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TUCoPS :: Linux :: Apps A-M :: bt315.txt

gcc (<3.2.3) implicit struct copy exploit

There is a bug in GCC, prior to version 3.2.3, which meant that
performing an implicit struct copy several times in succession would
result in data from different struct copy operations overwriting each

This problem is present in at least gcc-3.2 and gcc-3.2.2, i.e. the gcc
present in RH8.x and RH9.

This bug is potentially a security risk, because data is unintentionally
"overlapped" between subsequent struct copies.  A carefully crafted
exploit may be able to obtain sensitive information, or run arbitrary
code (in the case where a struct contains a function pointer).

Here is some code which illustrates the vulnerability:


 Compile with: gcc -Wall prog.c -o prog && ./prog

 I'm using gcc version 3.2 20020903 (Red Hat Linux 8.0 3.2-7)
 Also tested on gcc version 3.2.2 20030222 (Red Hat Linux 3.2.2-5) [RH9]
 This problem is solved in gcc version 3.2.3 [RawHide]

   Actual output:

    0 1 0
    1 0 0
    1 2 1
    2 3 4

   Expected output:

    2 2 3
    1 3 3
    1 2 4
    2 3 4


#include <stdio.h>

typedef struct {
  int _0, _1, _2;

POINT xform(POINT p) {
  return (POINT) { p._0 + 1, p._1 + 2, p._2 + 3 };

int main(void) {
  int i;
  POINT p[4] =
    { xform((POINT) { 1, 0, 0 }),
      xform((POINT) { 0, 1, 0 }),
      xform((POINT) { 0, 0, 1 }),
      xform((POINT) { 1, 1, 1 }) };

  for (i = 0; i < 4; i++)
    printf("    %d %d %d\n", p[i]._0, p[i]._1, p[i]._2);

  return 0;

I have reported this bug to RedHat:
however it is fixed in RawHide gcc (v.3.2.3), so the bug was closed.

It appears, however, from the RH bugzilla report, that there were
actually multiple struct-copy problems, one which was fixed by
gcc-3.2.2-5-rh, and one which was fixed by gcc-3.2.3.

Implicit struct copying is fortunately not used much by most C
programmers, although I have struck this problem myself.

If it is agreed that this bug poses a potential security risk, my
suggestion is that all code in gcc that deals with implicit struct
copying have statements added to send filenames/line numbers to a
special log file, and that all security-sensitive system packages be
built with this custom version of gcc, in order that a list of
potentially vulnerable source files be found.  [Unfortunately I do not
have the time or sufficient background to make these changes myself.] 
Hopefully this issue can be picked up by some interested party.


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