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