Annotation of wikisrc/pkgsrc/hardening.mdwn, revision 1.18

1.1       khorben     1: [[!meta title="Hardening pkgsrc"]]
                      2: 
1.9       khorben     3: A number of mechanisms are available in [pkgsrc](http://www.pkgsrc.org/) to
                      4: improve the security of the resulting system. They can be enabled individually
1.1       khorben     5: in `mk.conf`, and consist of:
                      6: 
                      7: * `PKGSRC_MKPIE`: forces the creation of PIE (Position Independent
                      8:   Executables) when supported on the current platform. This option is necessary
                      9:   to fully leverage ASLR as a mitigation for security vulnerabilities.
                     10: * `PKGSRC_USE_FORTIFY`: allows substitute wrappers to be used for commonly used
1.15      khorben    11:   functions that do not bounds checking regularly - but could in some cases (now
                     12:   enabled by default)
1.1       khorben    13: * `PKGSRC_USE_RELRO`: this also makes the exploitation of some security
                     14:   vulnerabilities more difficult in some cases.
1.16      khorben    15: * `PKGSRC_USE_SSP`: enables a stack-smashing protection mitigation (now enabled
                     16:   by default where known supported)
1.14      khorben    17: * `PKGSRC_USE_STACK_CHECK`: uses `-fstack-check` with GCC for another stack
1.16      khorben    18:   protection mitigation.
1.1       khorben    19: 
1.2       khorben    20: # Caveats
                     21: 
                     22: ## Problems with `PKGSRC_MKPIE`
                     23: 
                     24: ### No support for cwrappers
                     25: 
                     26: As of the time of this article `PKGSRC_MKPIE` is not supported by
                     27: `pkgtools/cwrappers` (`USE_CWRAPPERS` in `mk.conf`).
                     28: 
                     29: ### Packages failing to build
                     30: 
                     31: A number of packages may fail to build with this option enabled. The failures
1.18    ! khorben    32: are often related to the absence of the `-fPIC` compilation flag when building
        !            33: libraries or executables (or ideally `-fPIE` in the latter case). This flag is
1.2       khorben    34: added to the `CFLAGS` already, but requires the package to actually support it.
                     35: 
                     36: #### How to fix
                     37: 
                     38: These instructions are meant as a reference only; they likely need to be adapted
                     39: for many packages individually.
                     40: 
                     41: For packages using `Makefiles`:
                     42: 
                     43:     MAKE_FLAGS+=       CFLAGS=${CFLAGS:Q}
                     44:     MAKE_FLAGS+=       LDFLAGS=${LDFLAGS:Q}
                     45: 
                     46: For packages using `Imakefiles`:
                     47: 
                     48:     MAKE_FLAGS+=       CCOPTIONS=${CFLAGS:Q}
                     49:     MAKE_FLAGS+=       LOCAL_LDFLAGS=${LDFLAGS:Q}
                     50: 
                     51: ### Run-time crashes
                     52: 
                     53: Some programs may fail to run, or crash at random times once built as PIE. Two
                     54: scenarios are essentially possible:
                     55: 
                     56: * actual bug in the program crashing, exposed thanks to ASLR/mprotect;
                     57: * bug in the implementation of ASLR/mprotect in the Operating System.
                     58: 
1.4       khorben    59: ## Problems with `PKGSRC_USE_FORTIFY`
                     60: 
                     61: ### Packages failing to build
                     62: 
                     63: This feature makes use of pre-processing directives to look for hardened,
                     64: alternative implementations of essential library calls. Some programs may fail
                     65: to build as a result; this usually happens for those trying too hard to be
                     66: portable, or otherwise abusing definitions in the standard library.
                     67: 
                     68: This will require a modification to the program, or disabling this feature for
                     69: part or all of the build.
                     70: 
                     71: ### Run-time crashes
                     72: 
                     73: Just like with `PKGSRC_MKPIE` above, this feature may cause some programs to
                     74: crash, usually indicating an actual bug in the program. The fix will typically
                     75: involve patching the original program.
                     76: 
1.7       khorben    77: ## Problems with `PKGSRC_USE_RELRO`
                     78: 
                     79: ### Performance impact
                     80: 
                     81: For better protection, full RELRO requires every symbol to be resolved when the
1.11      khorben    82: program starts, rather than simply when required at run-time. This will have
                     83: more impact on programs using a lot of symbols, or linked to libraries exposing
                     84: a lot of symbols. Therefore, daemons or programs otherwise running in
                     85: background are affected only when started. Programs loading plug-ins at
                     86: run-time are affected when loading the plug-ins.
1.7       khorben    87: 
                     88: The impact is not expected to be noticeable on modern hardware, except in some
                     89: cases for big programs.
                     90: 
1.12      khorben    91: ### Run-time crashes
                     92: 
                     93: Some programs handle plug-ins and dependencies in a way that conflicts with
                     94: RELRO: for instance, with an initialization routine listing any other plug-in
                     95: required. With full RELRO, the missing symbols are resolved before the
                     96: initialization routine can run, and the dynamic loader will not be able to find
                     97: them directly and abort as a result. Unfortunately, this is how Xorg loads its
                     98: drivers. Partial RELRO can be applied instead in this case.
                     99: 
1.3       khorben   100: ## Problems with `PKGSRC_USE_SSP`
                    101: 
                    102: ### Packages failing to build
                    103: 
                    104: The stack-smashing protection provided by this option does not work for some
                    105: programs. The two most common situations in which this happens are:
                    106: 
                    107: * the program makes use of the `alloca(3)` library call (memory allocator on the
                    108:   stack)
                    109: * the program allocates variables on the stack, with the size determined at
                    110:   run-time.
                    111: 
                    112: Both cases will require a modification to the program, or disabling this feature
                    113: for part or all of the build.
                    114: 
                    115: ### Run-time crashes
                    116: 
1.4       khorben   117: Again, this feature may cause some programs to crash, usually indicating an
                    118: actual bug in the program. Patching the original program is then required.
1.3       khorben   119: 
1.8       khorben   120: ### Performance impact
                    121: 
                    122: The compiler emits extra code when using this feature: a check for buffer
                    123: overflows is performed when entering and exiting functions, requiring an extra
                    124: variable on the stack. The level of protection can otherwise be adjusted to
                    125: affect only those functions considered more sensitive by the compiler (with
                    126: `-fstack-protector` instead of `-fstack-protector-all`).
                    127: 
                    128: The impact is not expected to be noticeable on modern hardware. However,
                    129: programs with a hard requirement to run at the fastest possible speed should
                    130: avoid using this feature, or using libraries built with this feature.
                    131: 
1.5       khorben   132: # Auditing the system
                    133: 
                    134: The illusion of security is worse than having no security at all. This section
                    135: lists a number of ways to ensure the security features requested are actually
                    136: effective.
                    137: 
                    138: _These instructions were obtained and tested on a system derived from NetBSD 7
                    139: (amd64). YMMV._
                    140: 
                    141: ## Checking for PIE
                    142: 
                    143: The ELF executable type in use changes for binaries built as PIE; without:
                    144: 
                    145:     $ file /path/to/bin/ary
                    146:     /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
                    147: 
                    148: as opposed to the following binary, built as PIE:
                    149: 
                    150:     $ file /path/to/pie/bin/ary
                    151:     /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
                    152: 
                    153: The latter result is then what is expected.
                    154: 
1.13      khorben   155: ## Checking for partial RELRO
1.5       khorben   156: 
                    157: The following command should list a section called `RELRO`:
                    158: 
                    159:     $ objdump -p /path/to/bin/ary
                    160: 
                    161:     /path/to/bin/ary:     file format elf64-x86-64
                    162: 
                    163:     Program Header:
                    164:     [...]
                    165:        RELRO off    0x0000000000000d78 vaddr 0x0000000000600d78 paddr 0x0000000000600d78 align 2**0
1.6       khorben   166: 
1.17      khorben   167: This check is now performed automatically if `PKG_DEVELOPER` is set and `RELRO`
                    168: is enabled.
                    169: 
1.13      khorben   170: ## Checking for full RELRO
                    171: 
                    172: The dynamic loader will apply RELRO immediately when detecting the presence of
                    173: the `BIND_NOW` flag:
                    174: 
                    175:     $ objdump -x /path/to/bin/ary
                    176: 
                    177:     /path/to/bin/ary:     file format elf64-x86-64
                    178: 
                    179:     Dynamic Section:
                    180:     [...]
                    181:       BIND_NOW             0x0000000000000000
                    182: 
                    183: This has to be combined with partial RELRO (see above) to be fully efficient.
                    184: 
1.6       khorben   185: ## Checking for SSP
                    186: 
                    187: Building objects, binaries and libraries with SSP will affect the presence of
                    188: additional symbols in the resulting file:
                    189: 
                    190:     $ nm /path/to/bin/ary
                    191:     [...]
                    192:                      U __stack_chk_fail
                    193:     0000000000600ea0 B __stack_chk_guard
                    194: 
                    195: This is an indicator that the program was indeed built with support for SSP.
                    196: 
1.10      khorben   197: # References
                    198: 
                    199: * <http://tk-blog.blogspot.co.at/2009/02/relro-not-so-well-known-memory.html>
                    200: 

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