Access Violation in SNASERVR!SNPUGETC if Invalid Formatted GDS Variable Received (218178)

SYMPTOMS

The SNA Server service (Snaservr.exe) may fail unexpectedly with an access violation in function snpugetc. This failure may occur when receiving an invalid formatted General Data Stream (GDS) variable from a mainframe. If Drwtsn32.exe is configured as the default debugger on the SNA Server system, the Drwtsn32.log file may contain an entry similar to the following when this access violation occurs:

Application error:
        Application: exe\snaservr.dbg (pid=259)
        11/26/1998 @ 8:56:47.484
        Error: c0000005 (Access violation)
Function: snpugetc
        0100a5ca 8d4e06           lea     ecx,[esi+0x6]
ds:0120ea06=????????
        0100a5cd 2bdd             sub     ebx,ebp
        0100a5cf 0fbfef           movsx   ebp,di
        0100a5d2 3beb             cmp     ebp,ebx
        0100a5d4 7e27             jle     snpugetc+0x4d (0100a5fd)
        0100a5d6 85f6             test    esi,esi
        0100a5d8 7427             jz      snpugetc+0x51 (0100a601)
        0100a5da 668b00           mov     ax,[eax]
ds:019dff01=????
        0100a5dd 8b36             mov     esi,[esi]
ds:00000000=????????
        0100a5df 662b01           sub     ax,[ecx]
ds:019d61f6=010c
Fault ->0100a5e2 0fbf5e04         movsx   ebx,word ptr [esi+0x4]
ds:0120ea07=????
        0100a5e6 0fbf6e06         movsx   ebp,word ptr [esi+0x6]
ds:0120ea07=????
        0100a5ea 8d7c07ff         lea     edi,[edi+eax-0x1]
ds:02bee908=????????

*----> Stack Back Trace <----*

FramePtr ReturnAd Param#1  Param#2  Param#3  Param#4  Function Name
0118fd2c 0105f212 019d257c a3160e06 002d042c 002d0360 snaservr!snpugetc
(FPO: [EBP 0xa3160e06] [2,0,4])
0118fd94 0105c297 002d0360 0000125d 0118fdf8 01144bdd snaservr!s1pxntur
(FPO: [EBP 0x002d042c] [3,19,4])

In addition, an event similar to the following will be logged in the Windows NT Application Event Log:

Event ID: 624
Source: SNA Server
Description: Creating dump file C:\SNA\traces\snadump.log for
SNASERVR.EXE

CAUSE

The access violation is caused when SNA Server receives a badly formatted GDS variable. In this case, the format of the Sign-On GDS variable 0x'1221' sent by the host was invalid.

This problem started to occur with SNA Server 3.0 with the introduction of SNA Server single sign on support, where SNA Server now explicitly parses the user and password data within the Sign-On GDS message.

RESOLUTION

SNA Server 4.0

To resolve this problem, obtain the latest service pack for SNA Server version 4.0. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:

215838 How to Obtain the Latest SNA Server Version 4.0 Service Pack

SNA Server 3.0

To resolve this problem, obtain the latest service pack for SNA Server version 3.0. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:

184307 How to Obtain the Latest SNA Server Version 3.0 Service Pack

MORE INFORMATION

With Client message trace enabled, you may see output similar to the following:

 PVI   -----------------------------------------------
 PVI   9A000001->010AF5B1 LU 6.2
 PVI                      MSGID:SWAT   MSGTYP:NTEOD  Sense1:0000
 PVI                      Sense2:0700
 PVI
 PVI   ---- Header  at address 014D4B00, 14 elements ----
 PVI   0B010000 07000000 000E0000 01000301     <................>
 PVI
 PVI   ---- Element at address 019D257C, start 13, end 268 ----
 PVI   2A0502FF 0003D000 000406F3 F0F10019
 PVI   10C4C5E5 C2C7F0F3 F14BC1D7 D7C3C3D3
 PVI   F207CE0B 1A0DF500 01002033 1221202F
 PVI   FF002201 40404040 40404040 E5F0F0F1
 PVI   F0F0F0F0 F0F0F0F0 D3D6C7C9 D5404040
 PVI   40404040 40404040 40404040 40404040
 PVI   40404040 40404040 D9C5C3C5 C9D9C5C3
 PVI   C5C9F0F0 F0F0F0F0 F0F0F0F0 40404040
 PVI   40404040 40404040 40404040 40404040
 PVI   40404040 40404040 40404040 40404040
 PVI   40404040 40404040 40404040 40404040
 PVI   40404040 40404040 40404040 40404040
 PVI   40404040 40404040 40404040 40404040
 PVI   F0F0F0F0 F0F0F0F5 40404040 40404040
 PVI   40404040 40404040 E4E2D9C9 C4404040
 PVI   40404040 40404040 C3F0F0F0 F8F0D5D7

Decoding the data shows the following:

2033 1221 - Sign-on GDS variable of length 2033 (which is much too large).

202F FF00 - Sign-On Request Data of length 202F.

2201 - User ID subfield of length 22.

According to SNA Formats & Protocols, the user ID must be 1-10 bytes, so the only valid lengths are 3-12.

40404040 40404040 E5F0F0F1 F0F0F0F0 F0F0F0F0 D3D6C7C9 D5404040 40404040 - User ID

40404040 40404040 40404040 - This is supposed to be an 00,01 or 02 subfield, which it clearly isn't.

Therefore, the data within the above frame clearly shows incorrect SNA protocol.