lib: improve fletcher checksum validation

OVERVIEW

The checksum used in OSPF (rfc2328) is specified in rc905 annex B.  There is an
sample implementation in rfc1008 which forms the basis of the quagga
implementation.  This algorithm works perfectly when generating a checksum;
however, validation is a bit problematic.

The following LSA (generated by a non-quagga implementation) is received by
quagga and marked with an invalid checksum; however, it passes both the rfc905
and rfc1008 validation checks.

static uint8_t lsa_10_121_233_29[] = {
   0x0e, 0x10, 0x02, 0x03,
   0x09, 0x00, 0x35, 0x40,
   0x0a, 0x79, 0xe9, 0x1d,
   0x80, 0x00, 0x00, 0x03,
   0x00, 0x8a, 0x00, 0x1c,
   0xff, 0xff, 0xff, 0xe0,
   0x00, 0x00, 0x36, 0xb0
};

LS Type: Summary-LSA (IP network)
   LS Age: 3600 seconds
   Do Not Age: False
   Options: 0x02 (E)
   Link-State Advertisement Type: Summary-LSA (IP network) (3)
   Link State ID: 9.0.53.64
   Advertising Router: 10.121.233.29 (10.121.233.29)
   LS Sequence Number: 0x80000003
   LS Checksum: 0x008a
   Length: 28
   Netmask: 255.255.255.224
   Metric: 14000

You'll note that one byte of the checksum is 0x00; quagga would calculate the
checksum as 0xff8a.

It can be argued that the sourcing implementation generates an incorrect
checksum; however, rfc905 indicates that, for 1's complement arithmetic, the
value 255 shall be regarded as 0, thus either values are valid.

EXPLANATION

The quagga ospfd and ospf6d implementations operate by copying the PDU's
existing checksum in a holding variable, calculating the checksum, and comparing
the resulting checksum to the original.  As a note, this implementation has the
side effect of modifying the contents of the PDU.

Evaluation of both rfc905 and rfc1008 shows that checksum validation should
involve calculating the sum over the PDU and checking that both resulting C0 and
C1 values are zero.  This behavior is enacted in the rfc1008 implementation by
calling encodecc with k = 0 (checksum offset); however, this functionality had
been omitted from the quagga implementation.

PATCH

This patch adds the ability to call the quagga's fletcher_checksum() with a
checksum offset value of 0xffff (aka FLETCHER_CHECKSUM_VALIDATE) which returns
the sum over the buffer (a value of 0 indicates a valid checksum).  This is
similar to the mechanism in rfc1008 when called with k = 0.  The patch also
introduces ospf_lsa_checksum_valid().

ospf6d had it's own implementation of the fletcher checksum in
ospf6_lsa_checksum(); it's the same algorithm as in fletcher_checksum().  This
patch removes the local implementation in favor of the library's as well as creates
and uses ospf6_lsa_checksum_valid().

quagga's ISIS implementation suffers from the same problem; however, I do not
have the facilities to validate a fix to ISIS, thus this change has been left to
the ISIS maintainers.  The function iso_csum_verify() should be reduced to
running the fletcher checksum over the buffer using an offset of 0.

Signed-off-by: JR Rivers <jrrivers@cumulusnetworks.com>
Reviewed-by: Scott Feldman <sfeldma@cumulusnetworks.com>
Reviewed-by: Nolan Leake <nolan@cumulusnetworks.com>
Reviewed-by: Ayan Banerjee <ayan@cumulusnetworks.com>
Reviewed-by: Shrijeet Mukherjee <shm@cumulusnetworks.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>

2 files changed, 29 insertions, 18 deletions

diff --git a/lib/checksum.c b/lib/checksum.c
index 3ddde815..43940b71 100644
--- a/lib/checksum.c
+++ b/lib/checksum.c
@@ -51,6 +51,8 @@ in_cksum(void *parg, int nbytes)
 
 /* To be consistent, offset is 0-based index, rather than the 1-based 
    index required in the specification ISO 8473, Annex C.1 */
+/* calling with offset == FLETCHER_CHECKSUM_VALIDATE will validate the checksum
+   without modifying the buffer; a valid checksum returns 0 */
 u_int16_t
 fletcher_checksum(u_char * buffer, const size_t len, const uint16_t offset)
 {
@@ -62,13 +64,14 @@ fletcher_checksum(u_char * buffer, const size_t len, const uint16_t offset)
   
   checksum = 0;
 
-  assert (offset < len);
 
-  /*
-   * Zero the csum in the packet.
-   */
-  csum = (u_int16_t *) (buffer + offset);
-  *(csum) = 0;
+  if (offset != FLETCHER_CHECKSUM_VALIDATE)
+    /* Zero the csum in the packet. */
+    {
+      assert (offset < (len - 1)); /* account for two bytes of checksum */
+      csum = (u_int16_t *) (buffer + offset);
+      *(csum) = 0;
+    }
 
   p = buffer;
   c0 = 0;
@@ -89,7 +92,7 @@ fletcher_checksum(u_char * buffer, const size_t len, const uint16_t offset)
 
       left -= partial_len;
     }
-  
+
   /* The cast is important, to ensure the mod is taken as a signed value. */
   x = (int)((len - offset - 1) * c0 - c1) % 255;
 
@@ -98,17 +101,24 @@ fletcher_checksum(u_char * buffer, const size_t len, const uint16_t offset)
   y = 510 - c0 - x;
   if (y > 255)  
     y -= 255;
-  
-  /*
-   * Now we write this to the packet.
-   * We could skip this step too, since the checksum returned would
-   * be stored into the checksum field by the caller.
-   */
-  buffer[offset] = x;
-  buffer[offset + 1] = y;
-
-  /* Take care of the endian issue */
-  checksum = htons((x << 8) | (y & 0xFF));
+
+  if (offset == FLETCHER_CHECKSUM_VALIDATE)
+    {
+      checksum = (c1 << 8) + c0;
+    }
+  else
+    {
+      /*
+       * Now we write this to the packet.
+       * We could skip this step too, since the checksum returned would
+       * be stored into the checksum field by the caller.
+       */
+      buffer[offset] = x;
+      buffer[offset + 1] = y;
+
+      /* Take care of the endian issue */
+      checksum = htons((x << 8) | (y & 0xFF));
+    }
 
   return checksum;
 }
diff --git a/lib/checksum.h b/lib/checksum.h
index da1d3cba..b310f744 100644
--- a/lib/checksum.h
+++ b/lib/checksum.h
@@ -1,2 +1,3 @@
 extern int in_cksum(void *, int);
+#define FLETCHER_CHECKSUM_VALIDATE 0xffff
 extern u_int16_t fletcher_checksum(u_char *, const size_t len, const uint16_t offset);