ssp and entropy hints

Robert Connolly robert at
Thu Sep 30 22:19:54 PDT 2004

I updated the patches for lfs-6.0, etc. Please commit these attachments.

-------------- next part --------------
AUTHOR:		Robert Connolly <robert at linuxfromscratch dot org> (ashes)

DATE:		2004-10-01

LICENSE:	Public Domain

SYNOPSIS:	Random number generation


Many system components including smashing stack protector, mkstemp,
cryptography, depend on a supply of random bits to ensure data integrity.
In the Linux kernel a combination of input devices are used to gather
randomness from. This includes the keyboard, mouse, and hard disc.
On an idle system none of these devices are receiving input, and the entropy
(randomness) of the system is easy to deplete, especially with cryptography.

Hardware random:
Some systems have hardware devices for random numbers. The kernel supports
many of them. For more information check the above web site. Also see:

Fast and Economical Random number suite:
Frandom uses an arcfour stream cipher of seed data from the kernel's internal
pool. The advantage to frandom is that 256 bytes of kernel entropy can be
expanded into gigabytes of random output. Ideal for wiping discs, and maybe
even for online gaming. A new addition to the frandom package is erandom.
Economical random uses the state of frandom as a seed, and its use does not
drain any kernel entropy. This is done very efficiently, completely in the
kernel. Erandom is ideal for Smashing Stack Protector. Frandom now also
supports sysctl so SSP can use it regardless if /dev/erandom exists or not.
This is slightly faster and works through chroot. There is an mktemp patch
for frandom too.
Note: The name "frandom" is used because thats the name of the package. The
patches for mktemp and ssp used the erandom interface even though the patch
is named frandom. Also, if you expect to have very long uptimes, the frandom
device should be dumped into /dev/null once in a while to reseed erandom.
Something like 'dd if=/dev/frandom of=/dev/null count=1' should be added to
your random script on boot, and done daily or weekly in your system scripts;
to prevent the output of erandom from ever repeating itself over several years
of use.

LavaRnd Random Number Generator:
This uses hardware as a source of entropy much like Video Entropy Daemon.

audio/video entropy daemon:
This describes two daemons which use either the static noise from the
system audio, or the video frames from a video4linux device. These devices
have a never ending supply of randomness created by thermal fluctuation and
electric fields on the devices. These entropy gathering daemons depend on the
kernel driver for your hardware to work properly, be it your sound or video
card. These programs will re-seed the kernel entropy pool. The programs can
be used together in combination with the kernel's internal values to create
a very random pool from several different sources.



Audio entropy daemon:\

make &&
install -g 0 -o 0 -m 755 audio-entropyd /usr/sbin/audio-entropyd

Edit your /etc/rc.d/init.d/random and start audio-entropyd just after seeding
urandom, and stop it just after saving random-seed. The PID file will be in
/var/run. You don't need to reboot to use it, but you do need your sound card
driver loaded, and be root.

Video entropy daemon:\

make &&
install -g 0 -o 0 -m 755 video_entropyd /usr/sbin/video_entropyd

Add this to root's crontab every minute or so. It can not run as a daemon
because it will lock the video device. Depends on video4linux. Using one or
both of these daemons should be adequate for sustained moderate-to-heavy use.

Nothing else needs to be done, applications can continue to use /dev/random
and /dev/urandom normally. You should notice crypto keys get made faster.


And for LFS-6.0\

You don't need the frandom-0.8 source, its presented so you can read more
about it if you want. The Linux kernel patch is all we need.
Frandom is built in by default with this patch. It can be found in the
character devices menu. Build and install the new kernel.

cd linux-2.4.27
patch -Np1 -i ../linux-2.4.27-frandom-1.patch

mknod /dev/frandom c 235 11
mknod /dev/erandom c 235 12

To use it for SSP use the glibc-ssp-frandom patch.

To build mktemp with frandom instead of urandom do _not_ use the "--with-libc"
option during configure. Apply the patch and otherwise install normaly.

 - Testing entropy
You should try to test this on an idle machine. Nothing compiling in
background, no updatedb running, etc. Moving/clicking the mouse, keyboard, and
even network traffic will create entropy in the pool, and affect results.
Todo: Have tests for entropy quality, not just quantity.

Fetch this:

Open two windows with non-root login. This is easiest to do in X, else split
a console window in two. In one window do this:

sh ./

In the next window do something like this:

dd if=/dev/{u,f,e}random of=/dev/null bs=1 count=1024

If one or both of the entropyd programs are running you should see the pool
being refilled. Kill the entropyd program(s) and you should see it does not
refill so quickly. Move the mouse and play with it if you like. If you use a
small count like count=512 the entropyd program(s) may not refill immediately
because the pool is still large enough. This is to improve preformance.

You might want to delete entropy_avail.log when you're done.

* Thanks to Eli Billauer for the Frandom suite. -
* Thanks to hlfs-dev at

* Initial post
* Added test.
* Added frandom/erandom.
* Added hardware random url and notes.
* Switched the entropy_avail program to a more simple shell script.
* Added patch for kernel 2.6 and for mktemp.
* Added LavaRnd.
* Added libc-headers patch.
-------------- next part --------------
AUTHOR:		Robert Connolly <robert at linuxfromscratch dot org> (ashes)

DATE:		2004-10-01

LICENSE:	Public Domain

SYNOPSIS:	Smashing Stack Protector and Libsafe


Smashing Stack Protector is a C and C++ security extension for GCC.
Libsafe prevents format string attacks.

Based on StackGaurd, SSP was developed by IBM for protecting applications
from stack smashing attacks. This is the single largest class of attacks and
many security oriented vendors have added it to their default compiler. The
overhead lost to this type of guard is minimal. In practice if the entire
system is built with SSP users shouldn't notice any difference in preformance.

The official homepage for ProPolice Smashing Stack Srotector is at:\
"Hiroaki Etoh's ProPolice is a modification to the GNU C compiler that places a
random canary between any stack allocated character buffers and the return
pointer [5]. It then validates that the canary has not been dirtied by an
overflowed buffer before the function returns. ProPolice can also reorder local
variables to protect local pointers from being overwritten in a buffer overflow.
Also see:

The frandom kernel patch is now required for SSP. This provides the erandom
device and sysctl interface. Using erandom stops a serious entropy depletion
problem while still providing urandom quality random bytes. Idealy you should
reboot an frandom kernel before installing SSP, but SSP will build without it.
If the erandom sysctl interface is missing from the system (vanilla kernel)
then /dev/urandom will be used; if /dev/urandom is missing (chroot) then
gettimeofday() will be used. Read this to install frandom:
You will need the header from the frandom patch installed to build glibc.



		Extra security patches
	GCC 3.4 notes


Smashing Stack Protector

The GCC patch will add -fstack-protector-all, -fstack-protector, and
-fno-stack-protector to GCC extensions for C and C++; and
__guard_setup and __stack_smash_handler are defined in libgcc2.c. This code is
supplied by IBM, I have changed one definition to enable libc functions, and
added "ssp" to the version string. The gcc2 patch is only needed if you plan to
use gcc2 to build the kernel, and want stack protection in the kernel.

Note: gcc-3.3 patches apply to gcc-3.3.* too. Likewise with gcc-3.4 patches.\

The Glibc patch will define __guard_setup and __stack_smash_handler in
so the kill function can be kept in a shared object. In the Glibc patch the
erandom device is used to gather a small amount of random bits for the gaurd
value. /dev/log will also need to be present in chroot for syslog to log stack
overflows. It is reccomended intrusion detection systems monitor the system
logs for these alerts.\
	glibc-2.3.4-ssp_frandom-3.patch # This works for glibc-2.3.3 too.

This GCC Specs patch adds -fstack-protector-all to GCC's default compiler flags.
Filters prevent libraries and the kernel from being built with unnessesary
smash symbols. This patch will build all main executables with stack protection.
This patch makes using stack protector almost transparent. This gcc2 patch is
not nessesary for anyone using gcc3 as their main compiler, it is provided for

The Linux kernel patch adds support to the Linux kernel for smash symbols. It
can only build with -fstack-protector, not -fstack-protector-all, and is
therefore excluded from the default specs in the sspspecs patch.\
        linux-2.4.27-ssp-1.patch # or
        linux-2.6.5-ssp-1.patch # This still works on newer 2.6 kernels.\
        linux-2.4.26-frandom-1.patch # or

There is also an mktemp patch for frandom:\

The XFree86 patch disables stack protection for some modules. \

And for LFS-6.0\

Extra security patches
This patch fixes a bug in both glibc-2.3.2 and glibc-2.3.3. This bug can be
reproduced by bind9's testsuite.\

This patch adds a sanity check to malloc. Backported from the Owl project.

Note: This patch was integrated in the latest glibc-2.3.4 (cvs) package.\

Official site:

Note: Libsafe is obsolete, you can still use it if you wish.

Libsafe was developed by Avaya Labs to protect against format string
vulnerabilities. Though not widely used it has been widely tested. This
protection can be installed on an already running system, using
to watch applications at runtime for functions which are known to be vulnerable.
This of course only protects dynamically linked applications. There should not
be a noticeable performance decrease, and it also logs to syslog.

We get some errors if we install Libsafe early in the build.
FAIL: g++.dg/expr/anew1.C execution test
FAIL: g++.dg/expr/anew2.C execution test
FAIL: g++.dg/expr/anew3.C execution test
FAIL: g++.dg/expr/anew4.C execution test

FAIL: S-records
FAIL: S-records with constructors

To avoid these errors install Libsafe after GCC in chapter 6. Libsafe is
somewhat obsolete. Most modern software either doesn't use these strings, or
uses them properly. All of the example exploits in exploits/ will fail because
of SSP.

GCC 3.4 notes
The 3.4 series of GCC has a very picky testsuite. The sspspecs patch does
not disturb previous GCC testsuites, but -fstack-protector* causes many
testsuite failures with 3.4. There is a work around. The testsuite doesn't
respect `env CFLAGS` so between `make` and `make -k test` in both chapters
the specs file will need to be edited like so:

sed -e 's/%{!D__KERNEL__:.*//' -i gcc/specs

And after `make -k check` do this:

./gcc/xgcc -dumpspecs > gcc/specs

The testsuite should give normal results.


Chapter 5
Kernel headers
(See under PREREQUISITES above)
patch -Npq -i ../linux-2.4.27-frandom-1.patch

 - GCC pass 1
If the host system has SSP in Glibc already, then you can patch gcc
here. Otherwise do not. If in doubt, wait until pass two.
 - Glibc
patch -Np1 -i ../glibc-2.3.4-ssp-frandom-4.patch # or 2.3.2's patch

 - GCC pass 2
If you use sspspecs patch then a `make bootstrap` is a good idea too.
patch -Np1 -i ../gcc-3.3-ssp-3.patch
patch -Np1 -i ../gcc-3.3-sspspecs-3.patch

 - Binutils pass 2
Just for the testsuite.
make CFLAGS="-fno-stack-protector" check

Chapter 6
Make sure the frandom header get installed again.

 - Glibc
patch -Np1 -i ../glibc-2.3.4-ssp-frandom-3.patch

 - Binutils
make CFLAGS="-fno-stack-protector" check

 - GCC
hgcc -fa
patch -Np1 -i ../gcc-3.3-ssp-3.patch
patch -Np1 -i ../gcc-3.3-sspspecs-3.patch

 - Grub
CFLAGS="-fno-stack-protector" ./configure...

 - GCC 2.95.3
patch -Np1 -i ../gcc-2.95.3-ssp-3.patch

Chapter 8
Linux kernel

make mrproper &&
patch -Np1 -i ../linux-2.4.27-ssp-1.patch
patch -Np1 -i ../linux-2.4.27-frandom-1.patch

make menuconfig

make CC="/opt/gcc-2.95.3/bin/gcc -fstack-protector" dep
make CC="/opt/gcc-2.95.3/bin/gcc -fstack-protector" bzImage

There are a couple tests in this package which may also be usefull here.
There are also tests in the libsafe source.

This will test -fstack-protector-all

cat > fail.c << "EOF"
#include <stdio.h>
#include <unistd.h>

int foo(char *blah) {
  char buffer[7];
  sprintf(buffer, "12345678901234567890123456789012345678901234567890");

int main(int argc, char **argv) {
  printf("before foo()\n");
  printf("after foo()\n");

gcc -fstack-protector-all -o fail fail.c &&

This will display the __guard value. It should change each runtime.

cat > guard-test.c << "EOF"
extern unsigned long __guard[];
int main () {
        printf("__guard\t=\t0x%08x;\n", __guard[0]);
        return 0;


* Thanks to Hiroaki Etoh for providing the SSP patch to IBM
* Thanks to IBM for providing the SSP patch at
* Thanks to OpenBSD for their XFree86 code.
* Thanks to for this
* Thanks to and for this
* Thanks to for kernel patches.
* Thanks to Avaya Labs for Libsafe
* Thanks to Teemu Tervo for nptl hint
* Thanks to cross compiling hint \
* Thanks to for proof of concept tests.
* Thanks to Eli Billauer for the Frandom suite

* Debut
* Reformat hint
* Reformatted the patches so they're much easier to apply.
* Edit/rewrite hint & synopsis.
* Added caveat.
* Fixed URLS.
* Lite edit
* New bugs found.
* GCC 2.95.3 patches made.
* XFree86-4.3.0 patch made.
* Hint is now Beta - Need more feedback.
* Edit
* Reformatted patches.
* Reformat patches.
* Update/edit hint.
* Add new example tests.
* Reformat patches.
* Add homepage/mirror url.
* Small edit.
* Added Glibc and kernel patches.
* Rewrote install procedure.
* Try to be more informative.
* Removed Gentoo property.
* Added Libsafe.
* Added Pax.
* Added new versions of binutils and glibc.
* Added GCC PIE.
* Rename filename to winter.txt.
* Do not use "Enforce non-executable pages"
* Spell check.
* Fixed URL.
* Added LOPTS to Net-tools.
* Added LDFLAGS to Perl.
* More cflags.
* New tests.
* Renamed hint back to propolice.txt.
* Added back Gentoo property as optional.
* Added HCC
* Cleanup
* Update urls
* Convert propolice to ssp
* Update gcc-3.3.3 and linux-2.6.2 ssp patches
* Update linux-2.6.3 patch and hgcc url
* Add sspspecs patch. Update.
* Added entropy.txt link for erandom.
* Fix more/again for erandom.
* Update some patches.
* New patches.
* Added guard-test.c

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