GccSsp

Differences between revisions 13 and 14
Revision 13 as of 2006-06-21 17:21:53
Size: 4328
Editor: ALagny-109-1-2-202
Comment: small correction
Revision 14 as of 2006-06-21 17:23:01
Size: 4362
Editor: ALagny-109-1-2-202
Comment: small fixes
Deletions are marked like this. Additions are marked like this.
Line 67: Line 67:
 * Add gcc wrappers to `dpkg-dev`: `/usr/lib/dpkg-dev/{cc,c++,gcc,g++,gcc-4,1,g++-4,1}` and add this directory to `$PATH` in `dpkg-buildpackage`.
 * The wrappers call `/usr/bin/gcc -fstack-protector "$@"`.
 * Add gcc wrappers to `dpkg-dev`: `/usr/lib/dpkg-dev/{cc,c++,gcc,g++,gcc-4.1,g++-4,1}` and add this directory to `$PATH` in `dpkg-buildpackage`.
 * The wrappers call `/usr/bin/gcc -fstack-protector "$@"`. (Similar for other program names)

Summary

gcc 4.1 comes with SSP now, which is a nice technology to mitigate exploitability of many buffer overflows. This greatly enhances security in the time between publication of a vulnerability and the USN.

Rationale

A considerable percentage of arbitrary code execution vulnerabilities result from buffer overflows; about half of them occur due to overflowing a stack variable. While this does not avoid the necessity of issuing USNs, it provides effective zero-day protection for a subset of vulnerabilities, and since it is now in gcc upstream, it is very cheap for us to enable and support.

Use cases

  • Pitti reads about a stack buffer overflow in Firefox. However, backporting the fix to our current stable release takes a while. Users of our stable release can live for a while with Firefox crashing when visiting a malicious web site, but they do not want to be subject to arbitrary code execution until an USN is published.

Scope

This affects all packages in all components.

Design

SSP provides a technology to stop exploitability of this class of vulnerabilities by (1) reordering stack variables and (2) adding a 'canary' (a random number) to the bottom of the stack, which is checked when the function is returned, so that attempts to overwrite the return pointer are detected and stopped.

SSP itself has been around for some years and thus has matured quite a bit, and many distros are using it by default already. Therefore this should not be a major risk, since we are merely catching up with existing technology.

Implementation

RedHat and gentoo are using SSP by default for years, so we do not expect much breakage potential. So we will build packages with SSP enabled right from the start in Edgy, so that we have much time to discover breakage. If it is not trivial to fix a package for SSP, we can disable it by building it with fno-stack-protector.

We will not rebuild the archive in one shot. So in the unlikely event that SSP will cause many regressions, we can immediately revert enabling it and just have to rebuild a relatively low number of packages.

testing

The following dapper packages have been tested with SSP enabled (built with gcc-4.1 and -fstack-protector under edgy):

perl

ok

python

ok

apache2

ok

php5

ok

postgresql-8.1

ok

firefox

ok

mysql-dfsg-5.0

ok

glib2.0

ok

gtk+2.0

ok

glibc

ok

xorg

kernel

does not link since SSP uses external symbols provided by libssp.so

Before enabling SSP by default we will ensure that all those packages work flawlessly with SSP.

gcc changes

  • Merge back libssp-dev into gcc, so that we do not have to add it as build dependency to packages.

dpkg-dev changes

Changing gcc-4.1 to enable -fstack-protector by default has shown to be a suboptimal solution:

  • It breaks upstream kernel builds and potentially other direct usages of gcc.
  • It is not conformant to upstream behaviour.

Short-term solution for Edgy:

  • Add gcc wrappers to dpkg-dev: /usr/lib/dpkg-dev/{cc,c++,gcc,g++,gcc-4.1,g++-4,1} and add this directory to $PATH in dpkg-buildpackage.

  • The wrappers call /usr/bin/gcc -fstack-protector "$@". (Similar for other program names)

  • This will still break debian/rules build reproducability. We need to decide whether we officially declare that this use case is not supported (which is certainly fine for the edgy experiment), or it's temporarily broken until the long-term solution below is implemented.

Generic long-term solution:

  • Change all source packages to provide a general initialization hook (like using debian-source-make instead of make, or @includeing a common file). This should be done in accordance with Debian. MartinPitt will start a discussion with Debian.

  • This will allow us to do other global changes without changing all source packages again.


CategorySpec

GccSsp (last edited 2008-08-06 16:19:12 by localhost)