Re: [PATCH 2.6.21.3] ppp_mppe: account for osize too small errors in mppe_decompress()

Thanks for pointing this out. It seems that we would need to allocate
this extra byte somewhere in kernel ppp code.

However, I am not very familiar with that part of the code yet, and
I'll appreciate any help with finding parts of code where obuffer for
mppe_decompress is allocated.

On 6/20/07, Sergey Vlasov [off-list ref] wrote:

On Tue, 05 Jun 2007 22:23:02 +1100 Konstantin Sharlaimov wrote:

quoted

Prevent mppe_decompress() from generating "osize too small" errors when checking
for output buffer size. When receiving a packet of mru size the output buffer
for decrypted data is 1 byte too small since mppe_decompress() tries to account
for possible PFC, however later in code it is assumed no PFC.

Adjusting the check prevented there errors from occurring.

Signed-off-by: Konstantin Sharlaimov <redacted>
---

--- linux-2.6.21.3/drivers/net/ppp_mppe.c.orig        2007-06-01 20:57:04.000000000 +1100
+++ linux-2.6.21.3/drivers/net/ppp_mppe.c     2007-06-05 21:46:30.000000000 +1100

@@ -493,14 +493,14 @@ mppe_decompress(void *arg, unsigned char

      /*
       * Make sure we have enough room to decrypt the packet.
-      * Note that for our test we only subtract 1 byte whereas in
-      * mppe_compress() we added 2 bytes (+MPPE_OVHD);
-      * this is to account for possible PFC.
+      * To account for possible PFC we should only subtract 1
+      * byte whereas in mppe_compress() we added 2 bytes (+MPPE_OVHD);
+      * However, we assume no PFC, thus subtracting 2 bytes.
       */
-     if (osize < isize - MPPE_OVHD - 1) {
+     if (osize < isize - MPPE_OVHD - 2) {
              printk(KERN_DEBUG "mppe_decompress[%d]: osize too small! "
                     "(have: %d need: %d)\n", state->unit,
-                    osize, isize - MPPE_OVHD - 1);
+                    osize, isize - MPPE_OVHD - 2);
              return DECOMP_ERROR;
      }
      osize = isize - MPPE_OVHD - 2;  /* assume no PFC */

Seems that this patch is not correct - even though the comment above
says "assume no PFC", the subsequent code actually supports PFC:

        /*
         * Do PFC decompression.
         * This would be nicer if we were given the actual sk_buff
         * instead of a char *.
         */
        if ((obuf[0] & 0x01) != 0) {
                obuf[1] = obuf[0];
                obuf[0] = 0;
                obuf++;
                osize++;
        }

Therefore, depending on the packet contents, the decompressor may
really need that extra byte in the output buffer, and changing the
check may cause buffer overflows.