Annotation of wikisrc/pkgsrc/hardening.mdwn, revision 1.32
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:
1.29 khorben 58: * <https://en.wikipedia.org/wiki/Buffer_overflow_protection>
1.22 gdt 59:
60: ## Enabled by default in pkgsrc HEAD
61:
62: ## Not enabled by default
63:
1.23 gdt 64: ### PKGSRC_MKPIE
65:
66: This requests the the creation of PIE (Position Independent
1.25 khorben 67: Executables) for all executables. The PIE mechanism is normally used
1.23 gdt 68: for shared libraries so that they can be loaded at differing addresses
1.25 khorben 69: at runtime. PIE itself does not have useful security properties.
1.23 gdt 70: However, some operating systems support Address Space Layout
71: Randomization (ASLR), which causes different addresses to be used each
1.25 khorben 72: time a program is run. This makes it more difficult for an attacker
1.23 gdt 73: to guess addresses and thus makes exploits harder to construct.
74:
1.31 khorben 75: PIE executables will only be built for toolchains that are known to support PIE.
76: Currently, this means NetBSD on amd64 and i386.
1.23 gdt 77:
78: ### PKGSRC_USE_RELRO
79:
80: This also makes the exploitation of some security vulnerabilities more
81: difficult in some cases.
1.22 gdt 82:
1.24 gdt 83: TODO: Explain gcc vs clang, and whether this has broad support or just
84: a few platforms.
85:
86: TODO: Address "partial" vs "full"; which is this?
87:
88: TODO: Give a link to a comprehensive explanation.
89:
1.23 gdt 90: ### PKGSRC_USE_STACK_CHECK
1.22 gdt 91:
1.32 ! khorben 92: This uses `-fstack-check` with GCC for another stack protection mitigation.
! 93:
! 94: It asks the compiler to generate code verifying that it does not corrupt the
! 95: stack. According to GCC's manual page, this is really only useful for
! 96: multi-threaded programs.
1.1 khorben 97:
1.2 khorben 98: # Caveats
99:
100: ## Problems with `PKGSRC_MKPIE`
101:
1.19 khorben 102: ### Recent support for cwrappers
1.2 khorben 103:
1.19 khorben 104: `PKGSRC_MKPIE` is only supported by `pkgtools/cwrappers` from the 2017Q3
105: release on (`USE_CWRAPPERS` in `mk.conf`).
1.2 khorben 106:
107: ### Packages failing to build
108:
109: A number of packages may fail to build with this option enabled. The failures
1.18 khorben 110: are often related to the absence of the `-fPIC` compilation flag when building
111: libraries or executables (or ideally `-fPIE` in the latter case). This flag is
1.2 khorben 112: added to the `CFLAGS` already, but requires the package to actually support it.
113:
114: #### How to fix
115:
116: These instructions are meant as a reference only; they likely need to be adapted
117: for many packages individually.
118:
119: For packages using `Makefiles`:
120:
121: MAKE_FLAGS+= CFLAGS=${CFLAGS:Q}
122: MAKE_FLAGS+= LDFLAGS=${LDFLAGS:Q}
123:
124: For packages using `Imakefiles`:
125:
126: MAKE_FLAGS+= CCOPTIONS=${CFLAGS:Q}
127: MAKE_FLAGS+= LOCAL_LDFLAGS=${LDFLAGS:Q}
128:
129: ### Run-time crashes
130:
131: Some programs may fail to run, or crash at random times once built as PIE. Two
132: scenarios are essentially possible:
133:
134: * actual bug in the program crashing, exposed thanks to ASLR/mprotect;
135: * bug in the implementation of ASLR/mprotect in the Operating System.
136:
1.4 khorben 137: ## Problems with `PKGSRC_USE_FORTIFY`
138:
139: ### Packages failing to build
140:
141: This feature makes use of pre-processing directives to look for hardened,
142: alternative implementations of essential library calls. Some programs may fail
143: to build as a result; this usually happens for those trying too hard to be
144: portable, or otherwise abusing definitions in the standard library.
145:
146: This will require a modification to the program, or disabling this feature for
147: part or all of the build.
148:
149: ### Run-time crashes
150:
151: Just like with `PKGSRC_MKPIE` above, this feature may cause some programs to
152: crash, usually indicating an actual bug in the program. The fix will typically
153: involve patching the original program.
154:
1.28 khorben 155: ### Optimization is required
156:
157: At least in the case of GCC, FORTIFY will only be applied if optimization is
158: applied while compiling. This means that the CFLAGS should also contain -O, -O2
159: or another optimization level. This cannot easily be applied globally, as some
160: packages may require specific optimization levels.
161:
1.7 khorben 162: ## Problems with `PKGSRC_USE_RELRO`
163:
164: ### Performance impact
165:
166: For better protection, full RELRO requires every symbol to be resolved when the
1.11 khorben 167: program starts, rather than simply when required at run-time. This will have
168: more impact on programs using a lot of symbols, or linked to libraries exposing
169: a lot of symbols. Therefore, daemons or programs otherwise running in
170: background are affected only when started. Programs loading plug-ins at
171: run-time are affected when loading the plug-ins.
1.7 khorben 172:
173: The impact is not expected to be noticeable on modern hardware, except in some
174: cases for big programs.
175:
1.12 khorben 176: ### Run-time crashes
177:
178: Some programs handle plug-ins and dependencies in a way that conflicts with
179: RELRO: for instance, with an initialization routine listing any other plug-in
180: required. With full RELRO, the missing symbols are resolved before the
181: initialization routine can run, and the dynamic loader will not be able to find
182: them directly and abort as a result. Unfortunately, this is how Xorg loads its
183: drivers. Partial RELRO can be applied instead in this case.
184:
1.3 khorben 185: ## Problems with `PKGSRC_USE_SSP`
186:
187: ### Packages failing to build
188:
189: The stack-smashing protection provided by this option does not work for some
190: programs. The two most common situations in which this happens are:
191:
192: * the program makes use of the `alloca(3)` library call (memory allocator on the
193: stack)
194: * the program allocates variables on the stack, with the size determined at
195: run-time.
196:
197: Both cases will require a modification to the program, or disabling this feature
198: for part or all of the build.
199:
200: ### Run-time crashes
201:
1.4 khorben 202: Again, this feature may cause some programs to crash, usually indicating an
203: actual bug in the program. Patching the original program is then required.
1.3 khorben 204:
1.8 khorben 205: ### Performance impact
206:
207: The compiler emits extra code when using this feature: a check for buffer
208: overflows is performed when entering and exiting functions, requiring an extra
209: variable on the stack. The level of protection can otherwise be adjusted to
210: affect only those functions considered more sensitive by the compiler (with
211: `-fstack-protector` instead of `-fstack-protector-all`).
212:
213: The impact is not expected to be noticeable on modern hardware. However,
214: programs with a hard requirement to run at the fastest possible speed should
215: avoid using this feature, or using libraries built with this feature.
216:
1.5 khorben 217: # Auditing the system
218:
219: The illusion of security is worse than having no security at all. This section
220: lists a number of ways to ensure the security features requested are actually
221: effective.
222:
223: _These instructions were obtained and tested on a system derived from NetBSD 7
224: (amd64). YMMV._
225:
226: ## Checking for PIE
227:
228: The ELF executable type in use changes for binaries built as PIE; without:
229:
230: $ file /path/to/bin/ary
231: /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
232:
233: as opposed to the following binary, built as PIE:
234:
235: $ file /path/to/pie/bin/ary
236: /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
237:
238: The latter result is then what is expected.
239:
1.13 khorben 240: ## Checking for partial RELRO
1.5 khorben 241:
242: The following command should list a section called `RELRO`:
243:
244: $ objdump -p /path/to/bin/ary
245:
246: /path/to/bin/ary: file format elf64-x86-64
247:
248: Program Header:
249: [...]
250: RELRO off 0x0000000000000d78 vaddr 0x0000000000600d78 paddr 0x0000000000600d78 align 2**0
1.6 khorben 251:
1.17 khorben 252: This check is now performed automatically if `PKG_DEVELOPER` is set and `RELRO`
253: is enabled.
254:
1.13 khorben 255: ## Checking for full RELRO
256:
257: The dynamic loader will apply RELRO immediately when detecting the presence of
258: the `BIND_NOW` flag:
259:
260: $ objdump -x /path/to/bin/ary
261:
262: /path/to/bin/ary: file format elf64-x86-64
263:
264: Dynamic Section:
265: [...]
266: BIND_NOW 0x0000000000000000
267:
268: This has to be combined with partial RELRO (see above) to be fully efficient.
269:
1.6 khorben 270: ## Checking for SSP
271:
272: Building objects, binaries and libraries with SSP will affect the presence of
273: additional symbols in the resulting file:
274:
275: $ nm /path/to/bin/ary
276: [...]
277: U __stack_chk_fail
278: 0000000000600ea0 B __stack_chk_guard
279:
280: This is an indicator that the program was indeed built with support for SSP.
281:
1.10 khorben 282: # References
283:
284: * <http://tk-blog.blogspot.co.at/2009/02/relro-not-so-well-known-memory.html>
285:
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