openvswitch: fix GSO userspace truncation underflow

OVS_ACTION_ATTR_TRUNC currently stores a delta from the original skb
length in OVS_CB(skb)->cutlen. When a later userspace action segments a
GSO skb, queue_gso_packets() reuses that delta for each smaller segment.
A segment can then reach queue_userspace_packet() with cutlen greater
than skb->len, underflowing the length passed to skb_zerocopy().

Store the maximum preserved length instead and bound each consumer
against the current skb length. Use U32_MAX as the no-truncation
sentinel so the value remains valid if skb geometry changes before a
consumer handles it.

Fixes: f2a4d086ed4c ("openvswitch: Add packet truncation support.")
Cc: stable@vger.kernel.org
Assisted-by: Codex:gpt-5.5
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Reviewed-by: Ilya Maximets <i.maximets@ovn.org>
Reviewed-by: Aaron Conole <aconole@redhat.com>
Link: https://patch.msgid.link/20260707221635.27489-1-kylebot@openai.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

1 files changed, 14 insertions, 11 deletions

diff --git a/net/openvswitch/datapath.c b/net/openvswitch/datapath.c
index f0164817d9b7..eaf332b156d7 100644
--- a/net/openvswitch/datapath.c
+++ b/net/openvswitch/datapath.c
@@ -276,7 +276,7 @@ void ovs_dp_process_packet(struct sk_buff *skb, struct sw_flow_key *key)
 			upcall.portid = ovs_vport_find_upcall_portid(p, skb);
 
 		upcall.mru = OVS_CB(skb)->mru;
-		error = ovs_dp_upcall(dp, skb, key, &upcall, 0);
+		error = ovs_dp_upcall(dp, skb, key, &upcall, U32_MAX);
 		switch (error) {
 		case 0:
 		case -EAGAIN:
@@ -457,7 +457,8 @@ static int queue_userspace_packet(struct datapath *dp, struct sk_buff *skb,
 	struct sk_buff *nskb = NULL;
 	struct sk_buff *user_skb = NULL; /* to be queued to userspace */
 	struct nlattr *nla;
-	size_t len;
+	size_t msg_size;
+	size_t skb_len;
 	unsigned int hlen;
 	int err, dp_ifindex;
 	u64 hash;
@@ -478,7 +479,8 @@ static int queue_userspace_packet(struct datapath *dp, struct sk_buff *skb,
 		skb = nskb;
 	}
 
-	if (nla_attr_size(skb->len) > USHRT_MAX) {
+	skb_len = min(skb->len, cutlen);
+	if (nla_attr_size(skb_len) > USHRT_MAX) {
 		err = -EFBIG;
 		goto out;
 	}
@@ -493,13 +495,13 @@ static int queue_userspace_packet(struct datapath *dp, struct sk_buff *skb,
 	 * padding logic. Only perform zerocopy if padding is not required.
 	 */
 	if (dp->user_features & OVS_DP_F_UNALIGNED)
-		hlen = skb_zerocopy_headlen(skb);
+		hlen = min(skb_zerocopy_headlen(skb), cutlen);
 	else
-		hlen = skb->len;
+		hlen = skb_len;
 
-	len = upcall_msg_size(upcall_info, hlen - cutlen,
-			      OVS_CB(skb)->acts_origlen);
-	user_skb = genlmsg_new(len, GFP_ATOMIC);
+	msg_size = upcall_msg_size(upcall_info, hlen,
+				   OVS_CB(skb)->acts_origlen);
+	user_skb = genlmsg_new(msg_size, GFP_ATOMIC);
 	if (!user_skb) {
 		err = -ENOMEM;
 		goto out;
@@ -560,7 +562,7 @@ static int queue_userspace_packet(struct datapath *dp, struct sk_buff *skb,
 	}
 
 	/* Add OVS_PACKET_ATTR_LEN when packet is truncated */
-	if (cutlen > 0 &&
+	if (skb_len < skb->len &&
 	    nla_put_u32(user_skb, OVS_PACKET_ATTR_LEN, skb->len)) {
 		err = -ENOBUFS;
 		goto out;
@@ -585,9 +587,9 @@ static int queue_userspace_packet(struct datapath *dp, struct sk_buff *skb,
 		err = -ENOBUFS;
 		goto out;
 	}
-	nla->nla_len = nla_attr_size(skb->len - cutlen);
+	nla->nla_len = nla_attr_size(skb_len);
 
-	err = skb_zerocopy(user_skb, skb, skb->len - cutlen, hlen);
+	err = skb_zerocopy(user_skb, skb, skb_len, hlen);
 	if (err)
 		goto out;
 
@@ -644,6 +646,7 @@ static int ovs_packet_cmd_execute(struct sk_buff *skb, struct genl_info *info)
 		packet->ignore_df = 1;
 	}
 	OVS_CB(packet)->mru = mru;
+	OVS_CB(packet)->cutlen = U32_MAX;
 
 	if (a[OVS_PACKET_ATTR_HASH]) {
 		hash = nla_get_u64(a[OVS_PACKET_ATTR_HASH]);