[PATCH net] ipv4: validate checksum_start before completing checksum

["Michael S. Tsirkin" <mst@redhat.com>]

If a packet with bad checksum metadata gets into the ipv4 stack,
skb_checksum_help can corrupt the network header and cause a bunch of
mischief.

This was discovered and reported by Paulos, and has been reporoduced
by others independently since.

We really shouldn't allow such packets in, but as a defence
in depth measure, let's also check before we complete the checksum.

A more complete validation at input is forthcoming, but needs more work.

Assisted-by: LLM
Fixes: f43798c27684 ("tun: Allow GSO using virtio_net_hdr")
Fixes: bfd5f4a3d605 ("packet: Add GSO/csum offload support.")
Reported-by: Paulos Yibelo <habte.yibelo@gmail.com>
Closes: https://lore.kernel.org/netdev/20260922030310.8684-2-habte.yibelo@gmail.com/
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

Lightly tested.

 include/net/ip.h                           |  7 +++++++
 net/bridge/netfilter/nf_conntrack_bridge.c | 10 +++++++---
 net/ipv4/ip_output.c                       | 10 +++++++---
 net/netfilter/nfnetlink_queue.c            |  7 +++++++
 net/netfilter/xt_CHECKSUM.c                |  6 +++++-
 net/xfrm/xfrm_output.c                     | 16 +++++++++++-----
 6 files changed, 44 insertions(+), 12 deletions(-)

diff --git a/include/net/ip.h b/include/net/ip.h
index 6f602df72ee6..d07c2c573024 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -74,6 +74,13 @@ static inline unsigned int ip_hdrlen(const struct sk_buff *skb)
 	return ip_hdr(skb)->ihl * 4;
 }
 
+static inline int ip_check_csum_start(const struct sk_buff *skb)
+{
+	if (unlikely(skb->csum_start < skb->network_header + ip_hdrlen(skb)))
+		return -EINVAL;
+	return 0;
+}
+
 struct ipcm_cookie {
 	struct sockcm_cookie	sockc;
 	__be32			addr;
diff --git a/net/bridge/netfilter/nf_conntrack_bridge.c b/net/bridge/netfilter/nf_conntrack_bridge.c
index 7ecb8a26bfa3..b5444335b86f 100644
--- a/net/bridge/netfilter/nf_conntrack_bridge.c
+++ b/net/bridge/netfilter/nf_conntrack_bridge.c
@@ -39,9 +39,13 @@ static int nf_br_ip_fragment(struct net *net, struct sock *sk,
 	int err = 0;
 
 	/* for offloaded checksums cleanup checksum before fragmentation */
-	if (skb->ip_summed == CHECKSUM_PARTIAL &&
-	    (err = skb_checksum_help(skb)))
-		goto blackhole;
+	if (skb->ip_summed == CHECKSUM_PARTIAL) {
+		err = ip_check_csum_start(skb);
+		if (!err)
+			err = skb_checksum_help(skb);
+		if (err)
+			goto blackhole;
+	}
 
 	iph = ip_hdr(skb);
 
diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 74e095b6b7ca..bea79611617f 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -771,9 +771,13 @@ int ip_do_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
 	int err = 0;
 
 	/* for offloaded checksums cleanup checksum before fragmentation */
-	if (skb->ip_summed == CHECKSUM_PARTIAL &&
-	    (err = skb_checksum_help(skb)))
-		goto fail;
+	if (skb->ip_summed == CHECKSUM_PARTIAL) {
+		err = ip_check_csum_start(skb);
+		if (!err)
+			err = skb_checksum_help(skb);
+		if (err)
+			goto fail;
+	}
 
 	/*
 	 *	Point into the IP datagram header.
diff --git a/net/netfilter/nfnetlink_queue.c b/net/netfilter/nfnetlink_queue.c
index c727668b0c5b..5177e27174e7 100644
--- a/net/netfilter/nfnetlink_queue.c
+++ b/net/netfilter/nfnetlink_queue.c
@@ -40,6 +40,7 @@
 #include <linux/udp.h>
 #include <net/gre.h>
 #include <net/gso.h>
+#include <net/ip.h>
 #include <net/sock.h>
 #include <net/tcp_states.h>
 #include <net/netfilter/nf_queue.h>
@@ -674,6 +675,12 @@ static int nfqnl_put_bridge(struct nf_queue_entry *entry, struct sk_buff *skb)
 
 static int nf_queue_checksum_help(struct sk_buff *entskb)
 {
+	if (entskb->protocol == htons(ETH_P_IP)) {
+		if (!pskb_network_may_pull(entskb, sizeof(struct iphdr)) ||
+		    ip_check_csum_start(entskb))
+			return -EINVAL;
+	}
+
 	if (skb_csum_is_sctp(entskb))
 		return skb_crc32c_csum_help(entskb);
 
diff --git a/net/netfilter/xt_CHECKSUM.c b/net/netfilter/xt_CHECKSUM.c
index 9d99f5a3d176..1fb0f8118404 100644
--- a/net/netfilter/xt_CHECKSUM.c
+++ b/net/netfilter/xt_CHECKSUM.c
@@ -15,6 +15,7 @@
 
 #include <linux/netfilter_ipv4/ip_tables.h>
 #include <linux/netfilter_ipv6/ip6_tables.h>
+#include <net/ip.h>
 
 MODULE_LICENSE("GPL");
 MODULE_AUTHOR("Michael S. Tsirkin <mst@redhat.com>");
@@ -25,8 +26,11 @@ MODULE_ALIAS("ip6t_CHECKSUM");
 static unsigned int
 checksum_tg(struct sk_buff *skb, const struct xt_action_param *par)
 {
-	if (skb->ip_summed == CHECKSUM_PARTIAL && !skb_is_gso(skb))
+	if (skb->ip_summed == CHECKSUM_PARTIAL && !skb_is_gso(skb)) {
+		if (xt_family(par) == NFPROTO_IPV4 && ip_check_csum_start(skb))
+			return NF_DROP;
 		skb_checksum_help(skb);
+	}
 
 	return XT_CONTINUE;
 }
diff --git a/net/xfrm/xfrm_output.c b/net/xfrm/xfrm_output.c
index e305ba32e356..f1f612cd7b43 100644
--- a/net/xfrm/xfrm_output.c
+++ b/net/xfrm/xfrm_output.c
@@ -823,16 +823,22 @@ int xfrm_output(struct sock *sk, struct sk_buff *skb)
 	}
 
 	if (skb->ip_summed == CHECKSUM_PARTIAL) {
-		err = skb_checksum_help(skb);
-		if (err) {
-			XFRM_INC_STATS(net, LINUX_MIB_XFRMOUTERROR);
-			kfree_skb(skb);
-			return err;
+		if (skb->protocol == htons(ETH_P_IP)) {
+			err = ip_check_csum_start(skb);
+			if (err)
+				goto error;
 		}
+		err = skb_checksum_help(skb);
+		if (err)
+			goto error;
 	}
 
 out:
 	return xfrm_output2(net, sk, skb);
+error:
+	XFRM_INC_STATS(net, LINUX_MIB_XFRMOUTERROR);
+	kfree_skb(skb);
+	return err;
 }
 EXPORT_SYMBOL_GPL(xfrm_output);
 
-- 
MST