Diff for /wikisrc/pkgsrc/hardening.mdwn between versions 1.40 and 1.49

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 [[!meta title="Hardening pkgsrc"]]  This page has been moved to [the pkgsrc guide](//www.NetBSD.org/docs/pkgsrc/hardening.html).
   
 A number of mechanisms are available in  
 [pkgsrc](https://www.pkgsrc.org/) to improve the security of the  
 resulting system. This page describes the mechanisms, and gives hints  
 about detecting and fixing problems.  
   
 # Mechanisms  
   
 Mechanisms can be enabled individually in `mk.conf`, and are  
 individually described below. They are sorted by whether they are  
 enabled by default, and then by their ordering in `mk/defaults/mk.conf`.  
   
 Typically, a feature will cause some programs to fail to build or work  
 when first enabled. This can be due to latent problems in the  
 program, and can be due to other reasons. After enough testing to  
 have confidence that user problems will be quite rare, individual  
 mechanisms will be enabled by default.  
   
 For each mechanism, see the Caveats section below for an explanation  
 of what might go wrong at compile time and at run time, and how to  
 notice and address these problems.  
   
 ## Enabled by default  
   
 ### PKGSRC_USE_FORTIFY  
   
 This allows substitute wrappers to be used for some commonly used  
 library functions that do not have built-in bounds checking - but  
 could in some cases.  
   
 TODO: Explain FORTIFY_SOURCE 1 vs 2, and which is used. Give a link  
 to a good explanation of the technique. Explain if this is gcc specific.  
   
 It has been enabled by default since pkgsrc-2017Q3.  
   
 ### PKGSRC_USE_SSP  
   
 This enables a stack-smashing protection mitigation. It is done by adding a  
 guard variable to functions with vulnerable objects. The guards are initialized  
 when a function is entered and then checked when the function exits. The guard  
 check will fail and the program forcibly exited if the variable was modified in  
 the meantime. This can happen in case of buffer overflows or memory corruption,  
 and therefore exposing these bugs.  
   
 Different mitigation levels are available:  
   
 * the default ("yes"), which will only protect functions considered vulnerable  
   by the compiler;  
 * "all", which will protect every function;  
 * "strong", which will apply a better balance between the two settings above.  
   
 This mitigation is supported by both GCC and clang. It may be supported in  
 additional compilers, possibly under a different name. It is particularly useful  
 for unsafe programming languages, such as C/C++.  
   
 It is enabled by default where known supported since pkgsrc-2017Q3.  
   
 More details can be found here:  
   
 * <https://en.wikipedia.org/wiki/Buffer_overflow_protection>  
   
 ## Not enabled by default  
   
 ### PKGSRC_MKPIE  
   
 This requests the creation of PIE (Position Independent Executables) for all  
 executables. The PIE mechanism is normally used for shared libraries, so that  
 they can be loaded at differing addresses at runtime. PIE itself does not have  
 useful security properties; however, it is necessary to fully leverage some,  
 such as ASLR.  Some operating systems support Address Space Layout Randomization  
 (ASLR), which causes different addresses to be used each time a program is run.  
 This makes it more difficult for an attacker to guess addresses and thus makes  
 exploits harder to construct. With PIE, ASLR can really be applied to the entire  
 program, instead of the stack and heap only.  
   
 PIE executables will only be built for toolchains that are known to support PIE.  
 Currently, this means NetBSD on amd64 and i386.  
   
 ### PKGSRC_MKREPRO  
   
 With this option, pkgsrc will try to build packages reproducibly. This allows  
 packages built from the same tree and with the same options, to produce  
 identical results bit by bit. This option should be combined with ASLR and  
 `PKGSRC_MKPIE` to avoid predictable address offsets for attackers attempting to  
 exploit security vulnerabilities.  
   
 More details can be found here:  
   
 * <https://reproducible-builds.org/>  
   
 ### PKGSRC_USE_RELRO  
   
 This also makes the exploitation of some security vulnerabilities more  
 difficult in some cases.  
   
 Two different mitigation levels are available:  
   
 * partial: the ELF sections are reordered so that internal data sections  
   precede the program's own data sections, and non-PLT GOT is read-only;  
 * full: in addition to partial RELRO, every relocation is performed immediately  
   when starting the program (with a slight performance impact), allowing the  
   entire GOT to be read-only.  
   
 This is currently supported by GCC. Many software distributions now enable this  
 feature by default, at the "partial" level.  
   
 More details can be found here:  
   
 * <http://tk-blog.blogspot.co.at/2009/02/relro-not-so-well-known-memory.html>  
   
 ### PKGSRC_USE_STACK_CHECK  
   
 This uses `-fstack-check` with GCC for another stack protection mitigation.  
   
 It asks the compiler to generate code verifying that it does not corrupt the  
 stack. According to GCC's manual page, this is really only useful for  
 multi-threaded programs.  
   
 # Caveats  
   
 ## Problems with `PKGSRC_MKPIE`  
   
 ### Recent support for cwrappers  
   
 `PKGSRC_MKPIE` is only supported by `pkgtools/cwrappers` from the 2017Q3  
 release on (`USE_CWRAPPERS` in `mk.conf`).  
   
 ### Packages failing to build  
   
 A number of packages may fail to build with this option enabled. The failures  
 are often related to the absence of the `-fPIC` compilation flag when building  
 libraries or executables (or ideally `-fPIE` in the latter case). This flag is  
 added to the `CFLAGS` already, but requires the package to actually support it.  
   
 #### How to fix  
   
 These instructions are meant as a reference only; they likely need to be adapted  
 for many packages individually.  
   
 For packages using `Makefiles`:  
   
     MAKE_FLAGS+=        CFLAGS=${CFLAGS:Q}  
     MAKE_FLAGS+=        LDFLAGS=${LDFLAGS:Q}  
   
 For packages using `Imakefiles`:  
   
     MAKE_FLAGS+=        CCOPTIONS=${CFLAGS:Q}  
     MAKE_FLAGS+=        LOCAL_LDFLAGS=${LDFLAGS:Q}  
   
 ### Run-time crashes  
   
 Some programs may fail to run, or crash at random times once built as PIE. Two  
 scenarios are essentially possible:  
   
 * actual bug in the program crashing, exposed thanks to ASLR/mprotect;  
 * bug in the implementation of ASLR/mprotect in the Operating System.  
   
 ## Problems with `PKGSRC_USE_FORTIFY`  
   
 ### Packages failing to build  
   
 This feature makes use of pre-processing directives to look for hardened,  
 alternative implementations of essential library calls. Some programs may fail  
 to build as a result; this usually happens for those trying too hard to be  
 portable, or otherwise abusing definitions in the standard library.  
   
 This will require a modification to the program, or disabling this feature for  
 part or all of the build.  
   
 ### Run-time crashes  
   
 Just like with `PKGSRC_MKPIE` above, this feature may cause some programs to  
 crash, usually indicating an actual bug in the program. The fix will typically  
 involve patching the original program.  
   
 ### Optimization is required  
   
 At least in the case of GCC, FORTIFY will only be applied if optimization is  
 applied while compiling. This means that the CFLAGS should also contain -O, -O2  
 or another optimization level. This cannot easily be applied globally, as some  
 packages may require specific optimization levels.  
   
 ## Problems with `PKGSRC_USE_RELRO`  
   
 ### Performance impact  
   
 For better protection, full RELRO requires every symbol to be resolved when the  
 program starts, rather than simply when required at run-time. This will have  
 more impact on programs using a lot of symbols, or linked to libraries exposing  
 a lot of symbols. Therefore, daemons or programs otherwise running in  
 background are affected only when started. Programs loading plug-ins at  
 run-time are affected when loading the plug-ins.  
   
 The impact is not expected to be noticeable on modern hardware, except in some  
 cases for big programs.  
   
 ### Run-time crashes  
   
 Some programs handle plug-ins and dependencies in a way that conflicts with  
 RELRO: for instance, with an initialization routine listing any other plug-in  
 required. With full RELRO, the missing symbols are resolved before the  
 initialization routine can run, and the dynamic loader will not be able to find  
 them directly and abort as a result. Unfortunately, this is how Xorg loads its  
 drivers. Partial RELRO can be applied instead in this case.  
   
 ## Problems with `PKGSRC_USE_SSP`  
   
 ### Packages failing to build  
   
 The stack-smashing protection provided by this option does not work for some  
 programs. The two most common situations in which this happens are:  
   
 * the program makes use of the `alloca(3)` library call (memory allocator on the  
   stack)  
 * the program allocates variables on the stack, with the size determined at  
   run-time.  
   
 Both cases will require a modification to the program, or disabling this feature  
 for part or all of the build.  
   
 ### Run-time crashes  
   
 Again, this feature may cause some programs to crash via a SIGABRT,  
 usually indicating an actual bug in the program.  
   
 On NetBSD `LOG_CRIT` level `syslog()` messages are sent and - by  
 default - appended to `/var/log/messages`, e.g.:  
   
    Jan  6 15:42:51 <hostname> -: <hostname> <program> - - - buffer overflow detected; terminated  
   
 (where `<hostname>` is the `hostname(1)` and `<program>` is the  
 `basename(1)` of the program crashed).  
   
 Patching the original program is then required.  
   
 Rebuilding the package via:  
   
     % env CFLAGS=-g INSTALL_UNSTRIPPED=yes make replace  
   
 and inspecting the `backtrace` of the coredump via the debugger  
 should point out the problematic call by inspecting the frame  
 calling the `_chk()' (SSP) function.  
   
 ### Performance impact  
   
 The compiler emits extra code when using this feature: a check for buffer  
 overflows is performed when entering and exiting functions, requiring an extra  
 variable on the stack. The level of protection can otherwise be adjusted to  
 affect only those functions considered more sensitive by the compiler (with  
 `-fstack-protector` instead of `-fstack-protector-all`).  
   
 The impact is not expected to be noticeable on modern hardware. However,  
 programs with a hard requirement to run at the fastest possible speed should  
 avoid using this feature, or using libraries built with this feature.  
   
 # Auditing the system  
   
 The illusion of security is worse than having no security at all. This section  
 lists a number of ways to ensure the security features requested are actually  
 effective.  
   
 _These instructions were obtained and tested on a system derived from NetBSD 7  
 (amd64). YMMV._  
   
 ## Checking for PIE  
   
 The ELF executable type in use changes for binaries built as PIE; without:  
   
     $ file /path/to/bin/ary  
     /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  
   
 as opposed to the following binary, built as PIE:  
   
     $ file /path/to/pie/bin/ary  
     /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  
   
 The latter result is then what is expected.  
   
 ## Checking for partial RELRO  
   
 The following command should list a section called `RELRO`:  
   
     $ objdump -p /path/to/bin/ary  
   
     /path/to/bin/ary:     file format elf64-x86-64  
   
     Program Header:  
     [...]  
        RELRO off    0x0000000000000d78 vaddr 0x0000000000600d78 paddr 0x0000000000600d78 align 2**0  
   
 This check is now performed automatically if `PKG_DEVELOPER` is set and `RELRO`  
 is enabled.  
   
 ## Checking for full RELRO  
   
 The dynamic loader will apply RELRO immediately when detecting the presence of  
 the `BIND_NOW` flag:  
   
     $ objdump -x /path/to/bin/ary  
   
     /path/to/bin/ary:     file format elf64-x86-64  
   
     Dynamic Section:  
     [...]  
       BIND_NOW             0x0000000000000000  
   
 This has to be combined with partial RELRO (see above) to be fully efficient.  
   
 ## Checking for SSP  
   
 Building objects, binaries and libraries with SSP will affect the presence of  
 additional symbols in the resulting file:  
   
     $ nm /path/to/bin/ary  
     [...]  
                      U __stack_chk_fail  
     0000000000600ea0 B __stack_chk_guard  
   
 This is an indicator that the program was indeed built with support for SSP.  
   
 This check is now performed automatically (where supported) if `PKG_DEVELOPER`  
 is set and `SSP` is enabled.  

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