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Sun Jun 25 16:11:04 2017 UTC (4 years, 10 months ago) by khorben
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Document partial vs full RELRO

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

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