Annotation of wikisrc/pkgsrc/hardening.mdwn, revision 1.33
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:
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:
1.29 khorben 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++.
1.23 gdt 55:
1.29 khorben 56: It is enabled by default where known supported since pkgsrc-2017Q3.
1.23 gdt 57:
1.33 ! khorben 58: More details can be found here:
1.29 khorben 59: * <https://en.wikipedia.org/wiki/Buffer_overflow_protection>
1.22 gdt 60:
61: ## Enabled by default in pkgsrc HEAD
62:
63: ## Not enabled by default
64:
1.23 gdt 65: ### PKGSRC_MKPIE
66:
67: This requests the the creation of PIE (Position Independent
1.25 khorben 68: Executables) for all executables. The PIE mechanism is normally used
1.23 gdt 69: for shared libraries so that they can be loaded at differing addresses
1.25 khorben 70: at runtime. PIE itself does not have useful security properties.
1.23 gdt 71: However, some operating systems support Address Space Layout
72: Randomization (ASLR), which causes different addresses to be used each
1.25 khorben 73: time a program is run. This makes it more difficult for an attacker
1.23 gdt 74: to guess addresses and thus makes exploits harder to construct.
75:
1.31 khorben 76: PIE executables will only be built for toolchains that are known to support PIE.
77: Currently, this means NetBSD on amd64 and i386.
1.23 gdt 78:
79: ### PKGSRC_USE_RELRO
80:
81: This also makes the exploitation of some security vulnerabilities more
82: difficult in some cases.
1.22 gdt 83:
1.33 ! khorben 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.
1.24 gdt 90:
1.33 ! khorben 91: This is currently supported by GCC. Many software distributions now enable this
! 92: feature by default, at the "partial" level.
1.24 gdt 93:
1.33 ! khorben 94: More details can be found here:
! 95: * <http://tk-blog.blogspot.co.at/2009/02/relro-not-so-well-known-memory.html>
1.24 gdt 96:
1.23 gdt 97: ### PKGSRC_USE_STACK_CHECK
1.22 gdt 98:
1.32 khorben 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.
1.1 khorben 104:
1.2 khorben 105: # Caveats
106:
107: ## Problems with `PKGSRC_MKPIE`
108:
1.19 khorben 109: ### Recent support for cwrappers
1.2 khorben 110:
1.19 khorben 111: `PKGSRC_MKPIE` is only supported by `pkgtools/cwrappers` from the 2017Q3
112: release on (`USE_CWRAPPERS` in `mk.conf`).
1.2 khorben 113:
114: ### Packages failing to build
115:
116: A number of packages may fail to build with this option enabled. The failures
1.18 khorben 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
1.2 khorben 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:
1.4 khorben 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:
1.28 khorben 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:
1.7 khorben 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
1.11 khorben 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.
1.7 khorben 179:
180: The impact is not expected to be noticeable on modern hardware, except in some
181: cases for big programs.
182:
1.12 khorben 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:
1.3 khorben 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:
1.4 khorben 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.
1.3 khorben 211:
1.8 khorben 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:
1.5 khorben 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:
1.13 khorben 247: ## Checking for partial RELRO
1.5 khorben 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
1.6 khorben 258:
1.17 khorben 259: This check is now performed automatically if `PKG_DEVELOPER` is set and `RELRO`
260: is enabled.
261:
1.13 khorben 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:
1.6 khorben 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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