ipsec: allow to align IPv4 AH on 32 bits

The Linux IPv4 AH stack aligns the AH header on a 64 bit boundary
(like in IPv6). This is not RFC compliant (see RFC4302, Section
3.3.3.2.1), it should be aligned on 32 bits.

For most of the authentication algorithms, the ICV size is 96 bits.
The AH header alignment on 32 or 64 bits gives the same results.

However for SHA-256-128 for instance, the wrong 64 bit alignment results
in adding useless padding in IPv4 AH, which is forbidden by the RFC.

To avoid breaking backward compatibility, we use a new flag
(XFRM_STATE_ALIGN4) do change original behavior.

Initial patch from Dang Hongwu <hongwu.dang@6wind.com> and
Christophe Gouault <christophe.gouault@6wind.com>.
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/xfrm.h

@@ -350,6 +350,7 @@ struct xfrm_usersa_info {
 #define XFRM_STATE_WILDRECV	8
 #define XFRM_STATE_ICMP		16
 #define XFRM_STATE_AF_UNSPEC	32
+#define XFRM_STATE_ALIGN4	64
 };
 
 struct xfrm_usersa_id {

include/net/xfrm.h

@@ -36,6 +36,7 @@
 #define XFRM_PROTO_ROUTING	IPPROTO_ROUTING
 #define XFRM_PROTO_DSTOPTS	IPPROTO_DSTOPTS
 
+#define XFRM_ALIGN4(len)	(((len) + 3) & ~3)
 #define XFRM_ALIGN8(len)	(((len) + 7) & ~7)
 #define MODULE_ALIAS_XFRM_MODE(family, encap) \
 	MODULE_ALIAS("xfrm-mode-" __stringify(family) "-" __stringify(encap))

net/ipv4/ah4.c

@@ -201,7 +201,10 @@ static int ah_output(struct xfrm_state *x, struct sk_buff *skb)
 	top_iph->ttl = 0;
 	top_iph->check = 0;
 
-	ah->hdrlen  = (XFRM_ALIGN8(sizeof(*ah) + ahp->icv_trunc_len) >> 2) - 2;
+	if (x->props.flags & XFRM_STATE_ALIGN4)
+		ah->hdrlen  = (XFRM_ALIGN4(sizeof(*ah) + ahp->icv_trunc_len) >> 2) - 2;
+	else
+		ah->hdrlen  = (XFRM_ALIGN8(sizeof(*ah) + ahp->icv_trunc_len) >> 2) - 2;
 
 	ah->reserved = 0;
 	ah->spi = x->id.spi;
@@ -299,9 +302,15 @@ static int ah_input(struct xfrm_state *x, struct sk_buff *skb)
 	nexthdr = ah->nexthdr;
 	ah_hlen = (ah->hdrlen + 2) << 2;
 
-	if (ah_hlen != XFRM_ALIGN8(sizeof(*ah) + ahp->icv_full_len) &&
-	    ah_hlen != XFRM_ALIGN8(sizeof(*ah) + ahp->icv_trunc_len))
-		goto out;
+	if (x->props.flags & XFRM_STATE_ALIGN4) {
+		if (ah_hlen != XFRM_ALIGN4(sizeof(*ah) + ahp->icv_full_len) &&
+		    ah_hlen != XFRM_ALIGN4(sizeof(*ah) + ahp->icv_trunc_len))
+			goto out;
+	} else {
+		if (ah_hlen != XFRM_ALIGN8(sizeof(*ah) + ahp->icv_full_len) &&
+		    ah_hlen != XFRM_ALIGN8(sizeof(*ah) + ahp->icv_trunc_len))
+			goto out;
+	}
 
 	if (!pskb_may_pull(skb, ah_hlen))
 		goto out;
@@ -450,8 +459,12 @@ static int ah_init_state(struct xfrm_state *x)
 
 	BUG_ON(ahp->icv_trunc_len > MAX_AH_AUTH_LEN);
 
-	x->props.header_len = XFRM_ALIGN8(sizeof(struct ip_auth_hdr) +
-					  ahp->icv_trunc_len);
+	if (x->props.flags & XFRM_STATE_ALIGN4)
+		x->props.header_len = XFRM_ALIGN4(sizeof(struct ip_auth_hdr) +
+						  ahp->icv_trunc_len);
+	else
+		x->props.header_len = XFRM_ALIGN8(sizeof(struct ip_auth_hdr) +
+						  ahp->icv_trunc_len);
 	if (x->props.mode == XFRM_MODE_TUNNEL)
 		x->props.header_len += sizeof(struct iphdr);
 	x->data = ahp;