Annotation of wikisrc/pkgsrc/hardening.mdwn, revision 1.6

1.1       khorben     1: [[!meta title="Hardening pkgsrc"]]
                      2: 
                      3: [pkgsrc](http://www.pkgsrc.org/) supports a number of mechanisms that are meant
                      4: to improve the security of compiled binaries. They can be individually enabled
                      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: 
1.2       khorben    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: 
1.4       khorben    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: 
1.3       khorben    74: ## Problems with `PKGSRC_USE_SSP`
                     75: 
                     76: ### Packages failing to build
                     77: 
                     78: The stack-smashing protection provided by this option does not work for some
                     79: programs. The two most common situations in which this happens are:
                     80: 
                     81: * the program makes use of the `alloca(3)` library call (memory allocator on the
                     82:   stack)
                     83: * the program allocates variables on the stack, with the size determined at
                     84:   run-time.
                     85: 
                     86: Both cases will require a modification to the program, or disabling this feature
                     87: for part or all of the build.
                     88: 
                     89: ### Run-time crashes
                     90: 
1.4       khorben    91: Again, this feature may cause some programs to crash, usually indicating an
                     92: actual bug in the program. Patching the original program is then required.
1.3       khorben    93: 
1.5       khorben    94: # Auditing the system
                     95: 
                     96: The illusion of security is worse than having no security at all. This section
                     97: lists a number of ways to ensure the security features requested are actually
                     98: effective.
                     99: 
                    100: _These instructions were obtained and tested on a system derived from NetBSD 7
                    101: (amd64). YMMV._
                    102: 
                    103: ## Checking for PIE
                    104: 
                    105: The ELF executable type in use changes for binaries built as PIE; without:
                    106: 
                    107:     $ file /path/to/bin/ary
                    108:     /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
                    109: 
                    110: as opposed to the following binary, built as PIE:
                    111: 
                    112:     $ file /path/to/pie/bin/ary
                    113:     /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
                    114: 
                    115: The latter result is then what is expected.
                    116: 
                    117: ## Checking for RELRO
                    118: 
                    119: The following command should list a section called `RELRO`:
                    120: 
                    121:     $ objdump -p /path/to/bin/ary
                    122: 
                    123:     /path/to/bin/ary:     file format elf64-x86-64
                    124: 
                    125:     Program Header:
                    126:     [...]
                    127:        RELRO off    0x0000000000000d78 vaddr 0x0000000000600d78 paddr 0x0000000000600d78 align 2**0
1.6     ! khorben   128: 
        !           129: ## Checking for SSP
        !           130: 
        !           131: Building objects, binaries and libraries with SSP will affect the presence of
        !           132: additional symbols in the resulting file:
        !           133: 
        !           134:     $ nm /path/to/bin/ary
        !           135:     [...]
        !           136:                      U __stack_chk_fail
        !           137:     0000000000600ea0 B __stack_chk_guard
        !           138: 
        !           139: This is an indicator that the program was indeed built with support for SSP.
        !           140: 

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