[PATCH net] net/sched: act_nat: Fix missing headroom COW and integer underflow in header rewriting

[Muhammad Bilal <meatuni001@gmail.com>]

tcf_nat_act() accesses and rewrites IPv4, TCP, UDP, and ICMP headers
based on noff = skb_network_offset(skb).

When the network header resides in the headroom (noff < 0), two issues
occur:
1. sizeof(*iph) + noff can evaluate to a negative value or underflow
   when passed to functions expecting unsigned lengths, such as
   pskb_may_pull() and skb_try_make_writable().
2. skb_try_make_writable() only evaluates writability from skb->data
   forwards and does not invoke skb_cow() on the headroom. When modifying
   cloned SKBs (e.g. from packet sockets, tc mirred, or BPF redirects),
   in-place header modification via iph->saddr / iph->daddr mutates
   shared headroom data directly, leading to packet corruption and page
   cache corruption.

Fix this by introducing a helper nat_ensure_writable() that validates
headroom using skb_cow(skb, -offset) when offset is negative before
ensuring writability across the modified header length.

Fixes: b4219952356b ("[PKT_SCHED]: Add stateless NAT")
Signed-off-by: Muhammad Bilal <meatuni001@gmail.com>
---
 net/sched/act_nat.c | 29 ++++++++++++++++++-----------
 1 file changed, 18 insertions(+), 11 deletions(-)

diff --git a/net/sched/act_nat.c b/net/sched/act_nat.c
index 28cb48419616..1bf5d55b3dc4 100644
--- a/net/sched/act_nat.c
+++ b/net/sched/act_nat.c
@@ -112,6 +112,18 @@ static struct tc_action_ops act_nat_ops;
 
 static const struct rhashtable_params tcf_nat_ht_params;
 
+static int nat_ensure_writable(struct sk_buff *skb, int offset, size_t len)
+{
+	if (offset < 0) {
+		if (skb_cow(skb, -offset))
+			return -ENOMEM;
+		if (offset + (int)len > 0)
+			return skb_ensure_writable(skb, offset + len);
+		return 0;
+	}
+	return skb_ensure_writable(skb, offset + len);
+}
+
 TC_INDIRECT_SCOPE int tcf_nat_act(struct sk_buff *skb,
 				  const struct tc_action *a,
 				  struct tcf_result *res)
@@ -142,7 +154,7 @@ TC_INDIRECT_SCOPE int tcf_nat_act(struct sk_buff *skb,
 	egress = parms->flags & TCA_NAT_FLAG_EGRESS;
 
 	noff = skb_network_offset(skb);
-	if (!pskb_may_pull(skb, sizeof(*iph) + noff))
+	if (nat_ensure_writable(skb, noff, sizeof(*iph)))
 		goto drop;
 
 	iph = ip_hdr(skb);
@@ -153,9 +165,6 @@ TC_INDIRECT_SCOPE int tcf_nat_act(struct sk_buff *skb,
 		addr = iph->daddr;
 
 	if (!((old_addr ^ addr) & mask)) {
-		if (skb_try_make_writable(skb, sizeof(*iph) + noff))
-			goto drop;
-
 		new_addr &= mask;
 		new_addr |= addr & ~mask;
 
@@ -180,8 +189,7 @@ TC_INDIRECT_SCOPE int tcf_nat_act(struct sk_buff *skb,
 	{
 		struct tcphdr *tcph;
 
-		if (!pskb_may_pull(skb, ihl + sizeof(*tcph) + noff) ||
-		    skb_try_make_writable(skb, ihl + sizeof(*tcph) + noff))
+		if (nat_ensure_writable(skb, noff, ihl + sizeof(*tcph)))
 			goto drop;
 
 		tcph = (void *)(skb_network_header(skb) + ihl);
@@ -193,8 +201,7 @@ TC_INDIRECT_SCOPE int tcf_nat_act(struct sk_buff *skb,
 	{
 		struct udphdr *udph;
 
-		if (!pskb_may_pull(skb, ihl + sizeof(*udph) + noff) ||
-		    skb_try_make_writable(skb, ihl + sizeof(*udph) + noff))
+		if (nat_ensure_writable(skb, noff, ihl + sizeof(*udph)))
 			goto drop;
 
 		udph = (void *)(skb_network_header(skb) + ihl);
@@ -209,19 +216,19 @@ TC_INDIRECT_SCOPE int tcf_nat_act(struct sk_buff *skb,
 	{
 		struct icmphdr *icmph;
 
-		if (!pskb_may_pull(skb, ihl + sizeof(*icmph) + noff))
+		if (nat_ensure_writable(skb, noff, ihl + sizeof(*icmph)))
 			goto drop;
 
 		icmph = (void *)(skb_network_header(skb) + ihl);
 
 		if (!icmp_is_err(icmph->type))
 			break;
 
-		if (!pskb_may_pull(skb, ihl + sizeof(*icmph) + sizeof(*iph) +
-					noff))
+		if (nat_ensure_writable(skb, noff,
+					ihl + sizeof(*icmph) + sizeof(*iph)))
 			goto drop;
 
 		icmph = (void *)(skb_network_header(skb) + ihl);
 		iph = (void *)(icmph + 1);
 		if (egress)
 			addr = iph->daddr;
 		else
 			addr = iph->saddr;
 
 		if ((old_addr ^ addr) & mask)
 			break;
-
-		if (skb_try_make_writable(skb, ihl + sizeof(*icmph) +
-					  sizeof(*iph) + noff))
-			goto drop;
 
 		icmph = (void *)(skb_network_header(skb) + ihl);
 		iph = (void *)(icmph + 1);
-- 
2.43.0