File:  [NetBSD Developer Wiki] / wikisrc / pkgsrc / hardening.mdwn
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Tue Nov 7 02:18:46 2017 UTC (4 years, 6 months ago) by khorben
Branches: MAIN
CVS tags: HEAD
List the mitigation levels for PKGSRC_USE_SSP

    1: [[!meta title="Hardening pkgsrc"]]
    2: 
    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
   11: individually described below. They are sorted by whether they are
   12: enabled by default, and then by their ordering in `mk/defaults/mk.conf`.
   13: 
   14: Typically, a feature will cause some programs to fail to build or work
   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
   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.
   23: 
   24: ## Enabled by default in the stable branch
   25: 
   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: 
   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.
   34: 
   35: It has been enabled by default since pkgsrc-2017Q3.
   36: 
   37: ### PKGSRC_USE_SSP
   38: 
   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: 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: 
   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++.
   55: 
   56: It is enabled by default where known supported since pkgsrc-2017Q3.
   57: 
   58: * <https://en.wikipedia.org/wiki/Buffer_overflow_protection>
   59: 
   60: ## Enabled by default in pkgsrc HEAD
   61: 
   62: ## Not enabled by default
   63: 
   64: ### PKGSRC_MKPIE
   65: 
   66: This requests the the creation of PIE (Position Independent
   67: Executables) for all executables. The PIE mechanism is normally used
   68: for shared libraries so that they can be loaded at differing addresses
   69: at runtime. PIE itself does not have useful security properties.
   70: However, some operating systems support Address Space Layout
   71: Randomization (ASLR), which causes different addresses to be used each
   72: time a program is run. This makes it more difficult for an attacker
   73: to guess addresses and thus makes exploits harder to construct.
   74: 
   75: TODO/check: PIE executables will only be built for toolchains that
   76: support PIE and operating systems known to support ASLR. Currently,
   77: this means NetBSD 8 and later, i386 and amd64.
   78: 
   79: ### PKGSRC_USE_RELRO
   80: 
   81: This also makes the exploitation of some security vulnerabilities more
   82: difficult in some cases.
   83: 
   84: TODO: Explain gcc vs clang, and whether this has broad support or just
   85: a few platforms.
   86: 
   87: TODO: Address "partial" vs "full"; which is this?
   88: 
   89: TODO: Give a link to a comprehensive explanation.
   90: 
   91: ### PKGSRC_USE_STACK_CHECK
   92: 
   93: This uses `-fstack-check` with GCC for another stack protection
   94: mitigation.
   95: 
   96: # Caveats
   97: 
   98: ## Problems with `PKGSRC_MKPIE`
   99: 
  100: ### Recent support for cwrappers
  101: 
  102: `PKGSRC_MKPIE` is only supported by `pkgtools/cwrappers` from the 2017Q3
  103: release on (`USE_CWRAPPERS` in `mk.conf`).
  104: 
  105: ### Packages failing to build
  106: 
  107: A number of packages may fail to build with this option enabled. The failures
  108: are often related to the absence of the `-fPIC` compilation flag when building
  109: libraries or executables (or ideally `-fPIE` in the latter case). This flag is
  110: added to the `CFLAGS` already, but requires the package to actually support it.
  111: 
  112: #### How to fix
  113: 
  114: These instructions are meant as a reference only; they likely need to be adapted
  115: for many packages individually.
  116: 
  117: For packages using `Makefiles`:
  118: 
  119:     MAKE_FLAGS+=	CFLAGS=${CFLAGS:Q}
  120:     MAKE_FLAGS+=	LDFLAGS=${LDFLAGS:Q}
  121: 
  122: For packages using `Imakefiles`:
  123: 
  124:     MAKE_FLAGS+=	CCOPTIONS=${CFLAGS:Q}
  125:     MAKE_FLAGS+=	LOCAL_LDFLAGS=${LDFLAGS:Q}
  126: 
  127: ### Run-time crashes
  128: 
  129: Some programs may fail to run, or crash at random times once built as PIE. Two
  130: scenarios are essentially possible:
  131: 
  132: * actual bug in the program crashing, exposed thanks to ASLR/mprotect;
  133: * bug in the implementation of ASLR/mprotect in the Operating System.
  134: 
  135: ## Problems with `PKGSRC_USE_FORTIFY`
  136: 
  137: ### Packages failing to build
  138: 
  139: This feature makes use of pre-processing directives to look for hardened,
  140: alternative implementations of essential library calls. Some programs may fail
  141: to build as a result; this usually happens for those trying too hard to be
  142: portable, or otherwise abusing definitions in the standard library.
  143: 
  144: This will require a modification to the program, or disabling this feature for
  145: part or all of the build.
  146: 
  147: ### Run-time crashes
  148: 
  149: Just like with `PKGSRC_MKPIE` above, this feature may cause some programs to
  150: crash, usually indicating an actual bug in the program. The fix will typically
  151: involve patching the original program.
  152: 
  153: ### Optimization is required
  154: 
  155: At least in the case of GCC, FORTIFY will only be applied if optimization is
  156: applied while compiling. This means that the CFLAGS should also contain -O, -O2
  157: or another optimization level. This cannot easily be applied globally, as some
  158: packages may require specific optimization levels.
  159: 
  160: ## Problems with `PKGSRC_USE_RELRO`
  161: 
  162: ### Performance impact
  163: 
  164: For better protection, full RELRO requires every symbol to be resolved when the
  165: program starts, rather than simply when required at run-time. This will have
  166: more impact on programs using a lot of symbols, or linked to libraries exposing
  167: a lot of symbols. Therefore, daemons or programs otherwise running in
  168: background are affected only when started. Programs loading plug-ins at
  169: run-time are affected when loading the plug-ins.
  170: 
  171: The impact is not expected to be noticeable on modern hardware, except in some
  172: cases for big programs.
  173: 
  174: ### Run-time crashes
  175: 
  176: Some programs handle plug-ins and dependencies in a way that conflicts with
  177: RELRO: for instance, with an initialization routine listing any other plug-in
  178: required. With full RELRO, the missing symbols are resolved before the
  179: initialization routine can run, and the dynamic loader will not be able to find
  180: them directly and abort as a result. Unfortunately, this is how Xorg loads its
  181: drivers. Partial RELRO can be applied instead in this case.
  182: 
  183: ## Problems with `PKGSRC_USE_SSP`
  184: 
  185: ### Packages failing to build
  186: 
  187: The stack-smashing protection provided by this option does not work for some
  188: programs. The two most common situations in which this happens are:
  189: 
  190: * the program makes use of the `alloca(3)` library call (memory allocator on the
  191:   stack)
  192: * the program allocates variables on the stack, with the size determined at
  193:   run-time.
  194: 
  195: Both cases will require a modification to the program, or disabling this feature
  196: for part or all of the build.
  197: 
  198: ### Run-time crashes
  199: 
  200: Again, this feature may cause some programs to crash, usually indicating an
  201: actual bug in the program. Patching the original program is then required.
  202: 
  203: ### Performance impact
  204: 
  205: The compiler emits extra code when using this feature: a check for buffer
  206: overflows is performed when entering and exiting functions, requiring an extra
  207: variable on the stack. The level of protection can otherwise be adjusted to
  208: affect only those functions considered more sensitive by the compiler (with
  209: `-fstack-protector` instead of `-fstack-protector-all`).
  210: 
  211: The impact is not expected to be noticeable on modern hardware. However,
  212: programs with a hard requirement to run at the fastest possible speed should
  213: avoid using this feature, or using libraries built with this feature.
  214: 
  215: # Auditing the system
  216: 
  217: The illusion of security is worse than having no security at all. This section
  218: lists a number of ways to ensure the security features requested are actually
  219: effective.
  220: 
  221: _These instructions were obtained and tested on a system derived from NetBSD 7
  222: (amd64). YMMV._
  223: 
  224: ## Checking for PIE
  225: 
  226: The ELF executable type in use changes for binaries built as PIE; without:
  227: 
  228:     $ file /path/to/bin/ary
  229:     /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
  230: 
  231: as opposed to the following binary, built as PIE:
  232: 
  233:     $ file /path/to/pie/bin/ary
  234:     /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
  235: 
  236: The latter result is then what is expected.
  237: 
  238: ## Checking for partial RELRO
  239: 
  240: The following command should list a section called `RELRO`:
  241: 
  242:     $ objdump -p /path/to/bin/ary
  243: 
  244:     /path/to/bin/ary:     file format elf64-x86-64
  245: 
  246:     Program Header:
  247:     [...]
  248:        RELRO off    0x0000000000000d78 vaddr 0x0000000000600d78 paddr 0x0000000000600d78 align 2**0
  249: 
  250: This check is now performed automatically if `PKG_DEVELOPER` is set and `RELRO`
  251: is enabled.
  252: 
  253: ## Checking for full RELRO
  254: 
  255: The dynamic loader will apply RELRO immediately when detecting the presence of
  256: the `BIND_NOW` flag:
  257: 
  258:     $ objdump -x /path/to/bin/ary
  259: 
  260:     /path/to/bin/ary:     file format elf64-x86-64
  261: 
  262:     Dynamic Section:
  263:     [...]
  264:       BIND_NOW             0x0000000000000000
  265: 
  266: This has to be combined with partial RELRO (see above) to be fully efficient.
  267: 
  268: ## Checking for SSP
  269: 
  270: Building objects, binaries and libraries with SSP will affect the presence of
  271: additional symbols in the resulting file:
  272: 
  273:     $ nm /path/to/bin/ary
  274:     [...]
  275:                      U __stack_chk_fail
  276:     0000000000600ea0 B __stack_chk_guard
  277: 
  278: This is an indicator that the program was indeed built with support for SSP.
  279: 
  280: # References
  281: 
  282: * <http://tk-blog.blogspot.co.at/2009/02/relro-not-so-well-known-memory.html>
  283: 

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