Attachment 36941 Details for Bug 59164 – Updated pax-howto.xml

Updated pax-howto.xml

pax-howto.xml (text/plain), 7.77 KB, created by Sven Vermeulen (RETIRED) on 2004-08-07 05:44:34 UTC

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Creator: Sven Vermeulen (RETIRED)

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><?xml version='1.0' encoding="UTF-8"?>
>
>
><guide link="/proj/en/hardened/docs/pax-howto.xml">
><title>Hardened Gentoo PaX Quickstart</title>
>
><author title="Author">
>  <mail link="tseng@gentoo.org">Brandon Hale</mail>
></author>
><author title="Editor">
>  <mail link="blackace@gentoo.org">Blackace</mail>
></author>
>
>
>
><license/>
>
><abstract>
>A quickstart covering PaX and Hardened Gentoo.
></abstract>
>
><version>1.1</version>
><date>August 07, 2004</date>
>
><chapter>
><title>What is Hardened Gentoo?</title>
><section>
><body>
>
><p>
>Hardened Gentoo is a project interested in the hardening of a Gentoo system.
>Several different solutions are supported by us and there is a fair bit of
>flexibility to create your own setup. At the heart of Hardened Gentoo is
><e>PaX</e>.
></p>
>
></body>
></section>
></chapter>
>
><chapter>
><title>What is PaX?</title>
><section>
><body>
>
><p>
>PaX is a patch to the Linux kernel that provides hardening in two ways.
></p>
>
><p>
>The first, <e>ASLR</e> (Address Space Layout Randomization) provides a means to
>randomize the addressing scheme of all data loaded into memory. When an
>application is built as a <e>PIE</e> (Position Independent Executable), PaX is
>able to also randomize the addresses of the application base in addition.
></p>
>
><p>
>The second protection provided by PaX is non-executable memory. This prevents a
>common form of attack where executable code is inserted into memory by an
>attacker. More information on PaX can be found throughout this guide, but the
>homepage can be found at <uri>http://pax.grsecurity.net</uri>.
></p>
>
></body>
></section>
></chapter>
>
><chapter>
><title>An Introduction to PIE and SSP</title>
><section>
><body>
>
><p>
>As mentioned above, PaX is complemented by PIE. This method of building
>executables stores information needed to relocate parts of the executable in
>memory, hence the name <e>Position Independent</e>.
></p>
>
><p>
><e>SSP</e> (Stack Smashing Protector) is a second complementary technology we
>introduce at executable build time. SSP was originally introduced by IBM under
>the name <e>ProPolice</e>. It modifies the C compiler to insert initialization 
>code into functions that create a buffer in memory. 
></p>
>
><note>
>In newer versions of SSP, it is possible to apply SSP to all functions,
>adding protection to functions whose buffer would normally be below the size 
>limit for SSP. This is enabled via the CFLAG -fstack-protector-all.
></note>
>
><p>
>At run time, when a buffer is created, SSP adds a secret random value, the 
>canary, to the end of the buffer. When the function returns, SSP makes sure 
>that the canary is still intact. If an attacker were to perform a buffer 
>overflow, he would overwrite this value and trigger that stack smashing 
>handler. Currently this kills the target process.
></p>
>
><p>
><uri link="http://www.trl.ibm.com/projects/security/ssp/">Further reading on 
>SSP.</uri>
></p>
>
></body>
></section>
></chapter>
>
><chapter>
><title>Building a PaX-enabled Kernel</title>
><section>
><body>
>
><p>
>Several Gentoo kernel trees are already patched with PaX.
></p>
>
><p>
>For 2.4 based machines, the recommended kernels are <c>hardened-sources</c> or
><c>grsec-sources</c>. For 2.6 machines, <c>hardened-dev-sources</c> are
>recommended.
></p>
>
><p>
>Grab one of the recommended source trees, or apply the appropriate patch from
><uri>http://pax.grsecurity.net</uri> to your own tree and configure it as you
>normally would for the target machine.
></p>
>
><p>
>In <c>Security Options -&gt; PaX</c>, apply the options as shown below.
></p>
>
><pre caption="Kernel configuration">
>[*] Enable various PaX features
>
>PaX Control -&gt;
>
> [ ] Support soft mode
> [*] Use legacy ELF header marking
> [*] Use ELF program header marking
>     MAC system integration (none)  ---&gt;
>
>Non-executable page -&gt;
>
> [*] Enforce non-executable pages
> [*]   Paging based non-executable pages
> [*]   Segmentation based non-executable pages
> [*] Emulate trampolines
> [*] Restrict mprotect()
> [ ]   Disallow ELF text relocations
>
>Address Space Layout Randomization -&gt;
>
> [*] Address Space Layout Randomization
> [*]   Randomize kernel stack base
> [*]   Randomize user stack base
> [*]   Randomize mmap() base
> [*]     Randomize ET_EXEC base
></pre>
>
><p>
>Build this kernel as you normally would and install it to <path>/boot</path>.
></p>
>
></body>
></section>
></chapter>
>
><chapter>
><title>Building a PIE/SSP Enabled Userland</title>
><section>
><body>
>
><p>
>Hardened Gentoo has added support for transparent PIE/SSP building via GCC's
>specfile. This means that any users upgrading an older Hardened install should
>remove any LDFLAGS or CFLAGS used to trigger PIE/SSP. Also, the 
><c>hardened-gcc</c> package is now deprecated and should be unmerged 
>(version 5.0 is a dummy package). To get the current GCC, add 
><c>USE="hardened"</c> to <path>/etc/make.conf</path>.
></p>
>
><p>
>To maintain a consistant toolchain, first <c>emerge bintuils gcc glibc</c>.
>Next, rebuild the entire system with <c>emerge -e world</c>. All future packages
>will be built with PIE/SSP.
></p>
>
><warn>
>Both PIE and SSP are known to cause issues with some packages. If you come
>across a package that fails to compile, please file a bug report including a log
>of the failed compile and the output of <c>emerge info</c> to
><uri>http://bugs.gentoo.org/</uri>.
></warn>
>
></body>
></section>
></chapter>
>
><chapter>
><title>When Things Misbehave (PaX Control)</title>
><section>
><body>
>
><p>
>Some legitimate applications will attempt to generate code at run time which is
>executed out of memory. Naturally, PaX does not allow this and it will promptly
>kill the offending application.
></p>
>
><note>
>The most notable of these applications are XFree, mplayer and multimedia tools
>based on xine-lib. The easiest way around these problems are to disable PaX
>protections.
></note>
>
><p>
>Luckily there is a utility to toggle protections on a per-executable basis,
><e>paxctl</e>. As with any other package in Gentoo, install paxctl with the
>command <c>emerge paxctl</c>. Usage is show by <c>paxctl -h</c>.
></p>
>
><note>
>If you have an older version of binutils, you will need to use <e>chpax</e>,
>which edits the old-style PaX markings. Usage of chpax is largely the same as
>paxctl. This also requires legacy marking support built into your kernel.
></note>
>
><pre caption="paxctl -h">
>usage: paxctl &lt;options&gt; &lt;files&gt;
>
>options:
>        -p: disable PAGEEXEC            -P: enable PAGEEXEC
>        -e: disable EMUTRMAP            -E: enable EMUTRMAP
>        -m: disable MPROTECT            -M: enable MPROTECT
>        -r: disable RANDMMAP            -R: enable RANDMMAP
>        -x: disable RANDEXEC            -X: enable RANDEXEC
>        -s: disable SEGMEXEC            -S: enable SEGMEXEC
>
>        -v: view flags                  -z: restore default flags
>        -q: suppress error messages     -Q: report flags in short format flags
></pre>
>
><p>
>The first option we will note is <c>-v</c>, which can display flags set on a
>particular binary.
></p>
>
><pre caption="paxctl -v">
>y0shi brandon # paxctl -v /usr/X11R6/bin/XFree86
>PaX control v0.2
>Copyright 2004 PaX Team &lt;pageexec@freemail.hu&gt;
>
>- PaX flags: -p-sM--x-eR- [/usr/X11R6/bin/XFree86]
>        PAGEEXEC is disabled
>        SEGMEXEC is disabled
>        MPROTECT is enabled
>        RANDEXEC is disabled
>        EMUTRAMP is disabled
>        RANDMMAP is enabled
></pre>
>
><p>
>This shows an XFree binary with all protections disabled.
></p>
>
><p>
>To set flags on a binary, the <c>-z</c> flag is useful as it restores the
>default flags.
></p>
>
><p>
>To disable protections on XFree, run
><c>paxctl -zpeMRxs /usr/X11R6/bin/XFree86</c>.
></p>
>
><p>
>Play around with disabling/enabling protections to see what is the least needed
>to run.
></p>
>
></body>
></section>
></chapter>
></guide>

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