[PATCH net v3] net: openvswitch: make transport checksums writable

From: Kyle Zeng <hidden>
Date: 2026-11-13 23:22:05
Also in: lkml, stable

OVS can still rewrite an IP address after transport-header extraction
fails. The checksum helpers check skb->len, which includes non-linear
data, before directly accessing the L4 checksum. If the transport header
was not pulled into the linear head, the checksum update can read or
write beyond it.

Call skb_ensure_writable() before IPv4 TCP/UDP and IPv6 TCP/UDP/ICMPv6
checksum updates. Save the original addresses and reload header pointers,
since even a failed pull can move the skb head.

Keep failures nonfatal to preserve the existing policy of rewriting IP
addresses when the transport header is unavailable. A failed pull or
unshare may leave a stale checksum or CHECKSUM_PARTIAL seed; the receiver
may discard the packet.

The issue was found by an LLM.

Fixes: ccb1352e76cf ("net: Add Open vSwitch kernel components.")
Fixes: 3fdbd1ce11e5 ("openvswitch: add ipv6 'set' action")
Suggested-by: Ilya Maximets <i.maximets@ovn.org>
Cc: <redacted>
Signed-off-by: Kyle Zeng <redacted>
---
v3:
- Select source and destination addresses with booleans instead of
  calculating offsets. Keep the saved IPv6 address across head movement.
- Remove the redundant IPv6 fragment check and correct tab alignment.
- Move testing details below the separator and clean up attribution.

v2: https://lore.kernel.org/r/20261107040238.27606-1-kylebot@openai.com (local)
v1: https://lore.kernel.org/r/20261030042304.74291-1-kylebot@openai.com (local)

I have a reproducer for this issue and can share through DM.

 net/openvswitch/actions.c | 77 +++++++++++++++++++++++++--------------
 1 file changed, 49 insertions(+), 28 deletions(-)

diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
index 12e339cbf791..3b10b3fbadc3 100644
--- a/net/openvswitch/actions.c
+++ b/net/openvswitch/actions.c

@@ -319,20 +319,22 @@ static int pop_nsh(struct sk_buff *skb, struct sw_flow_key *key)
 	return 0;
 }
 
-static void update_ip_l4_checksum(struct sk_buff *skb, struct iphdr *nh,
-				  __be32 addr, __be32 new_addr)
+static void update_ip_l4_checksum(struct sk_buff *skb, __be32 addr,
+				  __be32 new_addr)
 {
-	int transport_len = skb->len - skb_transport_offset(skb);
+	struct iphdr *nh = ip_hdr(skb);
 
 	if (nh->frag_off & htons(IP_OFFSET))
 		return;
 
 	if (nh->protocol == IPPROTO_TCP) {
-		if (likely(transport_len >= sizeof(struct tcphdr)))
+		if (likely(!skb_ensure_writable(skb, skb_transport_offset(skb) +
+						sizeof(struct tcphdr))))
 			inet_proto_csum_replace4(&tcp_hdr(skb)->check, skb,
 						 addr, new_addr, true);
 	} else if (nh->protocol == IPPROTO_UDP) {
-		if (likely(transport_len >= sizeof(struct udphdr))) {
+		if (likely(!skb_ensure_writable(skb, skb_transport_offset(skb) +
+						sizeof(struct udphdr)))) {
 			struct udphdr *uh = udp_hdr(skb);
 
 			if (uh->check || skb->ip_summed == CHECKSUM_PARTIAL) {

@@ -345,32 +347,34 @@ static void update_ip_l4_checksum(struct sk_buff *skb, struct iphdr *nh,
 	}
 }
 
-static void set_ip_addr(struct sk_buff *skb, struct iphdr *nh,
-			__be32 *addr, __be32 new_addr)
+/* May move the skb head; callers must reload packet pointers. */
+static void set_ip_addr(struct sk_buff *skb, __be32 new_addr, bool saddr)
 {
-	update_ip_l4_checksum(skb, nh, *addr, new_addr);
-	csum_replace4(&nh->check, *addr, new_addr);
+	__be32 old_addr = saddr ? ip_hdr(skb)->saddr : ip_hdr(skb)->daddr;
+
+	update_ip_l4_checksum(skb, old_addr, new_addr);
+	csum_replace4(&ip_hdr(skb)->check, old_addr, new_addr);
+
 	skb_clear_hash(skb);
 	ovs_ct_clear(skb, NULL);
-	*addr = new_addr;
+
+	if (saddr)
+		ip_hdr(skb)->saddr = new_addr;
+	else
+		ip_hdr(skb)->daddr = new_addr;
 }
 
 static void update_ipv6_checksum(struct sk_buff *skb, u8 l4_proto,
 				 __be32 addr[4], const __be32 new_addr[4])
 {
-	int transport_len;
-
-	if (l4_proto == NEXTHDR_FRAGMENT)
-		return;
-
-	transport_len = skb->len - skb_transport_offset(skb);
-
 	if (l4_proto == NEXTHDR_TCP) {
-		if (likely(transport_len >= sizeof(struct tcphdr)))
+		if (likely(!skb_ensure_writable(skb, skb_transport_offset(skb) +
+						sizeof(struct tcphdr))))
 			inet_proto_csum_replace16(&tcp_hdr(skb)->check, skb,
 						  addr, new_addr, true);
 	} else if (l4_proto == NEXTHDR_UDP) {
-		if (likely(transport_len >= sizeof(struct udphdr))) {
+		if (likely(!skb_ensure_writable(skb, skb_transport_offset(skb) +
+						sizeof(struct udphdr)))) {
 			struct udphdr *uh = udp_hdr(skb);
 
 			if (uh->check || skb->ip_summed == CHECKSUM_PARTIAL) {

@@ -381,7 +385,8 @@ static void update_ipv6_checksum(struct sk_buff *skb, u8 l4_proto,
 			}
 		}
 	} else if (l4_proto == NEXTHDR_ICMP) {
-		if (likely(transport_len >= sizeof(struct icmp6hdr)))
+		if (likely(!skb_ensure_writable(skb, skb_transport_offset(skb) +
+						sizeof(struct icmp6hdr))))
 			inet_proto_csum_replace16(&icmp6_hdr(skb)->icmp6_cksum,
 						  skb, addr, new_addr, true);
 	}

@@ -396,16 +401,28 @@ static void mask_ipv6_addr(const __be32 old[4], const __be32 addr[4],
 	masked[3] = OVS_MASKED(old[3], addr[3], mask[3]);
 }
 
+/* May move the skb head; callers must reload packet pointers. */
 static void set_ipv6_addr(struct sk_buff *skb, u8 l4_proto,
-			  __be32 addr[4], const __be32 new_addr[4],
+			  const __be32 new_addr[4], bool saddr,
 			  bool recalculate_csum)
 {
-	if (recalculate_csum)
-		update_ipv6_checksum(skb, l4_proto, addr, new_addr);
+	struct ipv6hdr *nh = ipv6_hdr(skb);
+	__be32 old_addr[4];
+
+	if (recalculate_csum) {
+		memcpy(old_addr, saddr ? &nh->saddr : &nh->daddr,
+		       sizeof(old_addr));
+		update_ipv6_checksum(skb, l4_proto, old_addr, new_addr);
+		nh = ipv6_hdr(skb);
+	}
 
 	skb_clear_hash(skb);
 	ovs_ct_clear(skb, NULL);
-	memcpy(addr, new_addr, sizeof(__be32[4]));
+
+	if (saddr)
+		memcpy(&nh->saddr, new_addr, sizeof(__be32[4]));
+	else
+		memcpy(&nh->daddr, new_addr, sizeof(__be32[4]));
 }
 
 static void set_ipv6_dsfield(struct sk_buff *skb, struct ipv6hdr *nh, u8 ipv6_tclass, u8 mask)

@@ -479,16 +496,18 @@ static int set_ipv4(struct sk_buff *skb, struct sw_flow_key *flow_key,
 		new_addr = OVS_MASKED(nh->saddr, key->ipv4_src, mask->ipv4_src);
 
 		if (unlikely(new_addr != nh->saddr)) {
-			set_ip_addr(skb, nh, &nh->saddr, new_addr);
+			set_ip_addr(skb, new_addr, true);
 			flow_key->ipv4.addr.src = new_addr;
+			nh = ip_hdr(skb);
 		}
 	}
 	if (mask->ipv4_dst) {
 		new_addr = OVS_MASKED(nh->daddr, key->ipv4_dst, mask->ipv4_dst);
 
 		if (unlikely(new_addr != nh->daddr)) {
-			set_ip_addr(skb, nh, &nh->daddr, new_addr);
+			set_ip_addr(skb, new_addr, false);
 			flow_key->ipv4.addr.dst = new_addr;
+			nh = ip_hdr(skb);
 		}
 	}
 	if (mask->ipv4_tos) {

@@ -533,10 +552,11 @@ static int set_ipv6(struct sk_buff *skb, struct sw_flow_key *flow_key,
 		mask_ipv6_addr(saddr, key->ipv6_src, mask->ipv6_src, masked);
 
 		if (unlikely(memcmp(saddr, masked, sizeof(masked)))) {
-			set_ipv6_addr(skb, flow_key->ip.proto, saddr, masked,
+			set_ipv6_addr(skb, flow_key->ip.proto, masked, true,
 				      true);
 			memcpy(&flow_key->ipv6.addr.src, masked,
 			       sizeof(flow_key->ipv6.addr.src));
+			nh = ipv6_hdr(skb);
 		}
 	}
 	if (is_ipv6_mask_nonzero(mask->ipv6_dst)) {

@@ -555,10 +575,11 @@ static int set_ipv6(struct sk_buff *skb, struct sw_flow_key *flow_key,
 							     NULL, &flags)
 					       != NEXTHDR_ROUTING);
 
-			set_ipv6_addr(skb, flow_key->ip.proto, daddr, masked,
+			set_ipv6_addr(skb, flow_key->ip.proto, masked, false,
 				      recalc_csum);
 			memcpy(&flow_key->ipv6.addr.dst, masked,
 			       sizeof(flow_key->ipv6.addr.dst));
+			nh = ipv6_hdr(skb);
 		}
 	}
 	if (mask->ipv6_tclass) {

base-commit: 2800868f63227c8237d70c41e70d441c919620d6
-- 
2.55.0.openai.911.g06873a30d2f3

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