Annotation of wikisrc/pkgsrc/hardening.mdwn, revision 1.31

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: 
1.30      khorben    46: Different mitigation levels are available:
                     47: * the default ("yes"), which will only protect functions considered vulnerable
                     48:   by the compiler;
                     49: * "all", which will protect every function;
                     50: * "strong", which will apply a better balance between the two settings above.
                     51: 
1.29      khorben    52: This mitigation is supported by both GCC and clang. It may be supported in
                     53: additional compilers, possibly under a different name. It is particularly useful
                     54: for unsafe programming languages, such as C/C++.
1.23      gdt        55: 
1.29      khorben    56: It is enabled by default where known supported since pkgsrc-2017Q3.
1.23      gdt        57: 
1.29      khorben    58: * <https://en.wikipedia.org/wiki/Buffer_overflow_protection>
1.22      gdt        59: 
                     60: ## Enabled by default in pkgsrc HEAD
                     61: 
                     62: ## Not enabled by default
                     63: 
1.23      gdt        64: ### PKGSRC_MKPIE
                     65: 
                     66: This requests the the creation of PIE (Position Independent
1.25      khorben    67: Executables) for all executables. The PIE mechanism is normally used
1.23      gdt        68: for shared libraries so that they can be loaded at differing addresses
1.25      khorben    69: at runtime. PIE itself does not have useful security properties.
1.23      gdt        70: However, some operating systems support Address Space Layout
                     71: Randomization (ASLR), which causes different addresses to be used each
1.25      khorben    72: time a program is run. This makes it more difficult for an attacker
1.23      gdt        73: to guess addresses and thus makes exploits harder to construct.
                     74: 
1.31    ! khorben    75: PIE executables will only be built for toolchains that are known to support PIE.
        !            76: Currently, this means NetBSD on amd64 and i386.
1.23      gdt        77: 
                     78: ### PKGSRC_USE_RELRO
                     79: 
                     80: This also makes the exploitation of some security vulnerabilities more
                     81: difficult in some cases.
1.22      gdt        82: 
1.24      gdt        83: TODO: Explain gcc vs clang, and whether this has broad support or just
                     84: a few platforms.
                     85: 
                     86: TODO: Address "partial" vs "full"; which is this?
                     87: 
                     88: TODO: Give a link to a comprehensive explanation.
                     89: 
1.23      gdt        90: ### PKGSRC_USE_STACK_CHECK
1.22      gdt        91: 
1.23      gdt        92: This uses `-fstack-check` with GCC for another stack protection
                     93: mitigation.
1.1       khorben    94: 
1.2       khorben    95: # Caveats
                     96: 
                     97: ## Problems with `PKGSRC_MKPIE`
                     98: 
1.19      khorben    99: ### Recent support for cwrappers
1.2       khorben   100: 
1.19      khorben   101: `PKGSRC_MKPIE` is only supported by `pkgtools/cwrappers` from the 2017Q3
                    102: release on (`USE_CWRAPPERS` in `mk.conf`).
1.2       khorben   103: 
                    104: ### Packages failing to build
                    105: 
                    106: A number of packages may fail to build with this option enabled. The failures
1.18      khorben   107: are often related to the absence of the `-fPIC` compilation flag when building
                    108: libraries or executables (or ideally `-fPIE` in the latter case). This flag is
1.2       khorben   109: added to the `CFLAGS` already, but requires the package to actually support it.
                    110: 
                    111: #### How to fix
                    112: 
                    113: These instructions are meant as a reference only; they likely need to be adapted
                    114: for many packages individually.
                    115: 
                    116: For packages using `Makefiles`:
                    117: 
                    118:     MAKE_FLAGS+=       CFLAGS=${CFLAGS:Q}
                    119:     MAKE_FLAGS+=       LDFLAGS=${LDFLAGS:Q}
                    120: 
                    121: For packages using `Imakefiles`:
                    122: 
                    123:     MAKE_FLAGS+=       CCOPTIONS=${CFLAGS:Q}
                    124:     MAKE_FLAGS+=       LOCAL_LDFLAGS=${LDFLAGS:Q}
                    125: 
                    126: ### Run-time crashes
                    127: 
                    128: Some programs may fail to run, or crash at random times once built as PIE. Two
                    129: scenarios are essentially possible:
                    130: 
                    131: * actual bug in the program crashing, exposed thanks to ASLR/mprotect;
                    132: * bug in the implementation of ASLR/mprotect in the Operating System.
                    133: 
1.4       khorben   134: ## Problems with `PKGSRC_USE_FORTIFY`
                    135: 
                    136: ### Packages failing to build
                    137: 
                    138: This feature makes use of pre-processing directives to look for hardened,
                    139: alternative implementations of essential library calls. Some programs may fail
                    140: to build as a result; this usually happens for those trying too hard to be
                    141: portable, or otherwise abusing definitions in the standard library.
                    142: 
                    143: This will require a modification to the program, or disabling this feature for
                    144: part or all of the build.
                    145: 
                    146: ### Run-time crashes
                    147: 
                    148: Just like with `PKGSRC_MKPIE` above, this feature may cause some programs to
                    149: crash, usually indicating an actual bug in the program. The fix will typically
                    150: involve patching the original program.
                    151: 
1.28      khorben   152: ### Optimization is required
                    153: 
                    154: At least in the case of GCC, FORTIFY will only be applied if optimization is
                    155: applied while compiling. This means that the CFLAGS should also contain -O, -O2
                    156: or another optimization level. This cannot easily be applied globally, as some
                    157: packages may require specific optimization levels.
                    158: 
1.7       khorben   159: ## Problems with `PKGSRC_USE_RELRO`
                    160: 
                    161: ### Performance impact
                    162: 
                    163: For better protection, full RELRO requires every symbol to be resolved when the
1.11      khorben   164: program starts, rather than simply when required at run-time. This will have
                    165: more impact on programs using a lot of symbols, or linked to libraries exposing
                    166: a lot of symbols. Therefore, daemons or programs otherwise running in
                    167: background are affected only when started. Programs loading plug-ins at
                    168: run-time are affected when loading the plug-ins.
1.7       khorben   169: 
                    170: The impact is not expected to be noticeable on modern hardware, except in some
                    171: cases for big programs.
                    172: 
1.12      khorben   173: ### Run-time crashes
                    174: 
                    175: Some programs handle plug-ins and dependencies in a way that conflicts with
                    176: RELRO: for instance, with an initialization routine listing any other plug-in
                    177: required. With full RELRO, the missing symbols are resolved before the
                    178: initialization routine can run, and the dynamic loader will not be able to find
                    179: them directly and abort as a result. Unfortunately, this is how Xorg loads its
                    180: drivers. Partial RELRO can be applied instead in this case.
                    181: 
1.3       khorben   182: ## Problems with `PKGSRC_USE_SSP`
                    183: 
                    184: ### Packages failing to build
                    185: 
                    186: The stack-smashing protection provided by this option does not work for some
                    187: programs. The two most common situations in which this happens are:
                    188: 
                    189: * the program makes use of the `alloca(3)` library call (memory allocator on the
                    190:   stack)
                    191: * the program allocates variables on the stack, with the size determined at
                    192:   run-time.
                    193: 
                    194: Both cases will require a modification to the program, or disabling this feature
                    195: for part or all of the build.
                    196: 
                    197: ### Run-time crashes
                    198: 
1.4       khorben   199: Again, this feature may cause some programs to crash, usually indicating an
                    200: actual bug in the program. Patching the original program is then required.
1.3       khorben   201: 
1.8       khorben   202: ### Performance impact
                    203: 
                    204: The compiler emits extra code when using this feature: a check for buffer
                    205: overflows is performed when entering and exiting functions, requiring an extra
                    206: variable on the stack. The level of protection can otherwise be adjusted to
                    207: affect only those functions considered more sensitive by the compiler (with
                    208: `-fstack-protector` instead of `-fstack-protector-all`).
                    209: 
                    210: The impact is not expected to be noticeable on modern hardware. However,
                    211: programs with a hard requirement to run at the fastest possible speed should
                    212: avoid using this feature, or using libraries built with this feature.
                    213: 
1.5       khorben   214: # Auditing the system
                    215: 
                    216: The illusion of security is worse than having no security at all. This section
                    217: lists a number of ways to ensure the security features requested are actually
                    218: effective.
                    219: 
                    220: _These instructions were obtained and tested on a system derived from NetBSD 7
                    221: (amd64). YMMV._
                    222: 
                    223: ## Checking for PIE
                    224: 
                    225: The ELF executable type in use changes for binaries built as PIE; without:
                    226: 
                    227:     $ file /path/to/bin/ary
                    228:     /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
                    229: 
                    230: as opposed to the following binary, built as PIE:
                    231: 
                    232:     $ file /path/to/pie/bin/ary
                    233:     /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
                    234: 
                    235: The latter result is then what is expected.
                    236: 
1.13      khorben   237: ## Checking for partial RELRO
1.5       khorben   238: 
                    239: The following command should list a section called `RELRO`:
                    240: 
                    241:     $ objdump -p /path/to/bin/ary
                    242: 
                    243:     /path/to/bin/ary:     file format elf64-x86-64
                    244: 
                    245:     Program Header:
                    246:     [...]
                    247:        RELRO off    0x0000000000000d78 vaddr 0x0000000000600d78 paddr 0x0000000000600d78 align 2**0
1.6       khorben   248: 
1.17      khorben   249: This check is now performed automatically if `PKG_DEVELOPER` is set and `RELRO`
                    250: is enabled.
                    251: 
1.13      khorben   252: ## Checking for full RELRO
                    253: 
                    254: The dynamic loader will apply RELRO immediately when detecting the presence of
                    255: the `BIND_NOW` flag:
                    256: 
                    257:     $ objdump -x /path/to/bin/ary
                    258: 
                    259:     /path/to/bin/ary:     file format elf64-x86-64
                    260: 
                    261:     Dynamic Section:
                    262:     [...]
                    263:       BIND_NOW             0x0000000000000000
                    264: 
                    265: This has to be combined with partial RELRO (see above) to be fully efficient.
                    266: 
1.6       khorben   267: ## Checking for SSP
                    268: 
                    269: Building objects, binaries and libraries with SSP will affect the presence of
                    270: additional symbols in the resulting file:
                    271: 
                    272:     $ nm /path/to/bin/ary
                    273:     [...]
                    274:                      U __stack_chk_fail
                    275:     0000000000600ea0 B __stack_chk_guard
                    276: 
                    277: This is an indicator that the program was indeed built with support for SSP.
                    278: 
1.10      khorben   279: # References
                    280: 
                    281: * <http://tk-blog.blogspot.co.at/2009/02/relro-not-so-well-known-memory.html>
                    282: 

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