ppp_mppe: account for osize too small errors in mppe_decompress()

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 <[email protected]>

---

drivers/net/ppp_mppe.c renamed from drivers/net/ppp_mppe.c.orig +3 -3

@@ -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.

Quoted 21 lines · On Tue, 05 Jun 2007 22:23:02 +1100 Konstantin Sharlaimov wrote:

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 <[email protected]>

---

drivers/net/ppp_mppe.c 2007-06-05 21:46:30.000000000 +1100 renamed from drivers/net/ppp_mppe.c.orig 2007-06-01 20:57:04.000000000 +1100 +3 -3

@@ -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.

drivers/net/ppp_mppe.c 2007-06-05 21:46:30.000000000 +1100 mppe_decompress(void *arg, unsigned char · line 506

Quoted 10 lines

drivers/net/ppp_mppe.c 2007-06-05 21:46:30.000000000 +1100 renamed from drivers/net/ppp_mppe.c.orig 2007-06-01 20:57:04.000000000 +1100 +2 -2

       */
-     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.