[PATCH net v3 7/8] net: add ip4_parse()

["Michael S. Tsirkin" <mst@redhat.com>]

Multiple places in the net stack open-code the same minimal IPv4 header
validation: pull the fixed header, check version 4 and IHL >= 5 (where 5
is the fixed header size in 32-bit units) - and locate the transport
header at IHL * 4 past the network header.

Add a helper for doing that: it returns the validated header and
the transport header offset, or NULL if truncated or malformed.

As noted by Phil Sutter, there are more places that could use this
helper, e.g. ip_mc_check_iphdr(). They will be converted in a follow-up
patch.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Assisted-by: LLM
Reviewed-by: Björn Töpel <bjorn@kernel.org>
Reviewed-by: Phil Sutter <phil@nwl.cc>
---
 include/linux/ip.h                  | 27 +++++++++++++++++++++++++++
 include/linux/virtio_net.h          | 28 +---------------------------
 net/bridge/br_netfilter_hooks.c     | 15 +++++----------
 net/ipv4/ip_input.c                 | 13 +++++--------
 net/ipv4/netfilter/nf_reject_ipv4.c | 13 +++++--------
 5 files changed, 43 insertions(+), 53 deletions(-)

diff --git a/include/linux/ip.h b/include/linux/ip.h
index d11c25f5030a..7dc666e55a72 100644
--- a/include/linux/ip.h
+++ b/include/linux/ip.h
@@ -56,4 +56,31 @@ static inline void iph_set_totlen(struct iphdr *iph, unsigned int len)
 {
 	iph->tot_len = len <= IP_MAX_MTU ? htons(len) : 0;
 }
+
+/**
+ * ip4_parse - pull and minimally validate an IPv4 header
+ * @skb: buffer to parse
+ * @nhoff: offset of the IPv4 header within the linear data
+ * @thoff: returns the transport header offset
+ *
+ * Only the fixed header is guaranteed linear on return: callers
+ * that need the options or the transport header must pull
+ * further.
+ */
+static inline struct iphdr *ip4_parse(struct sk_buff *skb,
+				      unsigned int nhoff,
+				      unsigned int *thoff)
+{
+	struct iphdr *iph;
+
+	if (!pskb_may_pull(skb, nhoff + sizeof(struct iphdr)))
+		return NULL;
+
+	iph = (struct iphdr *)(skb->data + nhoff);
+	if (iph->version != 4 || iph->ihl < sizeof(struct iphdr) / sizeof(u32))
+		return NULL;
+
+	*thoff = nhoff + iph->ihl * sizeof(u32);
+	return iph;
+}
 #endif	/* _LINUX_IP_H */
diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index 25aa95c11e31..beaecd9cd28d 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -53,32 +53,6 @@ static inline int virtio_net_hdr_set_proto(struct sk_buff *skb,
 	return 0;
 }
 
-/* TODO: nothing virtio specific here, move to ip.h */
-
-/*
- * Pull and minimally validate the IPv4 header at @nhoff.
- * Returns the header and stores the transport header offset in
- * @thoff, or NULL if truncated or malformed.  Only the fixed header
- * is guaranteed linear; callers that need the options or the
- * transport header must pull further.
- */
-static inline struct iphdr *virtio_net_ip4_parse(struct sk_buff *skb,
-						  unsigned int nhoff,
-						  unsigned int *thoff)
-{
-	struct iphdr *iph;
-
-	if (!pskb_may_pull(skb, nhoff + sizeof(struct iphdr)))
-		return NULL;
-
-	iph = (struct iphdr *)(skb->data + nhoff);
-	if (iph->version != 4 ||
-	    iph->ihl < sizeof(struct iphdr) / sizeof(u32))
-		return NULL;
-
-	*thoff = nhoff + iph->ihl * sizeof(u32);
-	return iph;
-}
 /* Defined in net/core/dev.c, enabled while any network namespace
  * has net.core.virtio_net_hdr_csum_check set to 0.
  */
@@ -153,7 +127,7 @@ virtio_net_l4_start(struct sk_buff *skb, unsigned int nhoff, __be16 proto,
 		    unsigned int *thoff, int *ip_proto)
 {
 	if (proto == htons(ETH_P_IP)) {
-		const struct iphdr *iph = virtio_net_ip4_parse(skb, nhoff, thoff);
+		const struct iphdr *iph = ip4_parse(skb, nhoff, thoff);
 
 		if (!iph || ip_is_fragment(iph))
 			return false;
diff --git a/net/bridge/br_netfilter_hooks.c b/net/bridge/br_netfilter_hooks.c
index 0a394e5f4391..e89da0e41542 100644
--- a/net/bridge/br_netfilter_hooks.c
+++ b/net/bridge/br_netfilter_hooks.c
@@ -206,18 +206,13 @@ static inline void nf_bridge_pull_encap_header_rcsum(struct sk_buff *skb)
 static int br_validate_ipv4(struct net *net, struct sk_buff *skb)
 {
 	const struct iphdr *iph;
-	u32 len;
+	u32 len, thoff;
 
-	if (!pskb_may_pull(skb, sizeof(struct iphdr)))
+	iph = ip4_parse(skb, 0, &thoff);
+	if (!iph)
 		goto inhdr_error;
 
-	iph = ip_hdr(skb);
-
-	/* Basic sanity checks */
-	if (iph->ihl < 5 || iph->version != 4)
-		goto inhdr_error;
-
-	if (!pskb_may_pull(skb, iph->ihl*4))
+	if (!pskb_may_pull(skb, thoff))
 		goto inhdr_error;
 
 	iph = ip_hdr(skb);
@@ -228,7 +223,7 @@ static int br_validate_ipv4(struct net *net, struct sk_buff *skb)
 	if (skb->len < len) {
 		__IP_INC_STATS(net, IPSTATS_MIB_INTRUNCATEDPKTS);
 		goto drop;
-	} else if (len < (iph->ihl*4))
+	} else if (len < thoff)
 		goto inhdr_error;
 
 	if (pskb_trim_rcsum(skb, len)) {
diff --git a/net/ipv4/ip_input.c b/net/ipv4/ip_input.c
index 9860178752b8..5adf01763e85 100644
--- a/net/ipv4/ip_input.c
+++ b/net/ipv4/ip_input.c
@@ -500,7 +500,7 @@ static struct sk_buff *ip_rcv_core(struct sk_buff *skb, struct net *net)
 {
 	const struct iphdr *iph;
 	int drop_reason;
-	u32 len;
+	u32 len, thoff;
 
 	/* When the interface is in promisc. mode, drop all the crap
 	 * that it receives, do not try to analyse it.
@@ -520,10 +520,6 @@ static struct sk_buff *ip_rcv_core(struct sk_buff *skb, struct net *net)
 	}
 
 	drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
-	if (!pskb_may_pull(skb, sizeof(struct iphdr)))
-		goto inhdr_error;
-
-	iph = ip_hdr(skb);
 
 	/*
 	 *	RFC1122: 3.2.1.2 MUST silently discard any IP frame that fails the checksum.
@@ -536,7 +532,8 @@ static struct sk_buff *ip_rcv_core(struct sk_buff *skb, struct net *net)
 	 *	4.	Doesn't have a bogus length
 	 */
 
-	if (iph->ihl < 5 || iph->version != 4)
+	iph = ip4_parse(skb, 0, &thoff);
+	if (!iph)
 		goto inhdr_error;
 
 	BUILD_BUG_ON(IPSTATS_MIB_ECT1PKTS != IPSTATS_MIB_NOECTPKTS + INET_ECN_ECT_1);
@@ -546,7 +543,7 @@ static struct sk_buff *ip_rcv_core(struct sk_buff *skb, struct net *net)
 		       IPSTATS_MIB_NOECTPKTS + (iph->tos & INET_ECN_MASK),
 		       max_t(unsigned short, 1, skb_shinfo(skb)->gso_segs));
 
-	if (!pskb_may_pull(skb, iph->ihl*4))
+	if (!pskb_may_pull(skb, thoff))
 		goto inhdr_error;
 
 	iph = ip_hdr(skb);
@@ -559,7 +556,7 @@ static struct sk_buff *ip_rcv_core(struct sk_buff *skb, struct net *net)
 		drop_reason = SKB_DROP_REASON_PKT_TOO_SMALL;
 		__IP_INC_STATS(net, IPSTATS_MIB_INTRUNCATEDPKTS);
 		goto drop;
-	} else if (len < (iph->ihl*4))
+	} else if (len < thoff)
 		goto inhdr_error;
 
 	/* Our transport medium may have padded the buffer out. Now we know it
diff --git a/net/ipv4/netfilter/nf_reject_ipv4.c b/net/ipv4/netfilter/nf_reject_ipv4.c
index 59ec465a9df9..42d639854ddf 100644
--- a/net/ipv4/netfilter/nf_reject_ipv4.c
+++ b/net/ipv4/netfilter/nf_reject_ipv4.c
@@ -25,22 +25,19 @@ nf_reject_ip_tcphdr_get(struct sk_buff *oldskb,
 static int nf_reject_iphdr_validate(struct sk_buff *skb)
 {
 	struct iphdr *iph;
-	u32 len;
+	u32 len, thoff;
 
-	if (!pskb_may_pull(skb, sizeof(struct iphdr)))
-		return 0;
-
-	iph = ip_hdr(skb);
-	if (iph->ihl < 5 || iph->version != 4)
+	iph = ip4_parse(skb, 0, &thoff);
+	if (!iph)
 		return 0;
 
 	len = ntohs(iph->tot_len);
 	if (skb->len < len)
 		return 0;
-	else if (len < (iph->ihl*4))
+	else if (len < thoff)
 		return 0;
 
-	if (!pskb_may_pull(skb, iph->ihl*4))
+	if (!pskb_may_pull(skb, thoff))
 		return 0;
 
 	return 1;
-- 
MST