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. This can be done either through "dpkg-buildflags", or one of the packages provided by the "src:hardening-wrapper" source: "hardening-wrapper" or "hardening-includes".

Using "dpkg-buildflags" is the recommended way to incorporate the build flags in Debian.

"hardening-wrapper" is a set of scripts that divert the compiler, designed to help provide these options without requiring packaging changes.

"hardening-includes" is a single Makefile snippet that can be included in debian/rules from which the compile-time environment variables can be augmented to include the hardening options.

See ReleaseGoals/SecurityHardeningBuildFlags for additional information.

Choosing the appropriate method is up to you. For a step-by-step guide, see the HardeningWalkthrough.

dpkg-buildflags

To use "dpkg-buildflags", either switch to dh(1) to do builds (requires debian/compat=9), or use it directly in your builds to set the default compiler and linker flags:

CPPFLAGS:=$(shell dpkg-buildflags --get CPPFLAGS)
CFLAGS:=$(shell dpkg-buildflags --get CFLAGS)
CXXFLAGS:=$(shell dpkg-buildflags --get CXXFLAGS)
LDFLAGS:=$(shell dpkg-buildflags --get LDFLAGS)

hello.o: hello.c
    $(CC) $(CPPFLAGS) $(CFLAGS) $(LDFLAGS) -o hello.o hello.c

Or you can use the new buildflags.mk file (dpkg-dev >= 1.16.1~) to set all *FLAGS:

DPKG_EXPORT_BUILDFLAGS = 1
include /usr/share/dpkg/buildflags.mk

buildflags.mk overwrites the *FLAGS, so additions to the flags must happen after the include. Make sure to append to the *FLAGS instead of overwriting them, e.g. use CFLAGS += -Wextra instead of CFLAGS = -Wextra:

DPKG_EXPORT_BUILDFLAGS = 1
include /usr/share/dpkg/buildflags.mk

CFLAGS += -Wextra

This also works with DEB_BUILD_MAINT_OPTIONS, just declare it before the include (needs dpkg-dev >= 1.16.1.1):

export DEB_BUILD_MAINT_OPTIONS = hardening=+all
DPKG_EXPORT_BUILDFLAGS = 1
include /usr/share/dpkg/buildflags.mk

When building programs that handle untrusted data (parsers, network listeners, etc.), or run with elevated privileges (PAM, X, etc.), please enable "PIE" and "BINDNOW" in the build. The "all" option enables "PIE" and "BINDNOW" and future hardening flags:

export DEB_BUILD_MAINT_OPTIONS = hardening=+all

Enable or disable certain hardening features separately

Each hardening feature can be enabled and disabled in the DEB_BUILD_MAINT_OPTIONS environment variable's hardening value with the "+" and "-" modifier. For example, to enable the "pie" feature and disable the "fortify" feature you can do this in debian/rules:

export DEB_BUILD_MAINT_OPTIONS=hardening=+pie,-fortify

CDBS packages automatically export all dpkg-buildflags (Bug #651964 was fixed).

Due to #651966 it's not possible to use DEB_BUILD_MAINT_OPTIONS directly. To enable "PIE" and "BINDOW" use DEB_BUILD_MAINT_OPTIONS in combination with buildsystem.mk as explained above.

See the "HARDENING" section of the "dpkg-buildflags" manpage for more details.

Notes for packages using CMake

CMake silently ignores CPPFLAGS during build, this renders FORTIFY hardening isn't working as expected if you have just done like above. The question is not going to be easily answered because dpkg-buildflags set the relevant flag (-D_FORTIFY_SOURCE=2) in CPPFLAGS (bug 643632), while some CMake users rely on the behaviour of ignoring CPPFLAGS (comment from CMake bug 12928). The issue has been reported to Debian's CMake maintainers as (bug 653916), but anyway it's hard to resolve.

Debhelper (since 9.20120417, only with compat=9 and dh_auto* commands!) and cdbs (since 0.4.110) automatically append CPPFLAGS to CFLAGS and CXXFLAGS. The following workaround is not necessary anymore if the package uses debhelper (compat=9, dh_auto*) or cdbs (packages might require a new upload though).

A workaround is injecting CPPFLAGS to CFLAGS and CXXFLAGS in debian/rules.

CPPFLAGS:=$(shell dpkg-buildflags --get CPPFLAGS)
CFLAGS:=$(shell dpkg-buildflags --get CFLAGS) $(CPPFLAGS)
CXXFLAGS:=$(shell dpkg-buildflags --get CXXFLAGS) $(CPPFLAGS)
LDFLAGS:=$(shell dpkg-buildflags --get LDFLAGS)

DPKG_EXPORT_BUILDFLAGS = 1
include /usr/share/dpkg/buildflags.mk
CFLAGS+=$(CPPFLAGS)
CXXFLAGS+=$(CPPFLAGS)

Notes for packages using Vala

Use the following line, to tell the Vala compiler, that it should forward CPPFLAGS, CFLAGS and LDFLAGS to the C compiler:

VALAFLAGS:=$(foreach w,$(CPPFLAGS) $(CFLAGS) $(LDFLAGS),-X $(w))

hardening-wrapper

To use "hardening-wrapper", add it to the Build-Depends of your package, and enable it in the debian/rules file:

export DEB_BUILD_HARDENING=1

After that, any use of gcc, g++, or ld will have all hardening features enabled by default. Each feature can be disabled individually (via export DEB_BUILD_HARDENING_[feature]=0, see below), if there are complications with the build resulting from the new features. (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.

hardening-includes

To use "hardening-includes", add it to the Build-Depends of your package, include its Makefile snippet in debian/rules, and adjust the compiler flags to use it. For example, with a pure debhelper-7 style debian/rules:

include /usr/share/hardening-includes/hardening.make

CFLAGS=$(shell dpkg-buildflags --get CFLAGS)
LDFLAGS=$(shell dpkg-buildflags --get LDFLAGS)
CFLAGS+=$(HARDENING_CFLAGS)
LDFLAGS+=$(HARDENING_LDFLAGS)

%:
        dh $@

If you're building C++, use CXXFLAGS instead of CFLAGS above, etc. If building C and C++, use both.

Each hardening feature can be disabled individually (via export DEB_BUILD_HARDENING_[feature]=0, see below), if there are complications with the build resulting from the new features. (So far, only rare issues with stack protector and PIE support have been documented.)

See the Makefile snippet for more documentation on disabling features on a per-target basis, compiler option filtering, etc.

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

Validation

To verify that the resulting binary does, in fact, have hardening features enabled, you can use "hardening-check" from the "hardening-includes" package to test each ELF binary:

$ hardening-check /usr/sbin/sshd 
/usr/sbin/sshd:
 Position Independent Executable: yes
 Stack protected: yes
 Fortify Source functions: yes
 Read-only relocations: yes
 Immediate binding: yes

If your binary does not make use of character arrays on the stack, it's possible that "Stack protected" will report "no", since there was no stack it found to protect. If you absolutely want to protect all stacks, you can add "-fstack-protector-all", but this tends not to be needed, and there are some trade-offs on speed.

If your binary does not make use of FORTIFY_SOURCE-protected glibc routines, it's possible that "Fortify Source functions" will report "no", since there were no functions used that included the glibc fortification routines.

Environment variables

Both "hardening-wrapper" and "hardening-includes" react to the following set of environment variables and change the behavior of the compiler flags.

DEB_BUILD_HARDENING_FORMAT (gcc/g++ -Wformat -Wformat-security -Werror=format-security)

Quoting the gcc man page:

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)

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 -O1 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)

DEB_BUILD_HARDENING_STACKPROTECTOR (gcc/g++ -fstack-protector --param ssp-buffer-size=4)

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)

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. While ASLR can already be enforced for data areas in the stack and heap (brk and mmap), the code areas must be compiled as position-independent. Shared libraries already do this (-fPIC), so they gain ASLR automatically, but binary .text regions need to be build PIE to gain ASLR. When this happens, ROP attacks are much harder since there are no static locations to bounce off of during a memory corruption attack.

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)

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. This prevents some GOT (and .dtors) overwrite attacks, but at least the part of the GOT used by the dynamic linker (.got.plt) is still vulnerable.

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)

DEB_BUILD_HARDENING_BINDNOW (ld -z now)

During program load, all dynamic symbols are resolved, allowing for the complete GOT to be marked read-only (due to -z relro above). This prevents GOT overwrite attacks. For very large application, this can incur some performance loss during initial load while symbols are resolved, but this shouldn't be an issue for daemons.

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)

State of implementation

An email was sent to the debian-devel-announce list in 2006 introducing the hardening-wrapper package and describing the way to integrate this hardening features in Debian.

A discussion has been fired on the debian-gcc list, as well as several bugs (bug 552688, bug 489771) reported to decide the best way to enable hardening compiler options.

Packages that have a bug report asking for the inclusion of this features can be found on the bug tracker.

A list of packages including hardening-wrapper and hardening-includes in their build-deps can be retrieved with the folowing commands:

After their meeting on the 14-16 January 2011, the debian security team announced in an email they intend to push the inclusion of hardening features for the wheezy release. A Birds of a Feather-session will be organized during the 2011 debconf to setup a process.

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 -O1

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:

runtime memory allocation validation

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

Address Space Layout Randomization

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.

In Linux 3.6, and enabled by default! https://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=800179c9b8a1e796e441674776d11cd4c05d61d7

in sysctl via protected_hardlinks and proctected_symlinks

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 tpossible 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.

Documentation