reproducer
and complete runtime evidence remain available privately.

Validation included strict checkpatch, focused W=1 builds, a complete
build, two test boots, a legitimate fragmented CHECKSUM_PARTIAL control,
and both forged cases.

Paulos Yibelo (2):
  net: validate virtio checksum start after network header
  ipv4: reject partial checksums covering the IP header

 include/linux/virtio_net.h |  5 +++--
 net/ipv4/ip_output.c       | 23 +++++++++++++++++------
 2 files changed, 20 insertions(+), 8 deletions(-)

base-commit: 9d565b6b72fe3f41fd43636e143072848105189f

[PATCH net v5 0/2] net: prevent partial checksums from modifying network headers

Posted by Paulos Yibelo 3 days, 17 hours ago

A virtio-net header can supply CHECKSUM_PARTIAL metadata whose checksum
start resolves inside the network header after link-layer removal.
Software checksum completion can then modify header bytes which the stack
has already parsed.

Patch 1 validates the checksum start against an explicit data-relative L3
origin. It covers TUN/TAP, virtio-net, AF_PACKET, UML, nested VLAN
headers, and tunnel metadata. It does not rely on skb header state which
may not yet be established.

Patch 2 independently validates the checksum start against the parsed
IPv4 or IPv6 header length in all four IP fragmentation implementations
which complete partial checksums.

The v4 Sashiko findings were correct. Patch 1 used
skb_network_offset() before all receive callers had established it.
Patch 2 compared a signed checksum offset with an unsigned IPv4 header
length. This revision fixes both findings and covers the corresponding
bridge and IPv6 fragmentation paths.

Validation included strict checkpatch, focused x86 and UML W=1 builds,
an offset-boundary model, and application of the exact mail series to the
stated base.

Changes in v5:
- Pass an explicit data-relative L3 origin through the virtio-net
  converter and audit every in-tree caller.
- Parse Ethernet and nested VLAN headers without mutating skb header
  state.
- Propagate virtio-header conversion failures in UML.
- Keep the IPv4 comparison signed and add matching parsed-header checks
  to the IPv4/IPv6 output and bridge-netfilter fragmentation paths.
- Drop Michael S. Tsirkin's Acked-by and David Ahern's Reviewed-by tags
  because both patches changed materially.

Link: https://lore.kernel.org/netdev/20260920004733.6473-1-habte.yibelo@gmail.com/

Paulos Yibelo (2):
  net: validate virtio checksum start after network header
  ip: reject partial checksums covering network headers

 arch/um/drivers/vector_transports.c        | 10 ++-
 drivers/net/tun_vnet.h                     | 28 +++++++-
 drivers/net/virtio_net.c                   |  8 ++-
 include/linux/virtio_net.h                 | 76 ++++++++++++++++++----
 net/bridge/netfilter/nf_conntrack_bridge.c | 21 ++++--
 net/ipv4/ip_output.c                       | 23 +++++--
 net/ipv6/ip6_output.c                      | 12 +++-
 net/ipv6/netfilter.c                       | 12 +++-
 net/packet/af_packet.c                     |  6 +-
 9 files changed, 157 insertions(+), 39 deletions(-)


base-commit: 1e24c4f2ee44be0eee94092b5d13cbdb4bdf0d60
-- 
2.46.0

[PATCH net v5 1/2] net: validate virtio checksum start after network header

Posted by Paulos Yibelo 3 days, 17 hours ago

__virtio_net_hdr_to_skb() rejects a CHECKSUM_PARTIAL start smaller than
an estimated minimum network-header length. Its input offsets are relative
to skb->data.

Using skb_network_offset() here is unsafe. TUN/TAP, virtio-net, and UML
parse a received virtio header before skb->network_header is established.
On an skb with headroom, the resulting negative offset enlarges the
apparent distance to the transport header and can admit a checksum start
inside the network header.

Pass the data-relative L3 offset to the converter explicitly. IFF_TUN uses
zero, AF_PACKET supplies its established network offset, and Ethernet
receive paths parse Ethernet and nested VLAN headers with
skb_header_pointer(), without changing skb state. Use the same origin for
tunnel-offset validation, and make UML propagate conversion failures.

This does not require a virtual-machine guest. A TUN or TAP device with
virtio-net header support is sufficient to reach these paths.

Fixes: 49d14b54a527 ("net: test for not too small csum_start in virtio_net_hdr_to_skb()")
Fixes: a2fb4bc4e2a6 ("net: implement virtio helpers to handle UDP GSO tunneling.")
Reported-by: Paulos Yibelo <habte.yibelo@gmail.com>
Link: https://lore.kernel.org/netdev/20260920004733.6473-2-habte.yibelo@gmail.com/
Cc: stable@vger.kernel.org
Signed-off-by: Paulos Yibelo <habte.yibelo@gmail.com>
---
Changes in v5:
- Replace the not-yet-established skb network-header offset with an
  explicit data-relative L3 origin.
- Cover all in-tree callers, including Ethernet/VLAN receive paths,
  tunnel metadata, and UML error propagation.
- Drop the prior Acked-by and Reviewed-by tags because the code changed.

Changes in v4:
- State that a TUN device is sufficient and no guest is required, as
  noted by Michael S. Tsirkin.

Changes in v3:
- Keep the network-relative comparison on one line for readability, as
  requested by David Ahern.

Changes in v2:
- Make nh_min_len an int and remove the casts, as suggested by Michael S.
  Tsirkin.

 arch/um/drivers/vector_transports.c | 10 +++-
 drivers/net/tun_vnet.h              | 28 ++++++++++-
 drivers/net/virtio_net.c            |  8 ++-
 include/linux/virtio_net.h          | 76 +++++++++++++++++++++++------
 net/packet/af_packet.c              |  6 ++-
 5 files changed, 106 insertions(+), 22 deletions(-)

diff --git a/arch/um/drivers/vector_transports.c b/arch/um/drivers/vector_transports.c
index ddd127ee9..79bc05fc6 100644
--- a/arch/um/drivers/vector_transports.c
+++ b/arch/um/drivers/vector_transports.c
@@ -197,6 +197,7 @@ static int raw_verify_header(
 	uint8_t *header, struct sk_buff *skb, struct vector_private *vp)
 {
 	struct virtio_net_hdr *vheader = (struct virtio_net_hdr *) header;
+	int network_offset;
 
 	if ((vheader->gso_type != VIRTIO_NET_HDR_GSO_NONE) &&
 		(vp->req_size != 65536)) {
@@ -209,8 +210,13 @@ static int raw_verify_header(
 	if ((vheader->flags & VIRTIO_NET_HDR_F_DATA_VALID) > 0)
 		return 1;
 
-	virtio_net_hdr_to_skb(skb, vheader, virtio_legacy_is_little_endian());
-	return 0;
+	network_offset = virtio_net_hdr_get_l3_offset(skb, vheader);
+	if (network_offset < 0)
+		return network_offset;
+
+	return virtio_net_hdr_to_skb(skb, vheader,
+				     virtio_legacy_is_little_endian(),
+				     network_offset);
 }
 
 static bool get_uint_param(
diff --git a/drivers/net/tun_vnet.h b/drivers/net/tun_vnet.h
index f4c652b1f..1c83c359d 100644
--- a/drivers/net/tun_vnet.h
+++ b/drivers/net/tun_vnet.h
@@ -177,10 +177,27 @@ static inline int tun_vnet_hdr_put(int sz, struct iov_iter *iter,
 	return __tun_vnet_hdr_put(sz, 0, iter, hdr);
 }
 
+static inline int
+tun_vnet_hdr_get_l3_offset(unsigned int flags, const struct sk_buff *skb,
+			   const struct virtio_net_hdr *hdr)
+{
+	if ((flags & TUN_TYPE_MASK) != IFF_TAP)
+		return 0;
+
+	return virtio_net_hdr_get_l3_offset(skb, hdr);
+}
+
 static inline int tun_vnet_hdr_to_skb(unsigned int flags, struct sk_buff *skb,
 				      const struct virtio_net_hdr *hdr)
 {
-	return virtio_net_hdr_to_skb(skb, hdr, tun_vnet_is_little_endian(flags));
+	int network_offset = tun_vnet_hdr_get_l3_offset(flags, skb, hdr);
+
+	if (network_offset < 0)
+		return network_offset;
+
+	return virtio_net_hdr_to_skb(skb, hdr,
+				     tun_vnet_is_little_endian(flags),
+				     network_offset);
 }
 
 /*
@@ -199,10 +216,17 @@ tun_vnet_hdr_tnl_to_skb(unsigned int flags, netdev_features_t features,
 			struct sk_buff *skb,
 			const struct virtio_net_hdr_v1_hash_tunnel *hdr)
 {
+	const struct virtio_net_hdr *vnet_hdr = (const struct virtio_net_hdr *)hdr;
+	int network_offset = tun_vnet_hdr_get_l3_offset(flags, skb, vnet_hdr);
+
+	if (network_offset < 0)
+		return network_offset;
+
 	return virtio_net_hdr_tnl_to_skb(skb, hdr,
 				features & NETIF_F_GSO_UDP_TUNNEL,
 				features & NETIF_F_GSO_UDP_TUNNEL_CSUM,
-				tun_vnet_is_little_endian(flags));
+				tun_vnet_is_little_endian(flags),
+				network_offset);
 }
 
 static inline int tun_vnet_hdr_from_skb(unsigned int flags,
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index e34c52d05..059eeb18e 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -2502,6 +2502,7 @@ static void virtnet_receive_done(struct virtnet_info *vi, struct receive_queue *
 {
 	struct virtio_net_common_hdr *hdr;
 	struct net_device *dev = vi->dev;
+	int network_offset;
 
 	hdr = skb_vnet_common_hdr(skb);
 	if (dev->features & NETIF_F_RXHASH && vi->has_rss_hash_report)
@@ -2515,9 +2516,12 @@ static void virtnet_receive_done(struct virtnet_info *vi, struct receive_queue *
 		goto frame_err;
 	}
 
-	if (virtio_net_hdr_tnl_to_skb(skb, &hdr->tnl_hdr, vi->rx_tnl,
+	network_offset = virtio_net_hdr_get_l3_offset(skb, &hdr->hdr);
+	if (network_offset < 0 ||
+	    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),
+				      network_offset)) {
 		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/include/linux/virtio_net.h b/include/linux/virtio_net.h
index c381b916c..a4c005796 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -48,11 +48,49 @@ static inline int virtio_net_hdr_set_proto(struct sk_buff *skb,
 	return 0;
 }
 
+/*
+ * Return the L3 offset of an Ethernet frame starting at skb->data.
+ * The offset is unused without NEEDS_CSUM, so avoid parsing and return zero.
+ */
+static inline int
+virtio_net_hdr_get_l3_offset(const struct sk_buff *skb,
+			     const struct virtio_net_hdr *hdr)
+{
+	unsigned int parse_depth = VLAN_MAX_DEPTH;
+	const struct ethhdr *eth;
+	struct ethhdr ethbuf;
+	__be16 protocol;
+	int depth = ETH_HLEN;
+
+	if (!(hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM))
+		return 0;
+
+	eth = skb_header_pointer(skb, 0, sizeof(ethbuf), &ethbuf);
+	if (!eth)
+		return -EINVAL;
+
+	protocol = eth->h_proto;
+	while (eth_type_vlan(protocol)) {
+		const struct vlan_hdr *vh;
+		struct vlan_hdr vhdr;
+
+		vh = skb_header_pointer(skb, depth, sizeof(vhdr), &vhdr);
+		if (!vh || !--parse_depth)
+			return -EINVAL;
+
+		protocol = vh->h_vlan_encapsulated_proto;
+		depth += VLAN_HLEN;
+	}
+
+	return depth;
+}
+
 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)
+					  bool little_endian, u8 hdr_gso_type,
+					  int network_offset)
 {
-	unsigned int nh_min_len = sizeof(struct iphdr);
+	int nh_min_len = sizeof(struct iphdr);
 	unsigned int gso_type = 0;
 	unsigned int thlen = 0;
 	unsigned int p_off = 0;
@@ -98,16 +136,20 @@ static inline int __virtio_net_hdr_to_skb(struct sk_buff *skb,
 		u32 start = __virtio16_to_cpu(little_endian, hdr->csum_start);
 		u32 off = __virtio16_to_cpu(little_endian, hdr->csum_offset);
 		u32 needed = start + max_t(u32, thlen, off + sizeof(__sum16));
+		int transport_offset;
 
 		if (!pskb_may_pull(skb, needed))
 			return -EINVAL;
 
 		if (!skb_partial_csum_set(skb, start, off))
 			return -EINVAL;
-		if (skb_transport_offset(skb) < nh_min_len)
+
+		transport_offset = skb_transport_offset(skb);
+		if (transport_offset < nh_min_len || network_offset < 0 ||
+		    network_offset > transport_offset - nh_min_len)
 			return -EINVAL;
 
-		nh_min_len = skb_transport_offset(skb);
+		nh_min_len = transport_offset;
 		p_off = nh_min_len + thlen;
 		if (!pskb_may_pull(skb, p_off))
 			return -EINVAL;
@@ -206,9 +248,11 @@ static inline int __virtio_net_hdr_to_skb(struct sk_buff *skb,
 
 static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
 					const struct virtio_net_hdr *hdr,
-					bool little_endian)
+					bool little_endian,
+					int network_offset)
 {
-	return __virtio_net_hdr_to_skb(skb, hdr, little_endian, hdr->gso_type);
+	return __virtio_net_hdr_to_skb(skb, hdr, little_endian, hdr->gso_type,
+				       network_offset);
 }
 
 /* This function must be called after virtio_net_hdr_from_skb(). */
@@ -287,7 +331,7 @@ static inline int virtio_net_hdr_from_skb(const struct sk_buff *skb,
 	return 0;
 }
 
-static inline unsigned int virtio_l3min(bool is_ipv6)
+static inline int virtio_l3min(bool is_ipv6)
 {
 	return is_ipv6 ? sizeof(struct ipv6hdr) : sizeof(struct iphdr);
 }
@@ -297,18 +341,19 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
 			  const struct virtio_net_hdr_v1_hash_tunnel *vhdr,
 			  bool tnl_hdr_negotiated,
 			  bool tnl_csum_negotiated,
-			  bool little_endian)
+			  bool little_endian, int network_offset)
 {
 	const struct virtio_net_hdr *hdr = (const struct virtio_net_hdr *)vhdr;
-	unsigned int inner_nh, outer_th, inner_th;
-	unsigned int inner_l3min, outer_l3min;
 	u8 gso_inner_type, gso_tunnel_type;
 	bool outer_isv6, inner_isv6;
+	int inner_nh, outer_th, inner_th;
+	int inner_l3min, outer_l3min;
 	int ret;
 
 	gso_tunnel_type = hdr->gso_type & VIRTIO_NET_HDR_GSO_UDP_TUNNEL;
 	if (!gso_tunnel_type)
-		return virtio_net_hdr_to_skb(skb, hdr, little_endian);
+		return virtio_net_hdr_to_skb(skb, hdr, little_endian,
+					     network_offset);
 
 	/* Tunnel not supported/negotiated, but the hdr asks for it. */
 	if (!tnl_hdr_negotiated)
@@ -332,19 +377,22 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb,
 	outer_isv6 = gso_tunnel_type & VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV6;
 	inner_isv6 = gso_inner_type == VIRTIO_NET_HDR_GSO_TCPV6;
 	inner_l3min = virtio_l3min(inner_isv6);
-	outer_l3min = ETH_HLEN + virtio_l3min(outer_isv6);
+	outer_l3min = virtio_l3min(outer_isv6);
 
 	inner_th = __virtio16_to_cpu(little_endian, hdr->csum_start);
 	inner_nh = le16_to_cpu(vhdr->inner_nh_offset);
 	outer_th = le16_to_cpu(vhdr->outer_th_offset);
-	if (outer_th < outer_l3min ||
+	if (network_offset < 0 ||
+	    outer_th < outer_l3min ||
+	    network_offset > outer_th - outer_l3min ||
 	    inner_nh < outer_th + sizeof(struct udphdr) ||
 	    inner_th < inner_nh + inner_l3min)
 		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);
+				      hdr->gso_type & ~gso_tunnel_type,
+				      network_offset);
 	if (ret)
 		return ret;
 
diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 50cae32ae..04c80e23d 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2901,7 +2901,8 @@ 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(),
+						  skb_network_offset(skb))) {
 				tp_len = -EINVAL;
 				goto tpacket_error;
 			}
@@ -3103,7 +3104,8 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
 	packet_parse_headers(skb, sock);
 
 	if (vnet_hdr_sz) {
-		err = virtio_net_hdr_to_skb(skb, &vnet_hdr, vio_le());
+		err = virtio_net_hdr_to_skb(skb, &vnet_hdr, vio_le(),
+					    skb_network_offset(skb));
 		if (err)
 			goto out_free;
 		len += vnet_hdr_sz;
-- 
2.46.0