Annotation of wikisrc/pkgsrc/hardening.mdwn, revision 1.29

1.1       khorben     1: [[!meta title="Hardening pkgsrc"]]
                      2: 
1.22      gdt         3: A number of mechanisms are available in
                      4: [pkgsrc](https://www.pkgsrc.org/) to improve the security of the
                      5: resulting system. This page describes the mechanisms, and gives hints
                      6: about detecting and fixing problems.
                      7: 
                      8: # Mechanisms
                      9: 
                     10: Mechanisms can be enabled individually in `mk.conf`, and are
1.27      khorben    11: individually described below. They are sorted by whether they are
1.26      khorben    12: enabled by default, and then by their ordering in `mk/defaults/mk.conf`.
1.22      gdt        13: 
1.24      gdt        14: Typically, a feature will cause some programs to fail to build or work
1.25      khorben    15: when first enabled. This can be due to latent problems in the
                     16: program, and can be due to other reasons. After enough testing to
1.24      gdt        17: have confidence that user problems will be quite rare, individual
                     18: mechanisms will be enabled by default.
                     19: 
                     20: For each mechanism, see the Caveats section below for an explanation
                     21: of what might go wrong at compile time and at run time, and how to
                     22: notice and address these problems.
1.23      gdt        23: 
1.22      gdt        24: ## Enabled by default in the stable branch
1.1       khorben    25: 
1.23      gdt        26: ### PKGSRC_USE_FORTIFY
                     27: 
                     28: This allows substitute wrappers to be used for some commonly used
                     29: library functions that do not have built-in bounds checking - but
                     30: could in some cases.
                     31: 
1.25      khorben    32: TODO: Explain FORTIFY_SOURCE 1 vs 2, and which is used. Give a link
                     33: to a good explanation of the technique. Explain if this is gcc specific.
1.23      gdt        34: 
                     35: It has been enabled by default since pkgsrc-2017Q3.
                     36: 
                     37: ### PKGSRC_USE_SSP
1.22      gdt        38: 
1.29    ! khorben    39: This enables a stack-smashing protection mitigation. It is done by adding a
        !            40: guard variable to functions with vulnerable objects. The guards are initialized
        !            41: when a function is entered and then checked when the function exits. The guard
        !            42: check will fail and the program forcibly exited if the variable was modified in
        !            43: the meantime. This can happen in case of buffer overflows or memory corruption,
        !            44: and therefore exposing these bugs.
        !            45: 
        !            46: This mitigation is supported by both GCC and clang. It may be supported in
        !            47: additional compilers, possibly under a different name. It is particularly useful
        !            48: for unsafe programming languages, such as C/C++.
1.23      gdt        49: 
1.29    ! khorben    50: It is enabled by default where known supported since pkgsrc-2017Q3.
1.23      gdt        51: 
1.29    ! khorben    52: * <https://en.wikipedia.org/wiki/Buffer_overflow_protection>
1.22      gdt        53: 
                     54: ## Enabled by default in pkgsrc HEAD
                     55: 
                     56: ## Not enabled by default
                     57: 
1.23      gdt        58: ### PKGSRC_MKPIE
                     59: 
                     60: This requests the the creation of PIE (Position Independent
1.25      khorben    61: Executables) for all executables. The PIE mechanism is normally used
1.23      gdt        62: for shared libraries so that they can be loaded at differing addresses
1.25      khorben    63: at runtime. PIE itself does not have useful security properties.
1.23      gdt        64: However, some operating systems support Address Space Layout
                     65: Randomization (ASLR), which causes different addresses to be used each
1.25      khorben    66: time a program is run. This makes it more difficult for an attacker
1.23      gdt        67: to guess addresses and thus makes exploits harder to construct.
                     68: 
                     69: TODO/check: PIE executables will only be built for toolchains that
1.25      khorben    70: support PIE and operating systems known to support ASLR. Currently,
1.23      gdt        71: this means NetBSD 8 and later, i386 and amd64.
                     72: 
                     73: ### PKGSRC_USE_RELRO
                     74: 
                     75: This also makes the exploitation of some security vulnerabilities more
                     76: difficult in some cases.
1.22      gdt        77: 
1.24      gdt        78: TODO: Explain gcc vs clang, and whether this has broad support or just
                     79: a few platforms.
                     80: 
                     81: TODO: Address "partial" vs "full"; which is this?
                     82: 
                     83: TODO: Give a link to a comprehensive explanation.
                     84: 
1.23      gdt        85: ### PKGSRC_USE_STACK_CHECK
1.22      gdt        86: 
1.23      gdt        87: This uses `-fstack-check` with GCC for another stack protection
                     88: mitigation.
1.1       khorben    89: 
1.2       khorben    90: # Caveats
                     91: 
                     92: ## Problems with `PKGSRC_MKPIE`
                     93: 
1.19      khorben    94: ### Recent support for cwrappers
1.2       khorben    95: 
1.19      khorben    96: `PKGSRC_MKPIE` is only supported by `pkgtools/cwrappers` from the 2017Q3
                     97: release on (`USE_CWRAPPERS` in `mk.conf`).
1.2       khorben    98: 
                     99: ### Packages failing to build
                    100: 
                    101: A number of packages may fail to build with this option enabled. The failures
1.18      khorben   102: are often related to the absence of the `-fPIC` compilation flag when building
                    103: libraries or executables (or ideally `-fPIE` in the latter case). This flag is
1.2       khorben   104: added to the `CFLAGS` already, but requires the package to actually support it.
                    105: 
                    106: #### How to fix
                    107: 
                    108: These instructions are meant as a reference only; they likely need to be adapted
                    109: for many packages individually.
                    110: 
                    111: For packages using `Makefiles`:
                    112: 
                    113:     MAKE_FLAGS+=       CFLAGS=${CFLAGS:Q}
                    114:     MAKE_FLAGS+=       LDFLAGS=${LDFLAGS:Q}
                    115: 
                    116: For packages using `Imakefiles`:
                    117: 
                    118:     MAKE_FLAGS+=       CCOPTIONS=${CFLAGS:Q}
                    119:     MAKE_FLAGS+=       LOCAL_LDFLAGS=${LDFLAGS:Q}
                    120: 
                    121: ### Run-time crashes
                    122: 
                    123: Some programs may fail to run, or crash at random times once built as PIE. Two
                    124: scenarios are essentially possible:
                    125: 
                    126: * actual bug in the program crashing, exposed thanks to ASLR/mprotect;
                    127: * bug in the implementation of ASLR/mprotect in the Operating System.
                    128: 
1.4       khorben   129: ## Problems with `PKGSRC_USE_FORTIFY`
                    130: 
                    131: ### Packages failing to build
                    132: 
                    133: This feature makes use of pre-processing directives to look for hardened,
                    134: alternative implementations of essential library calls. Some programs may fail
                    135: to build as a result; this usually happens for those trying too hard to be
                    136: portable, or otherwise abusing definitions in the standard library.
                    137: 
                    138: This will require a modification to the program, or disabling this feature for
                    139: part or all of the build.
                    140: 
                    141: ### Run-time crashes
                    142: 
                    143: Just like with `PKGSRC_MKPIE` above, this feature may cause some programs to
                    144: crash, usually indicating an actual bug in the program. The fix will typically
                    145: involve patching the original program.
                    146: 
1.28      khorben   147: ### Optimization is required
                    148: 
                    149: At least in the case of GCC, FORTIFY will only be applied if optimization is
                    150: applied while compiling. This means that the CFLAGS should also contain -O, -O2
                    151: or another optimization level. This cannot easily be applied globally, as some
                    152: packages may require specific optimization levels.
                    153: 
1.7       khorben   154: ## Problems with `PKGSRC_USE_RELRO`
                    155: 
                    156: ### Performance impact
                    157: 
                    158: For better protection, full RELRO requires every symbol to be resolved when the
1.11      khorben   159: program starts, rather than simply when required at run-time. This will have
                    160: more impact on programs using a lot of symbols, or linked to libraries exposing
                    161: a lot of symbols. Therefore, daemons or programs otherwise running in
                    162: background are affected only when started. Programs loading plug-ins at
                    163: run-time are affected when loading the plug-ins.
1.7       khorben   164: 
                    165: The impact is not expected to be noticeable on modern hardware, except in some
                    166: cases for big programs.
                    167: 
1.12      khorben   168: ### Run-time crashes
                    169: 
                    170: Some programs handle plug-ins and dependencies in a way that conflicts with
                    171: RELRO: for instance, with an initialization routine listing any other plug-in
                    172: required. With full RELRO, the missing symbols are resolved before the
                    173: initialization routine can run, and the dynamic loader will not be able to find
                    174: them directly and abort as a result. Unfortunately, this is how Xorg loads its
                    175: drivers. Partial RELRO can be applied instead in this case.
                    176: 
1.3       khorben   177: ## Problems with `PKGSRC_USE_SSP`
                    178: 
                    179: ### Packages failing to build
                    180: 
                    181: The stack-smashing protection provided by this option does not work for some
                    182: programs. The two most common situations in which this happens are:
                    183: 
                    184: * the program makes use of the `alloca(3)` library call (memory allocator on the
                    185:   stack)
                    186: * the program allocates variables on the stack, with the size determined at
                    187:   run-time.
                    188: 
                    189: Both cases will require a modification to the program, or disabling this feature
                    190: for part or all of the build.
                    191: 
                    192: ### Run-time crashes
                    193: 
1.4       khorben   194: Again, this feature may cause some programs to crash, usually indicating an
                    195: actual bug in the program. Patching the original program is then required.
1.3       khorben   196: 
1.8       khorben   197: ### Performance impact
                    198: 
                    199: The compiler emits extra code when using this feature: a check for buffer
                    200: overflows is performed when entering and exiting functions, requiring an extra
                    201: variable on the stack. The level of protection can otherwise be adjusted to
                    202: affect only those functions considered more sensitive by the compiler (with
                    203: `-fstack-protector` instead of `-fstack-protector-all`).
                    204: 
                    205: The impact is not expected to be noticeable on modern hardware. However,
                    206: programs with a hard requirement to run at the fastest possible speed should
                    207: avoid using this feature, or using libraries built with this feature.
                    208: 
1.5       khorben   209: # Auditing the system
                    210: 
                    211: The illusion of security is worse than having no security at all. This section
                    212: lists a number of ways to ensure the security features requested are actually
                    213: effective.
                    214: 
                    215: _These instructions were obtained and tested on a system derived from NetBSD 7
                    216: (amd64). YMMV._
                    217: 
                    218: ## Checking for PIE
                    219: 
                    220: The ELF executable type in use changes for binaries built as PIE; without:
                    221: 
                    222:     $ file /path/to/bin/ary
                    223:     /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
                    224: 
                    225: as opposed to the following binary, built as PIE:
                    226: 
                    227:     $ file /path/to/pie/bin/ary
                    228:     /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
                    229: 
                    230: The latter result is then what is expected.
                    231: 
1.13      khorben   232: ## Checking for partial RELRO
1.5       khorben   233: 
                    234: The following command should list a section called `RELRO`:
                    235: 
                    236:     $ objdump -p /path/to/bin/ary
                    237: 
                    238:     /path/to/bin/ary:     file format elf64-x86-64
                    239: 
                    240:     Program Header:
                    241:     [...]
                    242:        RELRO off    0x0000000000000d78 vaddr 0x0000000000600d78 paddr 0x0000000000600d78 align 2**0
1.6       khorben   243: 
1.17      khorben   244: This check is now performed automatically if `PKG_DEVELOPER` is set and `RELRO`
                    245: is enabled.
                    246: 
1.13      khorben   247: ## Checking for full RELRO
                    248: 
                    249: The dynamic loader will apply RELRO immediately when detecting the presence of
                    250: the `BIND_NOW` flag:
                    251: 
                    252:     $ objdump -x /path/to/bin/ary
                    253: 
                    254:     /path/to/bin/ary:     file format elf64-x86-64
                    255: 
                    256:     Dynamic Section:
                    257:     [...]
                    258:       BIND_NOW             0x0000000000000000
                    259: 
                    260: This has to be combined with partial RELRO (see above) to be fully efficient.
                    261: 
1.6       khorben   262: ## Checking for SSP
                    263: 
                    264: Building objects, binaries and libraries with SSP will affect the presence of
                    265: additional symbols in the resulting file:
                    266: 
                    267:     $ nm /path/to/bin/ary
                    268:     [...]
                    269:                      U __stack_chk_fail
                    270:     0000000000600ea0 B __stack_chk_guard
                    271: 
                    272: This is an indicator that the program was indeed built with support for SSP.
                    273: 
1.10      khorben   274: # References
                    275: 
                    276: * <http://tk-blog.blogspot.co.at/2009/02/relro-not-so-well-known-memory.html>
                    277: 

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