[RFC] [PATCH 2.6.21.5] ppp: fix osize too small errors when decoding mppe

[Konstantin Sharlaimov <konstantin.sharlaimov@gmail.com>]

The mppe_decompress() function required a buffer that is 1 byte too small when
receiving a message of mru size. This fixes buffer allocation to prevent this
from occurring.

Signed-off-by: Konstantin Sharlaimov <konstantin.sharlaimov@gmail.com>
---
As Sergey Vlasov pointed out, ppp_mppe-account-for-osize-too-small-errors-in.patch
may cause buffer overflows on certain data. Here is another patch that should
eliminate the "osize too small" errors. Instead of patching mppe_decompress
itself, I have patched ppp_decompress_frame so it would allocate the required
extra byte when using mppe compression.

I didn't have a chance to check this patch carefully yet, but it seem to be
working as expected. Any comments would be greatly appreciated.

--- linux-2.6.21.3/drivers/net/ppp_generic.c.orig	2007-06-20 09:14:13.000000000
+1100
+++ linux-2.6.21.3/drivers/net/ppp_generic.c	2007-06-20 09:18:06.000000000 +1100
@@ -1711,7 +1711,18 @@ ppp_decompress_frame(struct ppp *ppp, st
 		goto err;

 	if (proto == PPP_COMP) {
-		ns = dev_alloc_skb(ppp->mru + PPP_HDRLEN);
+		int obuff_size;
+
+		switch(ppp->rcomp->compress_proto) {
+		case CI_MPPE:
+			obuff_size = ppp->mru + PPP_HDRLEN + 1;
+			break;
+		default:
+			obuff_size = ppp->mru + PPP_HDRLEN;
+			break;
+		}
+
+		ns = dev_alloc_skb(obuff_size);
 		if (ns == 0) {
 			printk(KERN_ERR "ppp_decompress_frame: no memory\n");
 			goto err;