tcf_skbmod_act() calls skb_ensure_writable(skb, max_edit_len) to ensure
the modified packet header is writable before rewriting Ethernet addresses
or setting ECN bits.

However, skb_ensure_writable() only pulls and COWs memory starting from
skb->data onwards. At ingress or on forwarded packets, the Ethernet
header (or network header) may reside in the headroom at a negative
offset (skb_mac_offset(skb) < 0).

Because skb_cow() is omitted for negative offsets, writes via
ether_addr_copy() or INET_ECN_set_ce() modify shared headroom in-place
on cloned SKBs (e.g., cloned by tc mirred, bpf_clone_redirect, or
packet capture sockets). This can result in silent packet corruption
and page cache corruption.

Fix this by ensuring that if the target header starts at a negative
offset in the headroom, skb_cow(skb, -offset) is called to unshare the
headroom before ensuring writability across the header span.

Fixes: 86da71b57383 ("net_sched: Introduce skbmod action")
Signed-off-by: Muhammad Bilal <meatuni001@gmail.com>
---
 net/sched/act_skbmod.c | 37 ++++++++++++++++++++++++-------------
 1 file changed, 24 insertions(+), 13 deletions(-)

diff --git a/net/sched/act_skbmod.c b/net/sched/act_skbmod.c
index a8e2b83ebae5..cd2a6e974e6f 100644
--- a/net/sched/act_skbmod.c
+++ b/net/sched/act_skbmod.c
@@ -22,13 +22,24 @@
 
 static struct tc_action_ops act_skbmod_ops;
 
+static int skbmod_ensure_writable(struct sk_buff *skb, int offset, int len)
+{
+	if (offset < 0) {
+		if (skb_cow(skb, -offset))
+			return -ENOMEM;
+		if (offset + len > 0)
+			return skb_ensure_writable(skb, offset + len);
+		return 0;
+	}
+	return skb_ensure_writable(skb, offset + len);
+}
+
 TC_INDIRECT_SCOPE int tcf_skbmod_act(struct sk_buff *skb,
 				     const struct tc_action *a,
 				     struct tcf_result *res)
 {
 	struct tcf_skbmod *d = to_skbmod(a);
 	struct tcf_skbmod_params *p;
-	int max_edit_len, err;
 	u64 flags;
 
 	tcf_lastuse_update(&d->tcf_tm);
@@ -38,7 +49,6 @@ TC_INDIRECT_SCOPE int tcf_skbmod_act(struct sk_buff *skb,
 	if (unlikely(p->action == TC_ACT_SHOT))
 		goto drop;
 
-	max_edit_len = skb_mac_header_len(skb);
 	flags = p->flags;
 
 	/* tcf_skbmod_init() guarantees "flags" to be one of the following:
@@ -52,19 +62,20 @@ TC_INDIRECT_SCOPE int tcf_skbmod_act(struct sk_buff *skb,
 		switch (skb_protocol(skb, true)) {
 		case cpu_to_be16(ETH_P_IP):
 		case cpu_to_be16(ETH_P_IPV6):
-			max_edit_len += skb_network_header_len(skb);
+			if (skbmod_ensure_writable(skb, skb_network_offset(skb),
+						   skb_network_header_len(skb)))
+				goto drop;
 			break;
 		default:
 			goto out;
 		}
-	} else if (!skb->dev || skb->dev->type != ARPHRD_ETHER) {
-		goto out;
-	}
-
-	err = skb_ensure_writable(skb, max_edit_len);
-	if (unlikely(err)) /* best policy is to drop on the floor */
-		goto drop;
+	} else {
+		if (!skb->dev || skb->dev->type != ARPHRD_ETHER)
+			goto out;
 
+		if (skbmod_ensure_writable(skb, skb_mac_offset(skb), ETH_HLEN))
+			goto drop;
+	}
 	if (flags & SKBMOD_F_DMAC)
 		ether_addr_copy(eth_hdr(skb)->h_dest, p->eth_dst);
 	if (flags & SKBMOD_F_SMAC)
-- 
2.43.0