[PATCH nf v2] netfilter: nft_exthdr: Fix non-linear header modification

[Xiao Liang <shaw.leon@gmail.com>]

Fix skb_ensure_writable() size. Don't use nft_tcp_header_pointer() to
make it explicit that pointers point to the packet (not local buffer).

Fixes: 99d1712bc41c ("netfilter: exthdr: tcp option set support")
Fixes: 7890cbea66e7 ("netfilter: exthdr: add support for tcp option removal")
Signed-off-by: Xiao Liang <shaw.leon@gmail.com>
---
 net/netfilter/nft_exthdr.c | 20 ++++++++------------
 1 file changed, 8 insertions(+), 12 deletions(-)

diff --git a/net/netfilter/nft_exthdr.c b/net/netfilter/nft_exthdr.c
index 7f856ceb3a66..a9844eefedeb 100644
--- a/net/netfilter/nft_exthdr.c
+++ b/net/netfilter/nft_exthdr.c
@@ -238,7 +238,12 @@ static void nft_exthdr_tcp_set_eval(const struct nft_expr *expr,
 	if (!tcph)
 		goto err;
 
+	if (skb_ensure_writable(pkt->skb, nft_thoff(pkt) + tcphdr_len))
+		goto err;
+
+	tcph = (struct tcphdr *)(pkt->skb->data + nft_thoff(pkt));
 	opt = (u8 *)tcph;
+
 	for (i = sizeof(*tcph); i < tcphdr_len - 1; i += optl) {
 		union {
 			__be16 v16;
@@ -253,15 +258,6 @@ static void nft_exthdr_tcp_set_eval(const struct nft_expr *expr,
 		if (i + optl > tcphdr_len || priv->len + priv->offset > optl)
 			goto err;
 
-		if (skb_ensure_writable(pkt->skb,
-					nft_thoff(pkt) + i + priv->len))
-			goto err;
-
-		tcph = nft_tcp_header_pointer(pkt, sizeof(buff), buff,
-					      &tcphdr_len);
-		if (!tcph)
-			goto err;
-
 		offset = i + priv->offset;
 
 		switch (priv->len) {
@@ -325,9 +321,9 @@ static void nft_exthdr_tcp_strip_eval(const struct nft_expr *expr,
 	if (skb_ensure_writable(pkt->skb, nft_thoff(pkt) + tcphdr_len))
 		goto drop;
 
-	opt = (u8 *)nft_tcp_header_pointer(pkt, sizeof(buff), buff, &tcphdr_len);
-	if (!opt)
-		goto err;
+	tcph = (struct tcphdr *)(pkt->skb->data + nft_thoff(pkt));
+	opt = (u8 *)tcph;
+
 	for (i = sizeof(*tcph); i < tcphdr_len - 1; i += optl) {
 		unsigned int j;
 
-- 
2.42.0