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<title>Gentoo Linux Documentation
--
  Gentoo revdep-pax introduction</title>
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<br><h1>Gentoo revdep-pax introduction</h1>
<form name="contents" action="http://www.gentoo.org">
<b>Content</b>:
        <select name="url" size="1" OnChange="location.href=form.url.options[form.url.selectedIndex].value" style="font-family:sans-serif,Arial,Helvetica"><option value="#doc_chap1">1. What's revdep-pax about?</option>
<option value="#doc_chap2">2. Using revdep-pax</option></select>
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<p class="chaphead"><a name="doc_chap1"></a><span class="chapnum">1.
            </span>What's revdep-pax about?</p>
<p class="epigraph">
Since the early days of PaX it was known that all programs were equal although
some were more equal than others and needed an environment with less
restrictions in order to be able to run. Thus, in order to have a secure way of
allowing system administrators and users telling the system which binaries
needed this lessened environment the PaX marks were created.
<br><br><span class="episig">—Geroge Orwell</span><br><br></p>
<p class="secthead"><a name="doc_chap1_sect2">A quick introduction to PaX markings.</a></p>
<p>
There are some programs which won't be able to run in an environment with all
the PaX features enabled, for example you may have a program which has so called
<span class="emphasis">text relocations</span> or you may have a language interpreter doing JIT code
compilation and requiring <span class="emphasis">RWX</span> mappings you may also have a program that
saves data including internal pointers into an mmaped file and which needs to be
restored in the same place no matter what. You could also be holding a security
competition and need to disable the execution restrictions and force it to
use fixed addresses on a particular program so it can be exploited doing a
simple nop sled based stack overflow to get to the next level. For taking into
account these issues binaries can be marked to force on or off some of the PaX
features.
</p>
<p>
Currently, the PaX features that can be lessened or enforced to allow programs
to run are:
</p>
<dl>
  <dt><b>PAGEEXEC</b></dt>
  <dd>Paging based execution restrictions. This is what other OSes know as
  <span class="emphasis">NX</span>.</dd>
  <dt><b>EMUTRAMP</b></dt>
  <dd>Trampoline emulation. Required by for amongst other things code with
  nested functions.</dd>
  <dt><b>MPROTECT</b></dt>
  <dd>Prevents the introduction of new executable code in the task. This is the
  one you are more likely to need disabling with libraries generating JIT code.
  </dd>
  <dt><b>RANDMMAP</b></dt>
  <dd>Randomizes the addresses where mappings are made unless the program
  explicitly requests one (using the MAP_FIXED flag).</dd>
  <dt><b>RANDEXEC</b></dt>
  <dd>This flag is currently deprecated and was used to enforce random placement
  of the executable part of the binary.</dd>
  <dt><b>SEGMEXEC</b></dt>
  <dd>This flag enables segmentation based execution protection. This feature is
  not available on the amd64 architecture so in that architecture is disables by
  default.</dd>
</dl>
<p>
There are various ways in which this advice to lessen the environment can be
provided to the system, amongst others Mandatory Access Control rules, extended
attributes and two kinds of markings on the binaries themselves, the legacy ones
which abuse an unused field in the ELF headers and the new ones which add a new
specific section to the ELF file with the markings.
</p>
<p>
All this markings though are only read in the executable and not in the
libraries linked by it to prevent some possible attacks (like libraries being
injected via LD_PRELOAD) and because it eases a lot the implementation since the
kernel shouldn't be aware of linking details.
</p>
<p>
This system has a problem: if we have a binary linking to a library which
requires, for example, trampoline emulation because it uses nested functions how
can we make sure the binary gets the propper markings? Yeah we could add PaX
marks to the library to state it needs trampoline emulation but still we haven't
fixed the issue since the kernel will only read the marks on the binary being
called. In order to solve this issue we have created <span class="code" dir="ltr">revdep-pax</span>.
</p>
<p class="secthead"><a name="doc_chap1_sect3">What's revdep-pax?</a></p>
<p>
<span class="code" dir="ltr">revdep-pax</span> is a tool that allows to check for differences in PaX markings
between elf objects linking to libraries (for example <span class="path" dir="ltr">/bin/bash</span>)
and the libraries themselves (for example <span class="path" dir="ltr">/lib64/libc.so.6</span>).
</p>
<p>
<span class="code" dir="ltr">revdep-pax</span> is able to do this in various ways, it can check for
differences <span class="emphasis">forward</span> from one binary to all the libraries it links and it
can also check for PaX marking differences <span class="emphasis">backwards</span> from one library to
all the binaries linking to it (which may include other libraries too). In a
similar way it is possible to have all the forward and reverse mappings in the
system checked to try finding issues.
</p>
<p>
<span class="code" dir="ltr">revdep-pax</span> is also able to propagate these markings both forward to the
libraries linked by an object and backwards to the objects linked by a library.
</p>
<p class="chaphead"><a name="doc_chap2"></a><span class="chapnum">2.
            </span>Using revdep-pax</p>
<p class="epigraph">
In order to witness the firepower of this fully ARMED and OPERATIONAL tool
you'll first need to learn how to use it, once you are done, you'll be
able to fire at will.
<br><br><span class="episig">—The Emperor</span><br><br></p>
<p class="secthead"><a name="doc_chap2_sect2">Propagating PaX marks backwards from a library to objects that link at it
</a></p>
<p>
This is going to be probably the main way in which you are going to use this
utility. What it does is check all the libraries linked statically by the
binaries using <span class="code" dir="ltr">ldd</span> and then smartly add the paxmarks of those libraries
to generate the new set. As a result if <span class="path" dir="ltr">/usr/games/bin/armagetronad</span>
links with <span class="path" dir="ltr">/usr/lib64/libGL.so.1</span> which has the <span class="emphasis">-m</span> PaX mark
(allow RWX mappings) because you are using a llvm requiring graphics driver
you'll get that binary marked with the <span class="emphasis">-m</span> PaX mark too since it needs it.
Below you can see how to run it.
</p>
<a name="doc_chap2_pre1"></a><table class="ntable" width="100%" cellspacing="0" cellpadding="0" border="0">
<tr><td bgcolor="#7a5ada"><p class="codetitle">Code Listing2.1: Propagating the PaX marks from the libraries</p></td></tr>
<tr><td bgcolor="#eeeeff" align="left" dir="ltr"><pre>
# <span class="code-input">mv /etc/grsec/learning.roles /etc/grsec/policy</span>
# <span class="code-input">chmod 0600 /etc/grsec/policy</span>
</pre></td></tr>
</table>
<table class="ncontent" width="100%" border="0" cellspacing="0" cellpadding="0"><tr><td bgcolor="#bbffbb"><p class="note"><b>Note: </b>
Due to the way in which <span class="code" dir="ltr">ldd</span> works you'll get all the libraries required
at runtime, even those required by libraries you link to (and so on recursively)
as a result <span class="code" dir="ltr">revdep-pax</span> will detect all dependencies in a single pass. If
the behaviour of <span class="code" dir="ltr">ldd</span> changes so may change the behaviour of
<span class="code" dir="ltr">revdep-pax</span>
</p></td></tr></table>
<p>
The <span class="code" dir="ltr">scanelf</span> application is part of the <span class="code" dir="ltr">app-misc/pax-utils</span> package.
With this application you can print out information specific to the ELF
structure of a binary. The following table sums up the various options.
</p>
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<tr><td class="topsep" align="center"><p class="alttext">Page updated February 19, 2012</p></td></tr>
<tr><td class="topsep" align="left"><p class="alttext"><b>Summary: </b>
This guide provides an introduction to revdep-pax and how to use it to propagate
the PaC markings caused by libraries requiring them, for example, libraries
requiring RWX memory in order to process JIT code.
</p></td></tr>
<tr><td align="left" class="topsep"><p class="alttext">
  <a href="mailto:klondike@gentoo.org" class="altlink"><b>Francisco Blas Izquierdo Riera</b></a>
<br><i>Author</i><br></p></td></tr>
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