[[!meta title="Hardening pkgsrc"]] A number of mechanisms are available in [pkgsrc](http://www.pkgsrc.org/) to improve the security of the resulting system. They can be enabled individually in `mk.conf`, and consist of: * `PKGSRC_MKPIE`: forces the creation of PIE (Position Independent Executables) when supported on the current platform. This option is necessary to fully leverage ASLR as a mitigation for security vulnerabilities. * `PKGSRC_USE_FORTIFY`: allows substitute wrappers to be used for commonly used functions that do not bounds checking regularly - but could in some cases. * `PKGSRC_USE_RELRO`: this also makes the exploitation of some security vulnerabilities more difficult in some cases. * `PKGSRC_USE_SSP`: enables stack-smashing protection (again, on supported platforms) # Caveats ## Problems with `PKGSRC_MKPIE` ### No support for cwrappers As of the time of this article `PKGSRC_MKPIE` is not supported by `pkgtools/cwrappers` (`USE_CWRAPPERS` in `mk.conf`). ### Packages failing to build A number of packages may fail to build with this option enabled. The failures are often related to the absence of the "-fPIC" compilation flag when building libraries or executables (or ideally "-fPIE" in the latter case). This flag is added to the `CFLAGS` already, but requires the package to actually support it. #### How to fix These instructions are meant as a reference only; they likely need to be adapted for many packages individually. For packages using `Makefiles`: MAKE_FLAGS+= CFLAGS=${CFLAGS:Q} MAKE_FLAGS+= LDFLAGS=${LDFLAGS:Q} For packages using `Imakefiles`: MAKE_FLAGS+= CCOPTIONS=${CFLAGS:Q} MAKE_FLAGS+= LOCAL_LDFLAGS=${LDFLAGS:Q} ### Run-time crashes Some programs may fail to run, or crash at random times once built as PIE. Two scenarios are essentially possible: * actual bug in the program crashing, exposed thanks to ASLR/mprotect; * bug in the implementation of ASLR/mprotect in the Operating System. ## Problems with `PKGSRC_USE_FORTIFY` ### Packages failing to build This feature makes use of pre-processing directives to look for hardened, alternative implementations of essential library calls. Some programs may fail to build as a result; this usually happens for those trying too hard to be portable, or otherwise abusing definitions in the standard library. This will require a modification to the program, or disabling this feature for part or all of the build. ### Run-time crashes Just like with `PKGSRC_MKPIE` above, this feature may cause some programs to crash, usually indicating an actual bug in the program. The fix will typically involve patching the original program. ## Problems with `PKGSRC_USE_RELRO` ### Performance impact For better protection, full RELRO requires every symbol to be resolved when the program starts, rather than simply when required at run-time. This will have more impact on programs using a lot of symbols, or linked to libraries exposing a lot of symbols. Therefore, daemons or programs otherwise running in background are affected only when started. Programs loading plug-ins at run-time are affected when loading the plug-ins. The impact is not expected to be noticeable on modern hardware, except in some cases for big programs. ### Run-time crashes Some programs handle plug-ins and dependencies in a way that conflicts with RELRO: for instance, with an initialization routine listing any other plug-in required. With full RELRO, the missing symbols are resolved before the initialization routine can run, and the dynamic loader will not be able to find them directly and abort as a result. Unfortunately, this is how Xorg loads its drivers. Partial RELRO can be applied instead in this case. ## Problems with `PKGSRC_USE_SSP` ### Packages failing to build The stack-smashing protection provided by this option does not work for some programs. The two most common situations in which this happens are: * the program makes use of the `alloca(3)` library call (memory allocator on the stack) * the program allocates variables on the stack, with the size determined at run-time. Both cases will require a modification to the program, or disabling this feature for part or all of the build. ### Run-time crashes Again, this feature may cause some programs to crash, usually indicating an actual bug in the program. Patching the original program is then required. ### Performance impact The compiler emits extra code when using this feature: a check for buffer overflows is performed when entering and exiting functions, requiring an extra variable on the stack. The level of protection can otherwise be adjusted to affect only those functions considered more sensitive by the compiler (with `-fstack-protector` instead of `-fstack-protector-all`). The impact is not expected to be noticeable on modern hardware. However, programs with a hard requirement to run at the fastest possible speed should avoid using this feature, or using libraries built with this feature. # Auditing the system The illusion of security is worse than having no security at all. This section lists a number of ways to ensure the security features requested are actually effective. _These instructions were obtained and tested on a system derived from NetBSD 7 (amd64). YMMV._ ## Checking for PIE The ELF executable type in use changes for binaries built as PIE; without: $ file /path/to/bin/ary /path/to/bin/ary: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for NetBSD 7.0, not stripped as opposed to the following binary, built as PIE: $ file /path/to/pie/bin/ary /path/to/pie/bin/ary: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for NetBSD 7.0, not stripped The latter result is then what is expected. ## Checking for RELRO The following command should list a section called `RELRO`: $ objdump -p /path/to/bin/ary /path/to/bin/ary: file format elf64-x86-64 Program Header: [...] RELRO off 0x0000000000000d78 vaddr 0x0000000000600d78 paddr 0x0000000000600d78 align 2**0 ## Checking for SSP Building objects, binaries and libraries with SSP will affect the presence of additional symbols in the resulting file: $ nm /path/to/bin/ary [...] U __stack_chk_fail 0000000000600ea0 B __stack_chk_guard This is an indicator that the program was indeed built with support for SSP. # References *