[PPP]: add PPP MPPE encryption module

From: Matt Domsch <Matt_Domsch@dell.com>

The patch below implements the Microsoft Point-to-Point Encryption method
as a PPP compressor/decompressor.  This is necessary for Linux clients and
servers to interoperate with Microsoft Point-to-Point Tunneling Protocol
(PPTP) servers (either Microsoft PPTP servers or the poptop project) which
use MPPE to encrypt data when creating a VPN.

This patch differs from the kernel_ppp_mppe DKMS pacakge at
pptpclient.sourceforge.net by utilizing the kernel crypto routines rather
than providing its own SHA1 and arcfour implementations.

Minor changes to ppp_generic.c try to prevent a link from disabling
compression (in our case, the encryption) after it has started using
compression (encryption).

Feedback to <pptpclient-devel@lists.sourceforge.net> please.

Signed-off-by: Matt Domsch <Matt_Domsch@dell.com>
Cc: James Cameron <james.cameron@hp.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Brice Goglin <Brice.Goglin@ens-lyon.org>
Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/Kconfig

@@ -2523,6 +2523,19 @@ config PPP_BSDCOMP
 	  module; it is called bsd_comp and will show up in the directory
 	  modules once you have said "make modules". If unsure, say N.
 
+config PPP_MPPE
+       tristate "PPP MPPE compression (encryption) (EXPERIMENTAL)"
+       depends on PPP && EXPERIMENTAL
+       select CRYPTO
+       select CRYPTO_SHA1
+       select CRYPTO_ARC4
+       ---help---
+         Support for the MPPE Encryption protocol, as employed by the
+	 Microsoft Point-to-Point Tunneling Protocol.
+
+	 See http://pptpclient.sourceforge.net/ for information on
+	 configuring PPTP clients and servers to utilize this method.
+
 config PPPOE
 	tristate "PPP over Ethernet (EXPERIMENTAL)"
 	depends on EXPERIMENTAL && PPP

drivers/net/Makefile

@@ -112,6 +112,7 @@ obj-$(CONFIG_PPP_ASYNC) += ppp_async.o
 obj-$(CONFIG_PPP_SYNC_TTY) += ppp_synctty.o
 obj-$(CONFIG_PPP_DEFLATE) += ppp_deflate.o
 obj-$(CONFIG_PPP_BSDCOMP) += bsd_comp.o
+obj-$(CONFIG_PPP_MPPE) += ppp_mppe.o
 obj-$(CONFIG_PPPOE) += pppox.o pppoe.o
 
 obj-$(CONFIG_SLIP) += slip.o

drivers/net/ppp_generic.c

@@ -137,13 +137,14 @@ struct ppp {
 
 /*
  * Bits in flags: SC_NO_TCP_CCID, SC_CCP_OPEN, SC_CCP_UP, SC_LOOP_TRAFFIC,
- * SC_MULTILINK, SC_MP_SHORTSEQ, SC_MP_XSHORTSEQ, SC_COMP_TCP, SC_REJ_COMP_TCP.
+ * SC_MULTILINK, SC_MP_SHORTSEQ, SC_MP_XSHORTSEQ, SC_COMP_TCP, SC_REJ_COMP_TCP,
+ * SC_MUST_COMP
  * Bits in rstate: SC_DECOMP_RUN, SC_DC_ERROR, SC_DC_FERROR.
  * Bits in xstate: SC_COMP_RUN
  */
 #define SC_FLAG_BITS	(SC_NO_TCP_CCID|SC_CCP_OPEN|SC_CCP_UP|SC_LOOP_TRAFFIC \
 			 |SC_MULTILINK|SC_MP_SHORTSEQ|SC_MP_XSHORTSEQ \
-			 |SC_COMP_TCP|SC_REJ_COMP_TCP)
+			 |SC_COMP_TCP|SC_REJ_COMP_TCP|SC_MUST_COMP)
 
 /*
  * Private data structure for each channel.
@@ -1027,6 +1028,56 @@ ppp_xmit_process(struct ppp *ppp)
 	ppp_xmit_unlock(ppp);
 }
 
+static inline struct sk_buff *
+pad_compress_skb(struct ppp *ppp, struct sk_buff *skb)
+{
+	struct sk_buff *new_skb;
+	int len;
+	int new_skb_size = ppp->dev->mtu +
+		ppp->xcomp->comp_extra + ppp->dev->hard_header_len;
+	int compressor_skb_size = ppp->dev->mtu +
+		ppp->xcomp->comp_extra + PPP_HDRLEN;
+	new_skb = alloc_skb(new_skb_size, GFP_ATOMIC);
+	if (!new_skb) {
+		if (net_ratelimit())
+			printk(KERN_ERR "PPP: no memory (comp pkt)\n");
+		return NULL;
+	}
+	if (ppp->dev->hard_header_len > PPP_HDRLEN)
+		skb_reserve(new_skb,
+			    ppp->dev->hard_header_len - PPP_HDRLEN);
+
+	/* compressor still expects A/C bytes in hdr */
+	len = ppp->xcomp->compress(ppp->xc_state, skb->data - 2,
+				   new_skb->data, skb->len + 2,
+				   compressor_skb_size);
+	if (len > 0 && (ppp->flags & SC_CCP_UP)) {
+		kfree_skb(skb);
+		skb = new_skb;
+		skb_put(skb, len);
+		skb_pull(skb, 2);	/* pull off A/C bytes */
+	} else if (len == 0) {
+		/* didn't compress, or CCP not up yet */
+		kfree_skb(new_skb);
+		new_skb = skb;
+	} else {
+		/*
+		 * (len < 0)
+		 * MPPE requires that we do not send unencrypted
+		 * frames.  The compressor will return -1 if we
+		 * should drop the frame.  We cannot simply test
+		 * the compress_proto because MPPE and MPPC share
+		 * the same number.
+		 */
+		if (net_ratelimit())
+			printk(KERN_ERR "ppp: compressor dropped pkt\n");
+		kfree_skb(skb);
+		kfree_skb(new_skb);
+		new_skb = NULL;
+	}
+	return new_skb;
+}
+
 /*
  * Compress and send a frame.
  * The caller should have locked the xmit path,
@@ -1113,29 +1164,14 @@ ppp_send_frame(struct ppp *ppp, struct sk_buff *skb)
 	/* try to do packet compression */
 	if ((ppp->xstate & SC_COMP_RUN) && ppp->xc_state != 0
 	    && proto != PPP_LCP && proto != PPP_CCP) {
-		new_skb = alloc_skb(ppp->dev->mtu + ppp->dev->hard_header_len,
-				    GFP_ATOMIC);
-		if (new_skb == 0) {
-			printk(KERN_ERR "PPP: no memory (comp pkt)\n");
+		if (!(ppp->flags & SC_CCP_UP) && (ppp->flags & SC_MUST_COMP)) {
+			if (net_ratelimit())
+				printk(KERN_ERR "ppp: compression required but down - pkt dropped.\n");
 			goto drop;
 		}
-		if (ppp->dev->hard_header_len > PPP_HDRLEN)
-			skb_reserve(new_skb,
-				    ppp->dev->hard_header_len - PPP_HDRLEN);
-
-		/* compressor still expects A/C bytes in hdr */
-		len = ppp->xcomp->compress(ppp->xc_state, skb->data - 2,
-					   new_skb->data, skb->len + 2,
-					   ppp->dev->mtu + PPP_HDRLEN);
-		if (len > 0 && (ppp->flags & SC_CCP_UP)) {
-			kfree_skb(skb);
-			skb = new_skb;
-			skb_put(skb, len);
-			skb_pull(skb, 2);	/* pull off A/C bytes */
-		} else {
-			/* didn't compress, or CCP not up yet */
-			kfree_skb(new_skb);
-		}
+		skb = pad_compress_skb(ppp, skb);
+		if (!skb)
+			goto drop;
 	}
 
 	/*
@@ -1155,7 +1191,8 @@ ppp_send_frame(struct ppp *ppp, struct sk_buff *skb)
 	return;
 
  drop:
-	kfree_skb(skb);
+	if (skb)
+		kfree_skb(skb);
 	++ppp->stats.tx_errors;
 }
 
@@ -1552,6 +1589,9 @@ ppp_receive_nonmp_frame(struct ppp *ppp, struct sk_buff *skb)
 	    && (ppp->rstate & (SC_DC_FERROR | SC_DC_ERROR)) == 0)
 		skb = ppp_decompress_frame(ppp, skb);
 
+	if (ppp->flags & SC_MUST_COMP && ppp->rstate & SC_DC_FERROR)
+		goto err;
+
 	proto = PPP_PROTO(skb);
 	switch (proto) {
 	case PPP_VJC_COMP: