File:  [NetBSD Developer Wiki] / wikisrc / pkgsrc / hardening.mdwn
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Tue Nov 7 02:37:43 2017 UTC (5 years ago) by khorben
Branches: MAIN
CVS tags: HEAD
Clarify RELRO

    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: More details can be found here:
   59: * <https://en.wikipedia.org/wiki/Buffer_overflow_protection>
   60: 
   61: ## Enabled by default in pkgsrc HEAD
   62: 
   63: ## Not enabled by default
   64: 
   65: ### PKGSRC_MKPIE
   66: 
   67: This requests the the creation of PIE (Position Independent
   68: Executables) for all executables. The PIE mechanism is normally used
   69: for shared libraries so that they can be loaded at differing addresses
   70: at runtime. PIE itself does not have useful security properties.
   71: However, some operating systems support Address Space Layout
   72: Randomization (ASLR), which causes different addresses to be used each
   73: time a program is run. This makes it more difficult for an attacker
   74: to guess addresses and thus makes exploits harder to construct.
   75: 
   76: PIE executables will only be built for toolchains that are known to support PIE.
   77: Currently, this means NetBSD on amd64 and i386.
   78: 
   79: ### PKGSRC_USE_RELRO
   80: 
   81: This also makes the exploitation of some security vulnerabilities more
   82: difficult in some cases.
   83: 
   84: Two different mitigation levels are available:
   85: * partial: the ELF sections are reordered so that internal data sections
   86:   precede the program's own data sections, and non-PLT GOT is read-only;
   87: * full: in addition to partial RELRO, every relocation is performed immediately
   88:   when starting the program (with a slight performance impact), allowing the
   89:   entire GOT to be read-only.
   90: 
   91: This is currently supported by GCC. Many software distributions now enable this
   92: feature by default, at the "partial" level.
   93: 
   94: More details can be found here:
   95: * <http://tk-blog.blogspot.co.at/2009/02/relro-not-so-well-known-memory.html>
   96: 
   97: ### PKGSRC_USE_STACK_CHECK
   98: 
   99: This uses `-fstack-check` with GCC for another stack protection mitigation.
  100: 
  101: It asks the compiler to generate code verifying that it does not corrupt the
  102: stack. According to GCC's manual page, this is really only useful for
  103: multi-threaded programs.
  104: 
  105: # Caveats
  106: 
  107: ## Problems with `PKGSRC_MKPIE`
  108: 
  109: ### Recent support for cwrappers
  110: 
  111: `PKGSRC_MKPIE` is only supported by `pkgtools/cwrappers` from the 2017Q3
  112: release on (`USE_CWRAPPERS` in `mk.conf`).
  113: 
  114: ### Packages failing to build
  115: 
  116: A number of packages may fail to build with this option enabled. The failures
  117: are often related to the absence of the `-fPIC` compilation flag when building
  118: libraries or executables (or ideally `-fPIE` in the latter case). This flag is
  119: added to the `CFLAGS` already, but requires the package to actually support it.
  120: 
  121: #### How to fix
  122: 
  123: These instructions are meant as a reference only; they likely need to be adapted
  124: for many packages individually.
  125: 
  126: For packages using `Makefiles`:
  127: 
  128:     MAKE_FLAGS+=	CFLAGS=${CFLAGS:Q}
  129:     MAKE_FLAGS+=	LDFLAGS=${LDFLAGS:Q}
  130: 
  131: For packages using `Imakefiles`:
  132: 
  133:     MAKE_FLAGS+=	CCOPTIONS=${CFLAGS:Q}
  134:     MAKE_FLAGS+=	LOCAL_LDFLAGS=${LDFLAGS:Q}
  135: 
  136: ### Run-time crashes
  137: 
  138: Some programs may fail to run, or crash at random times once built as PIE. Two
  139: scenarios are essentially possible:
  140: 
  141: * actual bug in the program crashing, exposed thanks to ASLR/mprotect;
  142: * bug in the implementation of ASLR/mprotect in the Operating System.
  143: 
  144: ## Problems with `PKGSRC_USE_FORTIFY`
  145: 
  146: ### Packages failing to build
  147: 
  148: This feature makes use of pre-processing directives to look for hardened,
  149: alternative implementations of essential library calls. Some programs may fail
  150: to build as a result; this usually happens for those trying too hard to be
  151: portable, or otherwise abusing definitions in the standard library.
  152: 
  153: This will require a modification to the program, or disabling this feature for
  154: part or all of the build.
  155: 
  156: ### Run-time crashes
  157: 
  158: Just like with `PKGSRC_MKPIE` above, this feature may cause some programs to
  159: crash, usually indicating an actual bug in the program. The fix will typically
  160: involve patching the original program.
  161: 
  162: ### Optimization is required
  163: 
  164: At least in the case of GCC, FORTIFY will only be applied if optimization is
  165: applied while compiling. This means that the CFLAGS should also contain -O, -O2
  166: or another optimization level. This cannot easily be applied globally, as some
  167: packages may require specific optimization levels.
  168: 
  169: ## Problems with `PKGSRC_USE_RELRO`
  170: 
  171: ### Performance impact
  172: 
  173: For better protection, full RELRO requires every symbol to be resolved when the
  174: program starts, rather than simply when required at run-time. This will have
  175: more impact on programs using a lot of symbols, or linked to libraries exposing
  176: a lot of symbols. Therefore, daemons or programs otherwise running in
  177: background are affected only when started. Programs loading plug-ins at
  178: run-time are affected when loading the plug-ins.
  179: 
  180: The impact is not expected to be noticeable on modern hardware, except in some
  181: cases for big programs.
  182: 
  183: ### Run-time crashes
  184: 
  185: Some programs handle plug-ins and dependencies in a way that conflicts with
  186: RELRO: for instance, with an initialization routine listing any other plug-in
  187: required. With full RELRO, the missing symbols are resolved before the
  188: initialization routine can run, and the dynamic loader will not be able to find
  189: them directly and abort as a result. Unfortunately, this is how Xorg loads its
  190: drivers. Partial RELRO can be applied instead in this case.
  191: 
  192: ## Problems with `PKGSRC_USE_SSP`
  193: 
  194: ### Packages failing to build
  195: 
  196: The stack-smashing protection provided by this option does not work for some
  197: programs. The two most common situations in which this happens are:
  198: 
  199: * the program makes use of the `alloca(3)` library call (memory allocator on the
  200:   stack)
  201: * the program allocates variables on the stack, with the size determined at
  202:   run-time.
  203: 
  204: Both cases will require a modification to the program, or disabling this feature
  205: for part or all of the build.
  206: 
  207: ### Run-time crashes
  208: 
  209: Again, this feature may cause some programs to crash, usually indicating an
  210: actual bug in the program. Patching the original program is then required.
  211: 
  212: ### Performance impact
  213: 
  214: The compiler emits extra code when using this feature: a check for buffer
  215: overflows is performed when entering and exiting functions, requiring an extra
  216: variable on the stack. The level of protection can otherwise be adjusted to
  217: affect only those functions considered more sensitive by the compiler (with
  218: `-fstack-protector` instead of `-fstack-protector-all`).
  219: 
  220: The impact is not expected to be noticeable on modern hardware. However,
  221: programs with a hard requirement to run at the fastest possible speed should
  222: avoid using this feature, or using libraries built with this feature.
  223: 
  224: # Auditing the system
  225: 
  226: The illusion of security is worse than having no security at all. This section
  227: lists a number of ways to ensure the security features requested are actually
  228: effective.
  229: 
  230: _These instructions were obtained and tested on a system derived from NetBSD 7
  231: (amd64). YMMV._
  232: 
  233: ## Checking for PIE
  234: 
  235: The ELF executable type in use changes for binaries built as PIE; without:
  236: 
  237:     $ file /path/to/bin/ary
  238:     /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
  239: 
  240: as opposed to the following binary, built as PIE:
  241: 
  242:     $ file /path/to/pie/bin/ary
  243:     /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
  244: 
  245: The latter result is then what is expected.
  246: 
  247: ## Checking for partial RELRO
  248: 
  249: The following command should list a section called `RELRO`:
  250: 
  251:     $ objdump -p /path/to/bin/ary
  252: 
  253:     /path/to/bin/ary:     file format elf64-x86-64
  254: 
  255:     Program Header:
  256:     [...]
  257:        RELRO off    0x0000000000000d78 vaddr 0x0000000000600d78 paddr 0x0000000000600d78 align 2**0
  258: 
  259: This check is now performed automatically if `PKG_DEVELOPER` is set and `RELRO`
  260: is enabled.
  261: 
  262: ## Checking for full RELRO
  263: 
  264: The dynamic loader will apply RELRO immediately when detecting the presence of
  265: the `BIND_NOW` flag:
  266: 
  267:     $ objdump -x /path/to/bin/ary
  268: 
  269:     /path/to/bin/ary:     file format elf64-x86-64
  270: 
  271:     Dynamic Section:
  272:     [...]
  273:       BIND_NOW             0x0000000000000000
  274: 
  275: This has to be combined with partial RELRO (see above) to be fully efficient.
  276: 
  277: ## Checking for SSP
  278: 
  279: Building objects, binaries and libraries with SSP will affect the presence of
  280: additional symbols in the resulting file:
  281: 
  282:     $ nm /path/to/bin/ary
  283:     [...]
  284:                      U __stack_chk_fail
  285:     0000000000600ea0 B __stack_chk_guard
  286: 
  287: This is an indicator that the program was indeed built with support for SSP.

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