[Bug c/30587] New: GCC4 reduces local variable padding, making off-by-one vulnerabilities once again critical

[defend dot the dot world at gmail dot com]

I've posted this to the mailing list and figured i might as well post a bug
report to accompany it. 

It's not as much of a bug as it a missing feature that gcc3 had.

Basically gcc v3 placed padding between the end of local variables and the
saved frame pointer or other control information and gcc4 no longer does that,
making an off-by-one much more dangerous again (as in gcc v2).

 Since a lot of security minded things are now included with default gcc and
some are default compile options, this is probably an unintended effect of code
rewrites.

Here's a link to my post that describes it in full:
http://gcc.gnu.org/ml/gcc/2007-01/msg00993.html

Again: 
in main(){ char buf[512]; }
buf[512] is the LSB of the saved frame pointer or other control information
(like %ecx in the new main function). On a gcc3 buf[512] would have just been
extra space, making an extra NULL byte not as security-critical. 

The "-mpreferred-stack-boundary" option should be used when padding is not
desired i think. I'm really unqualified though.

Good luck, and I appreciate the strides towards security.


-- 
           Summary: GCC4 reduces local variable padding, making off-by-one
                    vulnerabilities once again critical
           Product: gcc
           Version: 4.0.1
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: defend dot the dot world at gmail dot com
 GCC build triplet: gcc version 4.0.3 (Ubuntu 4.0.3-1ubuntu5)
  GCC host triplet: linux 32-bit x86
GCC target triplet: linux 32-bit x86


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30587