Annotation of wikisrc/pkgsrc/hardening.mdwn, revision 1.41

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: 
1.39      leot       24: ## Enabled by default
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:
1.34      khorben    47: 
1.30      khorben    48: * the default ("yes"), which will only protect functions considered vulnerable
                     49:   by the compiler;
                     50: * "all", which will protect every function;
                     51: * "strong", which will apply a better balance between the two settings above.
                     52: 
1.29      khorben    53: This mitigation is supported by both GCC and clang. It may be supported in
                     54: additional compilers, possibly under a different name. It is particularly useful
                     55: for unsafe programming languages, such as C/C++.
1.23      gdt        56: 
1.29      khorben    57: It is enabled by default where known supported since pkgsrc-2017Q3.
1.23      gdt        58: 
1.33      khorben    59: More details can be found here:
1.34      khorben    60: 
1.29      khorben    61: * <https://en.wikipedia.org/wiki/Buffer_overflow_protection>
1.22      gdt        62: 
                     63: ## Not enabled by default
                     64: 
1.23      gdt        65: ### PKGSRC_MKPIE
                     66: 
1.36      khorben    67: This requests the creation of PIE (Position Independent Executables) for all
1.37      khorben    68: executables. The PIE mechanism is normally used for shared libraries, so that
1.36      khorben    69: they can be loaded at differing addresses at runtime. PIE itself does not have
1.37      khorben    70: useful security properties; however, it is necessary to fully leverage some,
                     71: such as ASLR.  Some operating systems support Address Space Layout Randomization
                     72: (ASLR), which causes different addresses to be used each time a program is run.
                     73: This makes it more difficult for an attacker to guess addresses and thus makes
                     74: exploits harder to construct. With PIE, ASLR can really be applied to the entire
                     75: program, instead of the stack and heap only.
1.23      gdt        76: 
1.31      khorben    77: PIE executables will only be built for toolchains that are known to support PIE.
                     78: Currently, this means NetBSD on amd64 and i386.
1.23      gdt        79: 
1.38      khorben    80: ### PKGSRC_MKREPRO
                     81: 
                     82: With this option, pkgsrc will try to build packages reproducibly. This allows
                     83: packages built from the same tree and with the same options, to produce
                     84: identical results bit by bit. This option should be combined with ASLR and
                     85: `PKGSRC_MKPIE` to avoid predictable address offsets for attackers attempting to
                     86: exploit security vulnerabilities.
                     87: 
                     88: More details can be found here:
                     89: 
                     90: * <https://reproducible-builds.org/>
                     91: 
1.23      gdt        92: ### PKGSRC_USE_RELRO
                     93: 
                     94: This also makes the exploitation of some security vulnerabilities more
                     95: difficult in some cases.
1.22      gdt        96: 
1.33      khorben    97: Two different mitigation levels are available:
1.34      khorben    98: 
1.33      khorben    99: * partial: the ELF sections are reordered so that internal data sections
                    100:   precede the program's own data sections, and non-PLT GOT is read-only;
                    101: * full: in addition to partial RELRO, every relocation is performed immediately
                    102:   when starting the program (with a slight performance impact), allowing the
                    103:   entire GOT to be read-only.
1.24      gdt       104: 
1.33      khorben   105: This is currently supported by GCC. Many software distributions now enable this
                    106: feature by default, at the "partial" level.
1.24      gdt       107: 
1.33      khorben   108: More details can be found here:
1.34      khorben   109: 
1.33      khorben   110: * <http://tk-blog.blogspot.co.at/2009/02/relro-not-so-well-known-memory.html>
1.24      gdt       111: 
1.23      gdt       112: ### PKGSRC_USE_STACK_CHECK
1.22      gdt       113: 
1.32      khorben   114: This uses `-fstack-check` with GCC for another stack protection mitigation.
                    115: 
                    116: It asks the compiler to generate code verifying that it does not corrupt the
                    117: stack. According to GCC's manual page, this is really only useful for
                    118: multi-threaded programs.
1.1       khorben   119: 
1.2       khorben   120: # Caveats
                    121: 
                    122: ## Problems with `PKGSRC_MKPIE`
                    123: 
1.19      khorben   124: ### Recent support for cwrappers
1.2       khorben   125: 
1.19      khorben   126: `PKGSRC_MKPIE` is only supported by `pkgtools/cwrappers` from the 2017Q3
                    127: release on (`USE_CWRAPPERS` in `mk.conf`).
1.2       khorben   128: 
                    129: ### Packages failing to build
                    130: 
                    131: A number of packages may fail to build with this option enabled. The failures
1.18      khorben   132: are often related to the absence of the `-fPIC` compilation flag when building
                    133: libraries or executables (or ideally `-fPIE` in the latter case). This flag is
1.2       khorben   134: added to the `CFLAGS` already, but requires the package to actually support it.
                    135: 
                    136: #### How to fix
                    137: 
                    138: These instructions are meant as a reference only; they likely need to be adapted
                    139: for many packages individually.
                    140: 
                    141: For packages using `Makefiles`:
                    142: 
                    143:     MAKE_FLAGS+=       CFLAGS=${CFLAGS:Q}
                    144:     MAKE_FLAGS+=       LDFLAGS=${LDFLAGS:Q}
                    145: 
                    146: For packages using `Imakefiles`:
                    147: 
                    148:     MAKE_FLAGS+=       CCOPTIONS=${CFLAGS:Q}
                    149:     MAKE_FLAGS+=       LOCAL_LDFLAGS=${LDFLAGS:Q}
                    150: 
                    151: ### Run-time crashes
                    152: 
                    153: Some programs may fail to run, or crash at random times once built as PIE. Two
                    154: scenarios are essentially possible:
                    155: 
                    156: * actual bug in the program crashing, exposed thanks to ASLR/mprotect;
                    157: * bug in the implementation of ASLR/mprotect in the Operating System.
                    158: 
1.4       khorben   159: ## Problems with `PKGSRC_USE_FORTIFY`
                    160: 
                    161: ### Packages failing to build
                    162: 
                    163: This feature makes use of pre-processing directives to look for hardened,
                    164: alternative implementations of essential library calls. Some programs may fail
                    165: to build as a result; this usually happens for those trying too hard to be
                    166: portable, or otherwise abusing definitions in the standard library.
                    167: 
                    168: This will require a modification to the program, or disabling this feature for
                    169: part or all of the build.
                    170: 
                    171: ### Run-time crashes
                    172: 
                    173: Just like with `PKGSRC_MKPIE` above, this feature may cause some programs to
                    174: crash, usually indicating an actual bug in the program. The fix will typically
                    175: involve patching the original program.
                    176: 
1.28      khorben   177: ### Optimization is required
                    178: 
                    179: At least in the case of GCC, FORTIFY will only be applied if optimization is
                    180: applied while compiling. This means that the CFLAGS should also contain -O, -O2
                    181: or another optimization level. This cannot easily be applied globally, as some
                    182: packages may require specific optimization levels.
                    183: 
1.7       khorben   184: ## Problems with `PKGSRC_USE_RELRO`
                    185: 
                    186: ### Performance impact
                    187: 
                    188: For better protection, full RELRO requires every symbol to be resolved when the
1.11      khorben   189: program starts, rather than simply when required at run-time. This will have
                    190: more impact on programs using a lot of symbols, or linked to libraries exposing
                    191: a lot of symbols. Therefore, daemons or programs otherwise running in
                    192: background are affected only when started. Programs loading plug-ins at
                    193: run-time are affected when loading the plug-ins.
1.7       khorben   194: 
                    195: The impact is not expected to be noticeable on modern hardware, except in some
                    196: cases for big programs.
                    197: 
1.12      khorben   198: ### Run-time crashes
                    199: 
                    200: Some programs handle plug-ins and dependencies in a way that conflicts with
                    201: RELRO: for instance, with an initialization routine listing any other plug-in
                    202: required. With full RELRO, the missing symbols are resolved before the
                    203: initialization routine can run, and the dynamic loader will not be able to find
                    204: them directly and abort as a result. Unfortunately, this is how Xorg loads its
                    205: drivers. Partial RELRO can be applied instead in this case.
                    206: 
1.3       khorben   207: ## Problems with `PKGSRC_USE_SSP`
                    208: 
                    209: ### Packages failing to build
                    210: 
                    211: The stack-smashing protection provided by this option does not work for some
                    212: programs. The two most common situations in which this happens are:
                    213: 
                    214: * the program makes use of the `alloca(3)` library call (memory allocator on the
                    215:   stack)
                    216: * the program allocates variables on the stack, with the size determined at
                    217:   run-time.
                    218: 
                    219: Both cases will require a modification to the program, or disabling this feature
                    220: for part or all of the build.
                    221: 
                    222: ### Run-time crashes
                    223: 
1.40      leot      224: Again, this feature may cause some programs to crash via a SIGABRT,
                    225: usually indicating an actual bug in the program.
                    226: 
                    227: On NetBSD `LOG_CRIT` level `syslog()` messages are sent and - by
                    228: default - appended to `/var/log/messages`, e.g.:
                    229: 
1.41    ! leot      230:     Jan  6 15:42:51 <hostname> -: <hostname> <program> - - - buffer overflow detected; terminated
1.40      leot      231: 
                    232: (where `<hostname>` is the `hostname(1)` and `<program>` is the
                    233: `basename(1)` of the program crashed).
                    234: 
                    235: Patching the original program is then required.
                    236: 
                    237: Rebuilding the package via:
                    238: 
                    239:     % env CFLAGS=-g INSTALL_UNSTRIPPED=yes make replace
                    240: 
                    241: and inspecting the `backtrace` of the coredump via the debugger
                    242: should point out the problematic call by inspecting the frame
                    243: calling the `_chk()' (SSP) function.
1.3       khorben   244: 
1.8       khorben   245: ### Performance impact
                    246: 
                    247: The compiler emits extra code when using this feature: a check for buffer
                    248: overflows is performed when entering and exiting functions, requiring an extra
                    249: variable on the stack. The level of protection can otherwise be adjusted to
                    250: affect only those functions considered more sensitive by the compiler (with
                    251: `-fstack-protector` instead of `-fstack-protector-all`).
                    252: 
                    253: The impact is not expected to be noticeable on modern hardware. However,
                    254: programs with a hard requirement to run at the fastest possible speed should
                    255: avoid using this feature, or using libraries built with this feature.
                    256: 
1.5       khorben   257: # Auditing the system
                    258: 
                    259: The illusion of security is worse than having no security at all. This section
                    260: lists a number of ways to ensure the security features requested are actually
                    261: effective.
                    262: 
                    263: _These instructions were obtained and tested on a system derived from NetBSD 7
                    264: (amd64). YMMV._
                    265: 
                    266: ## Checking for PIE
                    267: 
                    268: The ELF executable type in use changes for binaries built as PIE; without:
                    269: 
                    270:     $ file /path/to/bin/ary
                    271:     /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
                    272: 
                    273: as opposed to the following binary, built as PIE:
                    274: 
                    275:     $ file /path/to/pie/bin/ary
                    276:     /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
                    277: 
                    278: The latter result is then what is expected.
                    279: 
1.13      khorben   280: ## Checking for partial RELRO
1.5       khorben   281: 
                    282: The following command should list a section called `RELRO`:
                    283: 
                    284:     $ objdump -p /path/to/bin/ary
                    285: 
                    286:     /path/to/bin/ary:     file format elf64-x86-64
                    287: 
                    288:     Program Header:
                    289:     [...]
                    290:        RELRO off    0x0000000000000d78 vaddr 0x0000000000600d78 paddr 0x0000000000600d78 align 2**0
1.6       khorben   291: 
1.17      khorben   292: This check is now performed automatically if `PKG_DEVELOPER` is set and `RELRO`
                    293: is enabled.
                    294: 
1.13      khorben   295: ## Checking for full RELRO
                    296: 
                    297: The dynamic loader will apply RELRO immediately when detecting the presence of
                    298: the `BIND_NOW` flag:
                    299: 
                    300:     $ objdump -x /path/to/bin/ary
                    301: 
                    302:     /path/to/bin/ary:     file format elf64-x86-64
                    303: 
                    304:     Dynamic Section:
                    305:     [...]
                    306:       BIND_NOW             0x0000000000000000
                    307: 
                    308: This has to be combined with partial RELRO (see above) to be fully efficient.
                    309: 
1.6       khorben   310: ## Checking for SSP
                    311: 
                    312: Building objects, binaries and libraries with SSP will affect the presence of
                    313: additional symbols in the resulting file:
                    314: 
                    315:     $ nm /path/to/bin/ary
                    316:     [...]
                    317:                      U __stack_chk_fail
                    318:     0000000000600ea0 B __stack_chk_guard
                    319: 
                    320: This is an indicator that the program was indeed built with support for SSP.
1.35      khorben   321: 
                    322: This check is now performed automatically (where supported) if `PKG_DEVELOPER`
                    323: is set and `SSP` is enabled.

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