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Date: Tue, 15 May 2012 08:13:31 -0400
From: Steve Grubb <sgrubb@...hat.com>
To: oss-security@...ts.openwall.com
Cc: Solar Designer <solar@...nwall.com>,
Keegan McAllister <mcallister.keegan@...il.com>
Subject: Re: Automatic binary hardening with Autoconf
On Monday, May 14, 2012 09:33:14 PM Solar Designer wrote:
> I'd like this sort of topics to be brought up in here, so I'll start by
> referring to some blog posts.
>
> Here's an interesting one by Keegan McAllister:
>
> http://mainisusuallyafunction.blogspot.com/2012/05/automatic-binary-hardeni
> ng-with.html
>
> This suggests (and shows how) individual programs that use autoconf may
> automatically enable the usual set of compile-time hardening settings
> that are otherwise normally provided by builds for/by/on hardened
> distros only. This is not rocket science, yet the provided examples may
> be reused and it may become a trend.
I think there are conflicting goals in projects like this. There are times when
someone may want to go all out and harden everything as much as possible. But
there is a cost to that...either startup or runtime. Not all programs have the
same threat model and consequence if attacked successfully. Apps that are at
greatest risk are: set[ug]id/fs based capabilities, network facing apps,
daemons, or parsers of untrusted media. It would be hard to argue that the "cat"
program needs full relro and bind now.
> Also interesting are the performance impact numbers (up to 30%), which
> are far worse than those I've seen posted before (up to 5.8%):
>
> http://d-sbd.alioth.debian.org/www/?page=pax_pie
This is because the stack canary is a random number. Imagine adding a "function
call" between every intended function call to grab a random number. Because of
this, we want sensible security. You have to live with the performance hit or
have some heuristic that you use to decide which apps get the special treatment.
Also, I have been studying the stack-protector, FORTIFY_SOURCE, ASLR (which is
how I found the ASLR bypass for fs capability programs recently), and RELRO. It
turns out there are many holes in the protection we all expect is working. Not
mentioned in the blog is the fact that with compiler optimizations being on,
stack-protector and FORTIFY_SOURCE do not work. There are many other loopholes,
like an array of structs do not get any stack-protector treatment. Nor do arrays
of ints. And the stack-protector is only checked on function exit. So, you may
have a function that gets overflowed and passes corrupted variables around and it
may do something irreversable like call execve or create a file based on those.
Consider these test cases which show how we can have stack overflows created that
aren't detected at the time they occur:
#include <stdio.h>
void *memset(void *dest, int c, size_t n);
void test2(char *buf1, char *buf2, char *buf3, char *buf4)
{
printf("buf1:%s, buf2:%s, buf3:%s, buf4:%s\n", buf1, buf2, buf3, buf4);
}
void test1(void)
{
char buf1[5], buf2[5], buf3[5], buf4[5];
sprintf(buf1, " ");
sprintf(buf2, " ");
sprintf(buf3, " ");
sprintf(buf4, " ");
memset(buf4, 'a', 80);
buf4[80] = 0;
test2(buf1, buf2, buf3, buf4);
puts("Finished test2");
}
int main(void)
{
test1();
puts("Finished test1");
return 0;
}
and
#include <stdio.h>
#include <execinfo.h>
void *memset(void *dest, int c, size_t n);
void test2(char *buf1, char *buf2, char *buf3, char *buf4)
{
void *array[10];
void *ptr = __builtin_frame_address(0);
printf("buf1: %p\n", buf1);
printf("buf2: %p\n", buf2);
printf("buf3: %p\n", buf3);
printf("buf4: %p\n", buf4);
printf("t2 frame: %p\n", ptr);
memset(buf1, 'a', 0x01a);
buf1[0x1a] = 0;
printf("buf1:%s, buf2:%s, buf3:%s, buf4:%s\n", buf1, buf2, buf3, buf4);
}
void test1(void)
{
char buf1[5], buf2[5], buf3[5], buf4[5];
void *ptr = __builtin_frame_address(0);
printf("t1 frame: %p\n", ptr);
sprintf(buf1, " ");
sprintf(buf2, " ");
sprintf(buf3, " ");
sprintf(buf4, " ");
test2(buf1, buf2, buf3, buf4);
puts("Finished test2");
}
int main(void)
{
test1();
puts("Finished test1");
return 0;
}
I'm slowly putting together a presentation about all the holes in our security
so that they can be explained and perhaps fixed. There are a lot. These ^^^ are
just 2 examples.
What I have been thinking about is that if we have a protected frame (a canary
is already existing) that it be checked prior to calling any function that uses
a variable from the protected frame if its been written to.
I've also concluded that FORTIFY_SOURCE is limited in its use. This is because
its factored out if it cannot determine the sizes at compile time. At runtime
the sizes may be easily determined, but its already not there.
> Perhaps this has to do with the specific code being protected and
> benchmarked (some crypto code in Mosh?) http://mosh.mit.edu
>
> An edit to this comment:
>
> https://github.com/keithw/mosh/issues/79#issuecomment-4683789
>
> says that the impact is less with Ubuntu 12.04's GCC 4.6.3 - but I think
> this may be because Ubuntu's GCC has some of the hardening enabled by
> default (so its baseline performance is worse, not the impact is less).
We found that compiler flags alone were not enough. For example, if you want to
set partial RELRO to be the default, there are a large number of programs which
do not take the LD_FLAGS environmental variable. The common fix appears to be to
patch binutils to add partial RELRO on all linking.
I created a shell script that can be used to "grade" an installed system based
on my own policy. http://people.redhat.com/sgrubb/files/rpm-chksec
This will take an rpm name as input and verify each ELF file to see if its
compiled with the intended flags to most effectively use PIE and RELRO. Green is
good, Orange could use work but is acceptable, and Red needs fixing. It has a
mode --all that is the equivalent of using rpm -qa and feeding the packages to
it. In this mode it will only give a summary result for the package. To find
which files don't comply, re-run using just the package name.
I want to raise the bar over a couple releases and will be updating the program
soon to grade F18 to a higher standard.
-Steve
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