[lwip-devel] [bug #51528] Bug when parsing WND_SCALE option

[Pascal Quantin]

URL:
  <http://savannah.nongnu.org/bugs/?51528>

                 Summary: Bug when parsing WND_SCALE option
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: pquantin
            Submitted on: Fri 21 Jul 2017 03:32:43 PM UTC
                Category: TCP
                Severity: 3 - Normal
              Item Group: Faulty Behaviour
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None
            lwIP version: 2.0.2

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Details:

With the current source code, when parsing the WND_SCALE, the window scaling
byte is only read if the current packet is a SYN one and not a
retransmission:

        /* If syn was received with wnd scale option,
           activate wnd scale opt, but only if this is not a retransmission
*/
        if ((flags & TCP_SYN) && !(pcb->flags & TF_WND_SCALE)) {
          /* An WND_SCALE option with the right option length. */
          data = tcp_getoptbyte();

But the data byte should always be read, even in case of TCP retransmission.
Otherwise tcp_optidx is not incremented and it breaks the rest of the options
parsing.

The proposed patch below solves my issue:

diff --git a/src/core/tcp_in.c b/src/core/tcp_in.c
index ba879284..93a07678 100644
--- a/src/core/tcp_in.c
+++ b/src/core/tcp_in.c
@@ -1749,11 +1749,11 @@ tcp_parseopt(struct tcp_pcb *pcb)
           LWIP_DEBUGF(TCP_INPUT_DEBUG, ("tcp_parseopt: bad length\n"));
           return;
         }
+        data = tcp_getoptbyte();
         /* If syn was received with wnd scale option,
            activate wnd scale opt, but only if this is not a retransmission
*/
         if ((flags & TCP_SYN) && !(pcb->flags & TF_WND_SCALE)) {
           /* An WND_SCALE option with the right option length. */
-          data = tcp_getoptbyte();
           pcb->snd_scale = data;
           if (pcb->snd_scale > 14U) {
             pcb->snd_scale = 14U;





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