[lwip-devel] [bug #57380] Assertion "split <= mss" failed

[Hiromasa Ito]

URL:
  <https://savannah.nongnu.org/bugs/?57380>

                 Summary: Assertion "split <= mss" failed
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: vhertz
            Submitted on: Sat 07 Dec 2019 01:23:36 PM UTC
                Category: TCP
                Severity: 3 - Normal
              Item Group: Crash Error
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None
            lwIP version: Other

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Details:

Hi, all.

This is one of the assertion failures I found by fuzzing (to lwIP
ver2.1.0.RC1).
The following LWIP_ASSERT() at lwip/src/core/tcp_out.c:859 fails.


LWIP_ASSERT("split <= mss", split <= pcb->mss);


>From my point of view, too small MSS causes this failure.
Incoming packets can set MSS in the range of 0 < MSS < TCP_MSS.
At tcp_in.c:1943, pcb->mss is set as below:


pcb->mss = ((mss > TCP_MSS) || (mss == 0)) ? TCP_MSS : mss;


Therefore, MSS can be set too small.
As a result, it can be smaller than `split` and the assertion fails.

You can reproduce this failure with 'crashed_inputs/009' attached to the
following message of lwip-devel:
https://lists.nongnu.org/archive/html/lwip-devel/2019-12/msg00013.html




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