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[Qemu-devel] [PATCH] vnc: segmentation fault caused by incorrect 'bytes'

From: Ulrich Obergfell
Subject: [Qemu-devel] [PATCH] vnc: segmentation fault caused by incorrect 'bytes' count calculated in tight_compress_data()
Date: Mon, 21 Mar 2011 04:48:10 -0400 (EDT)

tight_compress_data() calculates an incorrect 'bytes' count
if 'zstream->total_out' is greater than 0x7fffffff, because
the type of the variable 'previous_out' is 'int'.

    852     int previous_out;
    872     previous_out = zstream->total_out;
    881     bytes = zstream->total_out - previous_out;
    883     tight_send_compact_size(vs, bytes);
    884     vnc_write(vs, vs->tight.zlib.buffer, bytes);

The incorrect 'bytes' count causes segmentation faults in
functions called by tight_compress_data(). For example:

(gdb) bt
#0  0x000000396ab3d2b6 in __memcpy_ssse3_back () from /lib64/libc.so.6
#1  0x00000000004b3b91 in buffer_append (buffer=0x322d660, 
    data=<value optimized out>, len=<value optimized out>)
    at /usr/include/bits/string3.h:52
#2  0x00000000004bbf02 in tight_compress_data (vs=0x32215b0, 
    stream_id=<value optimized out>, bytes=0x100024881, 
    level=<value optimized out>, strategy=<value optimized out>)
    at ui/vnc-enc-tight.c:884
(gdb) x/i $rip
=> 0x396ab3d2b6 <__memcpy_ssse3_back+6710>:     movdqu -0x10(%rsi),%xmm0
(gdb) print $rsi+0x10
$1 = 0x103f2ab21
(gdb) x/xb 0x103f2ab21
0x103f2ab21:    Cannot access memory at address 0x103f2ab21

The following program illustrates the problem.

    $ cat t.c
    #include <stdio.h>

        int previous_out;
        unsigned long total_out = 0x80000000;
        size_t bytes;

        previous_out = total_out;
        total_out += 0x10000;
        bytes = total_out - previous_out;
        printf("%lx\n", bytes);
    $ cc t.c -o t
    $ ./t

The patch changes the type of 'previous_out' to 'uLong' which
is the same as the type of 'zstream->total_out'.

Signed-off-by: Ulrich Obergfell <address@hidden>

diff -up ./ui/vnc-enc-tight.c.orig0 ./ui/vnc-enc-tight.c
--- ./ui/vnc-enc-tight.c.orig0  2011-03-15 03:53:22.000000000 +0100
+++ ./ui/vnc-enc-tight.c        2011-03-20 12:14:48.013560009 +0100
@@ -849,7 +849,7 @@ static int tight_compress_data(VncState 
                                int level, int strategy)
     z_streamp zstream = &vs->tight.stream[stream_id];
-    int previous_out;
+    uLong previous_out;
     if (bytes < VNC_TIGHT_MIN_TO_COMPRESS) {
         vnc_write(vs, vs->tight.tight.buffer, vs->tight.tight.offset);

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