[RFC net-next 04/15] ipxlat: add IPv4 packet validation path

[Ralf Lici <ralf@mandelbit.com>]

Implement IPv4 packet parsing and validation, including option
inspection, fragment-sensitive L4 checks, and UDP checksum-zero handling
consistent with translator constraints. The parser populates skb
control-block metadata consumed by translation and marks RFC-driven drop
reasons for later action handling.

Signed-off-by: Ralf Lici <ralf@mandelbit.com>
---
 drivers/net/ipxlat/packet.c | 312 +++++++++++++++++++++++++++++++++++-
 1 file changed, 310 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ipxlat/packet.c b/drivers/net/ipxlat/packet.c
index f82c375255f3..0cc619dca147 100644
--- a/drivers/net/ipxlat/packet.c
+++ b/drivers/net/ipxlat/packet.c
@@ -11,6 +11,8 @@
  *		Ralf Lici <ralf@mandelbit.com>
  */
 
+#include <linux/icmp.h>
+
 #include "packet.h"
 
 /* Shift cached skb cb offsets by the L3 header delta after in-place rewrite.
@@ -88,9 +90,315 @@ bool ipxlat_cb_offsets_valid(const struct ipxlat_cb *cb)
 }
 #endif
 
-int ipxlat_v4_validate_skb(struct ipxlat_priv *ipxl, struct sk_buff *skb)
+static bool ipxlat_v4_validate_addr(__be32 addr4)
 {
-	return -EOPNOTSUPP;
+	return !(ipv4_is_zeronet(addr4) || ipv4_is_loopback(addr4) ||
+		 ipv4_is_multicast(addr4) || ipv4_is_lbcast(addr4));
+}
+
+/* RFC 7915 Section 4.1 requires ignoring IPv4 options unless an unexpired
+ * LSRR/SSRR is present, in which case we must send ICMPv4 SR_FAILED.
+ * We intentionally treat malformed option encoding as invalid input and
+ * drop early instead of continuing translation.
+ */
+static int ipxlat_v4_srr_check(struct sk_buff *skb, const struct iphdr *hdr)
+{
+	const u8 *opt, *end;
+	u8 type, len, ptr;
+
+	if (likely(hdr->ihl <= 5))
+		return 0;
+
+	opt = (const u8 *)(hdr + 1);
+	end = (const u8 *)hdr + (hdr->ihl << 2);
+
+	while (opt < end) {
+		type = opt[0];
+		if (type == IPOPT_END)
+			return 0;
+		if (type == IPOPT_NOOP) {
+			opt++;
+			continue;
+		}
+
+		if (unlikely(end - opt < 2))
+			return -EINVAL;
+
+		len = opt[1];
+		if (unlikely(len < 2 || opt + len > end))
+			return -EINVAL;
+
+		if (type == IPOPT_LSRR || type == IPOPT_SSRR) {
+			if (unlikely(len < 3))
+				return -EINVAL;
+
+			/* points to the beginning of the next IP addr */
+			ptr = opt[2];
+			if (unlikely(ptr < 4))
+				return -EINVAL;
+			if (unlikely(ptr > len))
+				return 0;
+			if (unlikely(ptr > len - 3))
+				return -EINVAL;
+
+			return -EINVAL;
+		}
+
+		opt += len;
+	}
+
+	return 0;
+}
+
+static int ipxlat_v4_pull_l3(struct sk_buff *skb, unsigned int l3_offset,
+			     bool inner)
+{
+	const struct iphdr *iph;
+	unsigned int tot_len;
+	int l3_len;
+
+	if (unlikely(!pskb_may_pull(skb, l3_offset + sizeof(*iph))))
+		return -EINVAL;
+
+	iph = (const struct iphdr *)(skb->data + l3_offset);
+	if (unlikely(iph->version != 4 || iph->ihl < 5))
+		return -EINVAL;
+
+	l3_len = iph->ihl << 2;
+	/* For inner packets use ntohs(iph->tot_len) instead of iph_totlen.
+	 * If inner iph->tot_len is zero, iph_totlen would fall back to outer
+	 * GSO metadata, which is unrelated to quoted inner packet length.
+	 */
+	tot_len = unlikely(inner) ? ntohs(iph->tot_len) : iph_totlen(skb, iph);
+	if (unlikely(tot_len < l3_len))
+		return -EINVAL;
+
+	if (unlikely(!pskb_may_pull(skb, l3_offset + l3_len)))
+		return -EINVAL;
+
+	return l3_len;
+}
+
+static int ipxlat_v4_pull_l4(struct sk_buff *skb, unsigned int l4_offset,
+			     u8 l4_proto, bool *is_icmp_err)
+{
+	struct icmphdr *icmp;
+	struct udphdr *udp;
+	struct tcphdr *tcp;
+
+	*is_icmp_err = false;
+
+	switch (l4_proto) {
+	case IPPROTO_TCP:
+		if (unlikely(!pskb_may_pull(skb, l4_offset + sizeof(*tcp))))
+			return -EINVAL;
+
+		tcp = (struct tcphdr *)(skb->data + l4_offset);
+		if (unlikely(tcp->doff < 5))
+			return -EINVAL;
+
+		return __tcp_hdrlen(tcp);
+	case IPPROTO_UDP:
+		if (unlikely(!pskb_may_pull(skb, l4_offset + sizeof(*udp))))
+			return -EINVAL;
+
+		udp = (struct udphdr *)(skb->data + l4_offset);
+		if (unlikely(ntohs(udp->len) < sizeof(*udp)))
+			return -EINVAL;
+
+		return sizeof(struct udphdr);
+	case IPPROTO_ICMP:
+		if (unlikely(!pskb_may_pull(skb, l4_offset + sizeof(*icmp))))
+			return -EINVAL;
+
+		icmp = (struct icmphdr *)(skb->data + l4_offset);
+		*is_icmp_err = icmp_is_err(icmp->type);
+		return sizeof(struct icmphdr);
+	default:
+		return 0;
+	}
+}
+
+static int ipxlat_v4_pull_icmp_inner(struct sk_buff *skb,
+				     unsigned int inner_l3_off)
+{
+	struct ipxlat_cb *cb = ipxlat_skb_cb(skb);
+	const struct iphdr *inner_l3_hdr;
+	unsigned int inner_l4_off;
+	int inner_l3_len, err;
+	bool is_icmp_err;
+
+	inner_l3_len = ipxlat_v4_pull_l3(skb, inner_l3_off, true);
+	if (unlikely(inner_l3_len < 0))
+		return inner_l3_len;
+	inner_l3_hdr = (const struct iphdr *)(skb->data + inner_l3_off);
+
+	/* accept non-first quoted fragments: only inner L3 is translatable */
+	inner_l4_off = inner_l3_off + inner_l3_len;
+	cb->inner_l3_offset = inner_l3_off;
+	cb->inner_l3_hdr_len = inner_l3_len;
+	cb->inner_l4_offset = inner_l4_off;
+
+	if (unlikely(!ipxlat_is_first_frag4(inner_l3_hdr)))
+		return 0;
+
+	err = ipxlat_v4_pull_l4(skb, inner_l4_off, inner_l3_hdr->protocol,
+				&is_icmp_err);
+	if (unlikely(err < 0))
+		return err;
+	if (unlikely(is_icmp_err))
+		return -EINVAL;
+
+	return 0;
+}
+
+static int ipxlat_v4_pull_hdrs(struct sk_buff *skb)
+{
+	const unsigned int l3_off = skb_network_offset(skb);
+	struct ipxlat_cb *cb = ipxlat_skb_cb(skb);
+	int err, l3_len, l4_len = 0;
+	const struct iphdr *l3_hdr;
+
+	/* parse IPv4 header and get its full length including options */
+	l3_len = ipxlat_v4_pull_l3(skb, l3_off, false);
+	if (unlikely(l3_len < 0))
+		return l3_len;
+	l3_hdr = ip_hdr(skb);
+
+	if (unlikely(!ipxlat_v4_validate_addr(l3_hdr->daddr)))
+		return -EINVAL;
+
+	/* RFC 7915 Section 4.1 */
+	if (unlikely(ipxlat_v4_srr_check(skb, l3_hdr)))
+		return -EINVAL;
+	if (unlikely(l3_hdr->ttl <= 1))
+		return -EINVAL;
+
+	/* RFC 7915 Section 1.2:
+	 * Fragmented ICMP/ICMPv6 packets will not be translated by IP/ICMP
+	 * translators.
+	 */
+	if (unlikely(l3_hdr->protocol == IPPROTO_ICMP &&
+		     ip_is_fragment(l3_hdr)))
+		return -EINVAL;
+
+	cb->l3_hdr_len = l3_len;
+	cb->l4_proto = l3_hdr->protocol;
+	cb->l4_off = l3_off + l3_len;
+	cb->payload_off = cb->l4_off;
+	cb->is_icmp_err = false;
+
+	/* only non fragmented packets or first fragments have transport hdrs */
+	if (unlikely(!ipxlat_is_first_frag4(l3_hdr))) {
+		if (unlikely(!ipxlat_v4_validate_addr(l3_hdr->saddr)))
+			return -EINVAL;
+		return 0;
+	}
+
+	l4_len = ipxlat_v4_pull_l4(skb, cb->l4_off, l3_hdr->protocol,
+				   &cb->is_icmp_err);
+	if (unlikely(l4_len < 0))
+		return l4_len;
+
+	/* RFC 7915 Section 4.1:
+	 * Illegal IPv4 sources are accepted only for ICMPv4 error translation.
+	 */
+	if (unlikely(!ipxlat_v4_validate_addr(l3_hdr->saddr) &&
+		     !cb->is_icmp_err))
+		return -EINVAL;
+
+	cb->payload_off = cb->l4_off + l4_len;
+
+	if (unlikely(cb->is_icmp_err)) {
+		/* validate the quoted packet in an ICMP error */
+		err = ipxlat_v4_pull_icmp_inner(skb, cb->payload_off);
+		if (unlikely(err))
+			return err;
+	}
+
+	return 0;
+}
+
+static int ipxlat_v4_validate_icmp_csum(const struct sk_buff *skb)
+{
+	__sum16 csum;
+
+	/* skip when checksum is not software-owned */
+	if (skb->ip_summed != CHECKSUM_NONE)
+		return 0;
+
+	/* compute checksum over ICMP header and payload, then fold to 16-bit
+	 * Internet checksum to validate it
+	 */
+	csum = csum_fold(skb_checksum(skb, skb_transport_offset(skb),
+				      ipxlat_skb_datagram_len(skb), 0));
+	return unlikely(csum) ? -EINVAL : 0;
+}
+
+/**
+ * ipxlat_v4_validate_skb - validate IPv4 input and fill parser metadata in cb
+ * @ipxlat: translator private context
+ * @skb: packet to validate
+ *
+ * Ensures required headers are present/consistent and stores parsed offsets
+ * into &struct ipxlat_cb for the translation path.
+ *
+ * Return: 0 on success, negative errno on validation failure.
+ */
+int ipxlat_v4_validate_skb(struct ipxlat_priv *ipxlat, struct sk_buff *skb)
+{
+	struct ipxlat_cb *cb = ipxlat_skb_cb(skb);
+	struct iphdr *l3_hdr;
+	struct udphdr *udph;
+	int err;
+
+	if (unlikely(skb_shared(skb)))
+		return -EINVAL;
+
+	err = ipxlat_v4_pull_hdrs(skb);
+	if (unlikely(err))
+		return err;
+
+	skb_set_transport_header(skb, cb->l4_off);
+
+	if (unlikely(cb->is_icmp_err)) {
+		if (unlikely(cb->l4_proto != IPPROTO_ICMP)) {
+			DEBUG_NET_WARN_ON_ONCE(1);
+			return -EINVAL;
+		}
+
+		/* Translation path recomputes ICMPv6 checksum from scratch.
+		 * Validate here so a corrupted ICMPv4 error is not converted
+		 * into a translated packet with a valid checksum.
+		 */
+		return ipxlat_v4_validate_icmp_csum(skb);
+	}
+
+	l3_hdr = ip_hdr(skb);
+	if (likely(cb->l4_proto != IPPROTO_UDP))
+		return 0;
+	if (unlikely(!ipxlat_is_first_frag4(l3_hdr)))
+		return 0;
+
+	udph = udp_hdr(skb);
+	if (likely(udph->check != 0))
+		return 0;
+
+	/* We are in the path where L4 header is present (unfragmented packets
+	 * or first fragments) and is UDP.
+	 * Fragmented checksum-less IPv4 UDP is rejected because 4->6 cannot
+	 * reliably translate it.
+	 */
+	if (unlikely(ip_is_fragment(l3_hdr)))
+		return -EINVAL;
+
+	/* udph->len bounds the span used to compute replacement checksum */
+	if (unlikely(ntohs(udph->len) > skb->len - cb->l4_off))
+		return -EINVAL;
+
+	cb->udp_zero_csum_len = ntohs(udph->len);
+
+	return 0;
 }
 
 int ipxlat_v6_validate_skb(struct sk_buff *skb)
-- 
2.53.0