Using Hardening Options

Several compile-time options (detailed below) can be used to help harden a resulting binary against memory corruption attacks, or provide additional warning messages during compiles. The "hardening-wrapper" set of scripts is designed to help provide these options without requiring packaging changes. The goal is to be able to use the wrapper on the buildds and local developer's systems to provide hardened builds.

After installing the "hardening-wrapper" package, it must be enabled:

 $ export DEB_BUILD_HARDENING=1

After that, any use of gcc, g++, or ld (e.g. through debuild or dpkg-buildpackage) will have all hardening options enabled by default. Each option can be disabled individually (via export DEB_BUILD_HARDENING_[feature]=0), if there are complications with the build resulting from the new options. (So far, only rare issues with stack protector and PIE support have been documented.)

To illustrate the effects of the options, a trivial C source (with Makefile) is used as an example.

DEB_BUILD_HARDENING_FORMAT (gcc/g++ -Wformat -Wformat-security)

Quoting the gcc man page:

• If -Wformat is specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, as in printf (foo);. This may be a security hole if the format string came from untrusted input and contains %n.

http://en.wikipedia.org/wiki/Format_string_attack

Default compile:

$ make trivial
cc -Wall -O2    trivial.c   -o trivial

Hardened compile:

$ DEB_BUILD_HARDENING=1 make trivial
cc -Wall -O2    trivial.c   -o trivial
trivial.c: In function 'main':
trivial.c:16: warning: format not a string literal and no format arguments

Known problems: (Common build failures, non-availability on some archs)

• None so far.

DEB_BUILD_HARDENING_FORTIFY (gcc/g++ -D_FORTIFY_SOURCE=2)

During code generation the compiler knows a great deal of information about buffer sizes (where possible), and attempts to replace insecure unlimited length buffer function calls with length-limited ones. This is especially useful for old, crufty code. Additionally, format strings in writable memory that contain '%n' are blocked. If an application depends on such a format string, it will need to be worked around.

Note that for this feature to be fully enabled, the source must also be compiled with -O2 or higher.

Default build:

$ make trivial
cc -Wall -O2    trivial.c   -o trivial
$ ./trivial $(perl -e 'print "A"x100')
Your first argument was: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
Segmentation fault (core dumped)

Hardened build:

$ DEB_BUILD_HARDENING=1 make trivial
cc -Wall -O2    trivial.c   -o trivial
trivial.c: In function 'main':
trivial.c:16: warning: format not a string literal and no format arguments
$ ./trivial $(perl -e 'print "A"x100')
*** buffer overflow detected ***: ./trivial terminated

Known problems: (Common build failures, non-availability on some archs)

• Code compiled with -Werror and using memcpy/strcpy with qualifier overrides will fail with FORTIFY enabled. See https://launchpad.net/bugs/217481

DEB_BUILD_HARDENING_STACKPROTECTOR (gcc/g++ -fstack-protector)

This is a mainline GCC feature, which adds safety checks against stack overwrites. This renders many potential code injection attacks into aborting situations. In the best case this turns code injection vulnerabilities into denial of service or into non-issues (depending on the application). http://en.wikipedia.org/wiki/Stack-smashing_protection

Default build:

$ make trivial
cc -Wall -O2    trivial.c   -o trivial
$ ./trivial $(perl -e 'print "A"x100')
Your first argument was: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
Segmentation fault (core dumped)

Hardened build (with FORTIFY disabled, since it catches the stack overflow before it happens):

$ DEB_BUILD_HARDENING=1 DEB_BUILD_HARDENING_FORTIFY=0 make trivial
cc -Wall -O2    trivial.c   -o trivial
trivial.c: In function 'main':
trivial.c:16: warning: format not a string literal and no format arguments
$ ./trivial $(perl -e 'print "A"x100')
Your first argument was: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
*** stack smashing detected ***: ./trivial terminated

Known problems: (Common build failures, non-availability on some archs)

• Not supported on ia64 and alpha. (disabled for these archs by default in hardening-wrapper 1.8)

  warning: -fstack-protector not supported for this target

• Not supported on mips and hppa. (disabled for these archs by default in hardening-wrapper 1.10)

  warning: -fstack-protector not supported for this target

• Broken on arm and armel (compiler supports the flag, but fails to produce sane executables -- they always segfault). Filed as bug 469517 (disabled for these archs by default in hardening-wrapper 1.9)

DEB_BUILD_HARDENING_PIE (gcc/g++ -fPIE -pie)

Position Independent Executable are needed to take advantage of Address Space Layout Randomization, supported by some kernel versions. http://en.wikipedia.org/wiki/ASLR

Default build:

$ make trivial
cc -Wall -O2    trivial.c   -o trivial
$ file trivial
trivial: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), for GNU/Linux 2.6.8, dynamically linked (uses shared libs), not stripped

Hardened build:

$ DEB_BUILD_HARDENING=1 make trivial
cc -Wall -O2    trivial.c   -o trivial
trivial.c: In function 'main':
trivial.c:16: warning: format not a string literal and no format arguments
$ file trivial
trivial: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), for GNU/Linux 2.6.8, not stripped

Known problems: (Common build failures, non-availability on some archs)

• Doesn't work on hppa and m68k (disabled for these archs by default in hardening-wrapper 1.6)
• Doesn't work on some assembler, due to lack of registers. e.g.:

  vf_decimate.c:26: error: can't find a register in class 'BREG' while reloading 'asm'

DEB_BUILD_HARDENING_RELRO (ld -z relro)

During program load, several ELF memory sections need to be written to by the linker, but can be turned read-only before turning over control to the program. Most notably this prevents GOT overwrite attacks.

Default build:

$ make trivial
cc -Wall -O2    trivial.c   -o trivial
$ objdump -x trivial | grep RELRO

Hardened build:

$ DEB_BUILD_HARDENING=1 make trivial
cc -Wall -O2    trivial.c   -o trivial
trivial.c: In function 'main':
trivial.c:16: warning: format not a string literal and no format arguments
$ objdump -x trivial | grep RELRO
   RELRO off    0x0000000000000de8 vaddr 0x0000000000200de8 paddr 0x0000000000200de8 align 2**0

Known problems: (Common build failures, non-availability on some archs)

• Unimplemented on ia64 (ld silently ignores the option).

DEB_BUILD_HARDENING_BINDNOW (ld -z now)

During program load, all dynamic symbols are resolved, allowing for the PLT to be marked read-only (due to -z relro above). For very large application, this can incur some performance loss during initial load while symbols are resolved.

Default build:

$ make trivial
cc -Wall -O2 trivial.c -o trivial
$ readelf -d trivial | grep BIND

Hardened build:

$ DEB_BUILD_HARDENING=1 make trivial
cc -Wall -O2 trivial.c -o trivial
trivial.c: In function 'main':
trivial.c:16: warning: format not a string literal and no format arguments
$ readelf -d trivial | grep BIND
 0x0000000000000018 (BIND_NOW)           

Known problems: (Common build failures, non-availability on some archs)

• None.

Notes on Memory Corruption Mitigation Methods

User Space

Stack Protector

gcc's -fstack-protector attempts to detect when a stack has been overwritten and aborts the program. Ubuntu has had this enabled by default since Edgy. Some programs do not play nice with it, and can be worked around with -fno-stack-protector. It would be nice to enable this by default, and for gcc to only attempt to use it when libc is being linked against.

Already done in sendmail.

heap protection

In glibc2.5, no additional work needed.

libc pointer encryption

In mainline glibc, as PTR_MANGLE.

gcc -D_FORTIFY_SOURCE=2 -O2

Compile-time protection against static sized buffer overflows. No known regressions or performance loss. This should be enabled system-wide

gcc -Wformat -Wformat-security

While not all programs correctly implement the printf hints (like glib's G_GNUC_PRINTF macro), adding this will at least call out simple printf format string vulnerabilities. Any programs whose builds become "noisy" as a result, should be fixed anyway.

gcc -pie -fPIE

This is especially difficult to plumb into packaging in a safe way, since it requires the executable be built with -fPIE for any .o files that are linked at the end with -pie. There is some amount of performance loss, but only due to the -fPIE, which is already true for all the linked libraries (via their -fPIC).

Already done with openssh, sendmail.

ld -z relro

(Or via gcc with -Wl,-z,relro) Already done with sendmail.

ld -z now

(Or via gcc with -Wl,-z,now).

Kernel Space

non-exec memory segmentation (ExecShield)

Stops execution of code in heap/stack. i386 specific (nx already does this for amd64), and introduces some small level of performance loss (5% for CPU-bound). Some people have worked on getting it pushed into the mainline kernel. Current state unknown -- would be very handy to have due to the popularity of i386. Marcus Better may be willing to continue to maintain the patchset for Debian.

Some applications appear to break when run in the protected memory layout. Most of these issues should be fixed due to RH (and SUSE?) already running with these protections.

Additional work for user-space is identifying programs that build assembly but fail to explicitly mark their stack as non-exec (gnupg, for example).

-fstack-protector

Is available for amd64 builds:

• config CC_STACKPROTECTOR

runtime memory allocation validation

Detect double-frees in kernel space. No idea where it stands.

Address Space Layout Randomization

• mmap: in mainline
• stack: in mainline
• vdso: in since 2.6.18 (COMPAT_VDSO disables it)
• heap/exec: in -mm, 2.6.24
• brk: 2.6.25

Having heap/exec ASLR is a prerequisite for -pie being useful. Presently, openssh is compiled with -pie.

/proc/$pid/maps protection

Present in 2.6.22; requires sysctl toggle (kernel.maps_protect = 1). Became non-optional in 2.6.27

/dev/mem protection

Included in 2.6.25.

link protections

From the GRSecurity patchset, protections against hardlink/symlink creation/following in world-writable areas. (Solves tmp races.) May potentially break things like postfix that manipulation hardlinks? Breaks POSIX. Getting taken in mainline may be possible with a build-time or proc toggle.

http://lkml.org/lkml/2005/3/10/101 http://lkml.org/lkml/2005/4/18/167

chroot, dmesg, fifo protections

Also from GRSecurity patchset.

Build Changes

Integrated build variables

Need to be able to enable/disable compile-time protections via debian/rules elements:

• -fstack-protector
• -pie (-fPIE)
• -D_FORTIFY_SOURCE=2 -O2
• -Wformat -Wformat-security
• -Wl,-z,relro
• -Wl,-z,now

Current implementation attempt at the user-space compiled hardening is http://svn.debian.org/wsvn/hardening which provides a gcc/g++/ld wrapper.

Documentation

• https://wiki.ubuntu.com/CompilerFlags

• http://people.redhat.com/drepper/nonselsec.pdf

• http://www.suse.de/~krahmer/no-nx.pdf

• http://www.neworder.box.sk/newsread.php?newsid=13007

• http://www.hackinthebox.org/modules.php?op=modload&name=News&file=article&sid=15604&mode=thread&order=0&thold=0

• http://www.phrack.org/archives/58/p58-0x04

• http://insecure.org/sploits/non-executable.stack.problems.html

• http://www.phrack.org/archives/59/p59-0x09.txt

• http://www.coresecurity.com/files/attachments/Richarte_Stackguard_2002.pdf

• http://www.redhat.com/archives/fedora-tools-list/2004-September/msg00002.html

• http://www.gentoo.org/proj/en/hardened/hardened-toolchain.xml

• http://labs.mwrinfosecurity.com/notices/security_mechanisms_in_linux_environment__part_1___userspace_memory_protection/

• http://labs.mwrinfosecurity.com/notices/assessing_the_tux_strength_part_2_into_the_kernel/