[PATCH nf] ipvs: revalidate ihl to prevent out-of-bounds access

While the outer IP header is already pulled into the skb head,
we must be careful and revalidate the embedded headers after
reading them from the skb frags to prevent out-of-bounds
access.

One such place reported by Sashiko is ip_vs_nat_icmp() where
local process can change the ihl field and after
skb_ensure_writable() we can see larger value which is a
problem for the ip_send_check(cih) calls.

Add check to drop the packet if the ihl field is changed.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Link: https://sashiko.dev/#/patchset/20260730183506.87473-1-ja%40ssi.bg
Signed-off-by: Julian Anastasov <[email protected]>
---
 include/net/ip_vs.h             |  2 +-
 net/netfilter/ipvs/ip_vs_core.c | 11 +++++++++--
 net/netfilter/ipvs/ip_vs_xmit.c |  3 ++-
 3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/include/net/ip_vs.h b/include/net/ip_vs.h
index fc2ef5ef31a6..be3a6617adf4 100644
--- a/include/net/ip_vs.h
+++ b/include/net/ip_vs.h
@@ -2068,7 +2068,7 @@ static inline bool ip_vs_conn_use_hash2(struct ip_vs_conn *cp)
 	       !(cp->flags & IP_VS_CONN_F_TEMPLATE);
 }
 
-void ip_vs_nat_icmp(struct sk_buff *skb, struct ip_vs_protocol *pp,
+bool ip_vs_nat_icmp(struct sk_buff *skb, struct ip_vs_protocol *pp,
 		    struct ip_vs_conn *cp, int dir, unsigned int toff,
 		    bool has_ports, struct ip_vs_iphdr *ciph);
 
diff --git a/net/netfilter/ipvs/ip_vs_core.c b/net/netfilter/ipvs/ip_vs_core.c
index 95af77b68851..1a0661d19d9f 100644
--- a/net/netfilter/ipvs/ip_vs_core.c
+++ b/net/netfilter/ipvs/ip_vs_core.c
@@ -923,7 +923,7 @@ static int ip_vs_route_me_harder(struct netns_ipvs *ipvs, int af,
  * Packet has been made sufficiently writable in caller
  * - inout: 1=in->out, 0=out->in
  */
-void ip_vs_nat_icmp(struct sk_buff *skb, struct ip_vs_protocol *pp,
+bool ip_vs_nat_icmp(struct sk_buff *skb, struct ip_vs_protocol *pp,
 		    struct ip_vs_conn *cp, int inout, unsigned int toff,
 		    bool has_ports, struct ip_vs_iphdr *ciph)
 {
@@ -931,6 +931,11 @@ void ip_vs_nat_icmp(struct sk_buff *skb, struct ip_vs_protocol *pp,
 	struct icmphdr *icmph	 = (struct icmphdr *)(skb->data + toff);
 	struct iphdr *cih	 = (struct iphdr *)(icmph + 1);
 
+	/* Before now we may used ihl from skb frag, revalidate it after
+	 * copying it into skb head to prevent out-of-bounds access
+	 */
+	if (cih->ihl * 4 != ciph->len - ciph->off)
+		return false;
 	if (inout) {
 		iph->saddr = cp->vaddr.ip;
 		ip_send_check(iph);
@@ -964,6 +969,7 @@ void ip_vs_nat_icmp(struct sk_buff *skb, struct ip_vs_protocol *pp,
 	else
 		IP_VS_DBG_PKT(11, AF_INET, pp, skb, ciph->off,
 			      "Forwarding altered incoming ICMP");
+	return true;
 }
 
 #ifdef CONFIG_IP_VS_IPV6
@@ -1055,7 +1061,8 @@ static int handle_response_icmp(int af, struct sk_buff *skb,
 		ip_vs_nat_icmp_v6(skb, pp, cp, 1, toff, has_ports, ciph);
 	else
 #endif
-		ip_vs_nat_icmp(skb, pp, cp, 1, toff, has_ports, ciph);
+		if (!ip_vs_nat_icmp(skb, pp, cp, 1, toff, has_ports, ciph))
+			goto out;
 
 	if (ip_vs_route_me_harder(cp->ipvs, af, skb, hooknum))
 		goto out;
diff --git a/net/netfilter/ipvs/ip_vs_xmit.c b/net/netfilter/ipvs/ip_vs_xmit.c
index 3d3c72c9dda4..f5f72a2135e1 100644
--- a/net/netfilter/ipvs/ip_vs_xmit.c
+++ b/net/netfilter/ipvs/ip_vs_xmit.c
@@ -1582,7 +1582,8 @@ ip_vs_icmp_xmit(struct sk_buff *skb, struct ip_vs_conn *cp,
 	if (skb_cow(skb, rt->dst.dev->hard_header_len))
 		goto tx_error;
 
-	ip_vs_nat_icmp(skb, pp, cp, 0, toff, has_ports, ciph);
+	if (!ip_vs_nat_icmp(skb, pp, cp, 0, toff, has_ports, ciph))
+		goto tx_error;
 
 	/* Another hack: avoid icmp_send in ip_fragment */
 	skb->ignore_df = 1;
-- 
2.55.0