Annotation of wikisrc/pkgsrc/hardening.mdwn, revision 1.46
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:
1.46 ! leot 32: **TODO**: Explain FORTIFY_SOURCE 1 vs 2, and which is used. Give a link
1.25 khorben 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:
1.43 leot 168: This will require a modification to the program, or disabling this feature
169: by adding in the package `Makefile`:
170:
171: FORTIFY_SUPPORTED= no
1.4 khorben 172:
173: ### Run-time crashes
174:
175: Just like with `PKGSRC_MKPIE` above, this feature may cause some programs to
176: crash, usually indicating an actual bug in the program. The fix will typically
177: involve patching the original program.
178:
1.28 khorben 179: ### Optimization is required
180:
181: At least in the case of GCC, FORTIFY will only be applied if optimization is
182: applied while compiling. This means that the CFLAGS should also contain -O, -O2
183: or another optimization level. This cannot easily be applied globally, as some
184: packages may require specific optimization levels.
185:
1.7 khorben 186: ## Problems with `PKGSRC_USE_RELRO`
187:
188: ### Performance impact
189:
190: For better protection, full RELRO requires every symbol to be resolved when the
1.11 khorben 191: program starts, rather than simply when required at run-time. This will have
192: more impact on programs using a lot of symbols, or linked to libraries exposing
193: a lot of symbols. Therefore, daemons or programs otherwise running in
194: background are affected only when started. Programs loading plug-ins at
195: run-time are affected when loading the plug-ins.
1.7 khorben 196:
197: The impact is not expected to be noticeable on modern hardware, except in some
198: cases for big programs.
199:
1.12 khorben 200: ### Run-time crashes
201:
202: Some programs handle plug-ins and dependencies in a way that conflicts with
203: RELRO: for instance, with an initialization routine listing any other plug-in
204: required. With full RELRO, the missing symbols are resolved before the
205: initialization routine can run, and the dynamic loader will not be able to find
206: them directly and abort as a result. Unfortunately, this is how Xorg loads its
207: drivers. Partial RELRO can be applied instead in this case.
208:
1.3 khorben 209: ## Problems with `PKGSRC_USE_SSP`
210:
211: ### Packages failing to build
212:
213: The stack-smashing protection provided by this option does not work for some
214: programs. The two most common situations in which this happens are:
215:
216: * the program makes use of the `alloca(3)` library call (memory allocator on the
1.44 leot 217: stack) (**TODO**: at least regarding build failure, this was a problem only
218: with older gcc version and probably do not apply in most common pkgsrc setup
219: (i.e. no PR/mails about that seen in the last years about that.))
1.3 khorben 220: * the program allocates variables on the stack, with the size determined at
221: run-time.
222:
223: Both cases will require a modification to the program, or disabling this feature
1.42 leot 224: by adding in the package `Makefile`:
225:
226: SSP_SUPPORTED= no
1.3 khorben 227:
228: ### Run-time crashes
229:
1.40 leot 230: Again, this feature may cause some programs to crash via a SIGABRT,
231: usually indicating an actual bug in the program.
232:
233: On NetBSD `LOG_CRIT` level `syslog()` messages are sent and - by
234: default - appended to `/var/log/messages`, e.g.:
235:
1.41 leot 236: Jan 6 15:42:51 <hostname> -: <hostname> <program> - - - buffer overflow detected; terminated
1.40 leot 237:
238: (where `<hostname>` is the `hostname(1)` and `<program>` is the
239: `basename(1)` of the program crashed).
240:
241: Patching the original program is then required.
242:
243: Rebuilding the package via:
244:
245: % env CFLAGS=-g INSTALL_UNSTRIPPED=yes make replace
246:
247: and inspecting the `backtrace` of the coredump via the debugger
248: should point out the problematic call by inspecting the frame
249: calling the `_chk()' (SSP) function.
1.3 khorben 250:
1.8 khorben 251: ### Performance impact
252:
253: The compiler emits extra code when using this feature: a check for buffer
254: overflows is performed when entering and exiting functions, requiring an extra
255: variable on the stack. The level of protection can otherwise be adjusted to
256: affect only those functions considered more sensitive by the compiler (with
257: `-fstack-protector` instead of `-fstack-protector-all`).
258:
259: The impact is not expected to be noticeable on modern hardware. However,
260: programs with a hard requirement to run at the fastest possible speed should
261: avoid using this feature, or using libraries built with this feature.
262:
1.5 khorben 263: # Auditing the system
264:
265: The illusion of security is worse than having no security at all. This section
266: lists a number of ways to ensure the security features requested are actually
267: effective.
268:
269: _These instructions were obtained and tested on a system derived from NetBSD 7
270: (amd64). YMMV._
271:
272: ## Checking for PIE
273:
274: The ELF executable type in use changes for binaries built as PIE; without:
275:
276: $ file /path/to/bin/ary
277: /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
278:
279: as opposed to the following binary, built as PIE:
280:
281: $ file /path/to/pie/bin/ary
282: /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
283:
284: The latter result is then what is expected.
285:
1.13 khorben 286: ## Checking for partial RELRO
1.5 khorben 287:
288: The following command should list a section called `RELRO`:
289:
290: $ objdump -p /path/to/bin/ary
291:
292: /path/to/bin/ary: file format elf64-x86-64
293:
294: Program Header:
295: [...]
296: RELRO off 0x0000000000000d78 vaddr 0x0000000000600d78 paddr 0x0000000000600d78 align 2**0
1.6 khorben 297:
1.17 khorben 298: This check is now performed automatically if `PKG_DEVELOPER` is set and `RELRO`
299: is enabled.
300:
1.13 khorben 301: ## Checking for full RELRO
302:
303: The dynamic loader will apply RELRO immediately when detecting the presence of
304: the `BIND_NOW` flag:
305:
306: $ objdump -x /path/to/bin/ary
307:
308: /path/to/bin/ary: file format elf64-x86-64
309:
310: Dynamic Section:
311: [...]
312: BIND_NOW 0x0000000000000000
313:
314: This has to be combined with partial RELRO (see above) to be fully efficient.
315:
1.6 khorben 316: ## Checking for SSP
317:
318: Building objects, binaries and libraries with SSP will affect the presence of
319: additional symbols in the resulting file:
320:
321: $ nm /path/to/bin/ary
322: [...]
323: U __stack_chk_fail
324: 0000000000600ea0 B __stack_chk_guard
325:
326: This is an indicator that the program was indeed built with support for SSP.
1.35 khorben 327:
328: This check is now performed automatically (where supported) if `PKG_DEVELOPER`
329: is set and `SSP` is enabled.
1.45 leot 330:
331: If it is needed to disable SSP check per-package, please add in the package
332: `Makefile`:
333:
334: CHECK_SSP_SUPPORTED= no
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