xfrm: Workaround incompatibility of ESN and async crypto

ESN for esp is defined in RFC 4303. This RFC assumes that the
sequence number counters are always up to date. However,
this is not true if an async crypto algorithm is employed.

If the sequence number counters are not up to date on sequence
number check, we may incorrectly update the upper 32 bit of
the sequence number. This leads to a DOS.

We workaround this by comparing the upper sequence number,
(used for authentication) with the upper sequence number
computed after the async processing. We drop the packet
if these numbers are different.

To do this, we introduce a recheck function that does this
check in the ESN case.
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/xfrm.h

@@ -273,6 +273,9 @@ struct xfrm_replay {
 	int	(*check)(struct xfrm_state *x,
 			 struct sk_buff *skb,
 			 __be32 net_seq);
+	int	(*recheck)(struct xfrm_state *x,
+			   struct sk_buff *skb,
+			   __be32 net_seq);
 	void	(*notify)(struct xfrm_state *x, int event);
 	int	(*overflow)(struct xfrm_state *x, struct sk_buff *skb);
 };

net/xfrm/xfrm_input.c

@@ -212,7 +212,7 @@ int xfrm_input(struct sk_buff *skb, int nexthdr, __be32 spi, int encap_type)
 		/* only the first xfrm gets the encap type */
 		encap_type = 0;
 
-		if (async && x->repl->check(x, skb, seq)) {
+		if (async && x->repl->recheck(x, skb, seq)) {
 			XFRM_INC_STATS(net, LINUX_MIB_XFRMINSTATESEQERROR);
 			goto drop_unlock;
 		}

net/xfrm/xfrm_replay.c

@@ -420,6 +420,18 @@ static int xfrm_replay_check_esn(struct xfrm_state *x,
 	return -EINVAL;
 }
 
+static int xfrm_replay_recheck_esn(struct xfrm_state *x,
+				   struct sk_buff *skb, __be32 net_seq)
+{
+	if (unlikely(XFRM_SKB_CB(skb)->seq.input.hi !=
+		     htonl(xfrm_replay_seqhi(x, net_seq)))) {
+			x->stats.replay_window++;
+			return -EINVAL;
+	}
+
+	return xfrm_replay_check_esn(x, skb, net_seq);
+}
+
 static void xfrm_replay_advance_esn(struct xfrm_state *x, __be32 net_seq)
 {
 	unsigned int bitnr, nr, i;
@@ -479,6 +491,7 @@ static void xfrm_replay_advance_esn(struct xfrm_state *x, __be32 net_seq)
 static struct xfrm_replay xfrm_replay_legacy = {
 	.advance	= xfrm_replay_advance,
 	.check		= xfrm_replay_check,
+	.recheck	= xfrm_replay_check,
 	.notify		= xfrm_replay_notify,
 	.overflow	= xfrm_replay_overflow,
 };
@@ -486,6 +499,7 @@ static struct xfrm_replay xfrm_replay_legacy = {
 static struct xfrm_replay xfrm_replay_bmp = {
 	.advance	= xfrm_replay_advance_bmp,
 	.check		= xfrm_replay_check_bmp,
+	.recheck	= xfrm_replay_check_bmp,
 	.notify		= xfrm_replay_notify_bmp,
 	.overflow	= xfrm_replay_overflow_bmp,
 };
@@ -493,6 +507,7 @@ static struct xfrm_replay xfrm_replay_bmp = {
 static struct xfrm_replay xfrm_replay_esn = {
 	.advance	= xfrm_replay_advance_esn,
 	.check		= xfrm_replay_check_esn,
+	.recheck	= xfrm_replay_recheck_esn,
 	.notify		= xfrm_replay_notify_bmp,
 	.overflow	= xfrm_replay_overflow_esn,
 };