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Please visit our
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</font></b>
<hr/><section id="nacl-sfi-model-on-x86-64-systems">
<h1 id="nacl-sfi-model-on-x86-64-systems">NaCl SFI model on x86-64 systems</h1>
<div class="contents local" id="contents" style="display: none">
<ul class="small-gap">
<li><a class="reference internal" href="#summary" id="id5">Summary</a></li>
<li><a class="reference internal" href="#binary-format" id="id6">Binary Format</a></li>
<li><a class="reference internal" href="#runtime-invariants" id="id7">Runtime Invariants</a></li>
<li><a class="reference internal" href="#text-segment-rules" id="id8">Text Segment Rules</a></li>
<li><a class="reference internal" href="#list-of-pseudo-instructions" id="id9">List of Pseudo-instructions</a></li>
</ul>
</div><h2 id="summary">Summary</h2>
<p>This document addresses the details of the Software Fault Isolation
(SFI) model for executable code that can be run in Native Client on an
x86-64 system. An overview of this model can be found in the paper:
<a class="reference external" href="https://research.google.com/pubs/archive/35649.pdf">Adapting Software Fault Isolation to Contemporary CPU Architectures</a>.
The primary focus of the SFI model is a Windows x86-64 system but the
same techniques can be applied to run identical x86-64 binaries on
other x86-64 systems such as Linux, Mac, FreeBSD, etc, so the
description of the SFI model tries to abstract away system
dependencies when possible.</p>
<p>Please note: throughout this document we use the AT&T notation for
assembler syntax, in which the target operand appears last, e.g. <code>mov
src, dst</code>.</p>
<h2 id="binary-format">Binary Format</h2>
<p>The format of Native Client executable binaries is identical to the
x86-64 ELF binary format (<a class="reference external" href="http://en.wikipedia.org/wiki/Executable_and_Linkable_Format">[0]</a>, <a class="reference external" href="http://www.sco.com/developers/devspecs/gabi41.pdf">[1]</a>, <a class="reference external" href="http://www.sco.com/developers/gabi/latest/contents.html">[2]</a>, <a class="reference external" href="http://downloads.openwatcom.org/ftp/devel/docs/elf-64-gen.pdf">[3]</a>) for
Linux or BSD with a few extra requirements. The additional rules that
a Native Client ELF binary must follow are:</p>
<ul class="small-gap">
<li>The ELF magic OS ABI field must be 123.</li>
<li>The ELF magic OS ABI VERSION field must be 5.</li>
<li>The ELF e_flags field must be 0x200000 (32-byte alignment).</li>
<li>There must be exactly one PT_LOAD text segment. It must begin at
0x20000 (128 kB) and be marked RX (no W). The contents of the text
segment must follow <a class="reference internal" href="#x86-64-text-segment-rules"><em>Text Segment Rules</em></a>.</li>
<li>There can be at most one PT_LOAD data segment marked R.</li>
<li>There can be at most one PT_LOAD data segment marked RW.</li>
<li>There can be at most one PT_GNU_STACK segment. It must be marked RW.</li>
<li>All segments must end before limit address (4 GiB).</li>
</ul>
<h2 id="runtime-invariants">Runtime Invariants</h2>
<p>To ensure fault isolation at runtime, the system must maintain a
number of runtime <em>invariants</em> across the lifetime of the running
program. Both the <em>Validator</em> and the <em>Service Runtime</em> are
responsible for maintaining the invariants. See the paper for the
rationale for the invariants:</p>
<ul class="small-gap">
<li><code>RIP</code> always points to valid instruction boundary (the validator must
ensure this with direct jumps and direct calls).</li>
<li><code>R15</code> (aka <code>RBASE</code> and <code>RZP</code>) is never modified by code (the
validator must ensure this). Low 32 bits of <code>RZP</code> are all zero
(loader must ensure this).</li>
<li><code>RIP</code>, <code>RBP</code> and <code>RSP</code> are always in the <strong>safe zone</strong>: between
<code>R15</code> and <code>R15+4GiB</code>.</li>
</ul>
<blockquote>
<div><ul class="small-gap">
<li>Exception: <code>RSP</code> and <code>RBP</code> are allowed to be in the range of
<code>0..4GiB</code> inside <em>pseudo-instructions</em>: <code>naclrestbp</code>,
<code>naclrestsp</code>, <code>naclspadj</code>, <code>naclasp</code>, <code>naclssp</code>.</li>
</ul>
</div></blockquote>
<ul class="small-gap">
<li>84GiB are allocated for NaCl module (i.e. <strong>untrusted region</strong>):</li>
</ul>
<blockquote>
<div><ul class="small-gap">
<li><code>R15-40GiB..R15</code> and <code>R15+4GIB..R15+44GiB</code> are buffer zones with
PROT_NONE flags.</li>
<li>The 4GB <em>safe zone</em> has pages with either PROT_WRITE or PROT_EXEC
but must not have PROT_WRITE+PROT_EXEC pages.</li>
<li>All executable code in PROT_EXEC pages is validatable and
guaranteed to obey the invariant.</li>
</ul>
</div></blockquote>
<ul class="small-gap">
<li>Trampoline/springboard code is mapped to a non-writable region in
the <em>untrusted 84GB region</em>; each trampoline/springboard is 32-byte
aligned and fits within a single <em>bundle</em>.</li>
<li>The OS must not put any internal structures/code into the untrusted
region at any time (not using OS dynamic linker, etc)</li>
</ul>
<h2 id="text-segment-rules"><span id="x86-64-text-segment-rules"></span>Text Segment Rules</h2>
<ul class="small-gap">
<li>The validation process must ensure that the text segment complies
with the following rules. The validation process must complete
successfully strictly before executing any instruction of the
untrusted code.</li>
<li>The following instructions are illegal and must be rejected by the
validator (the list is not exhaustive as the validator uses a
whiteist, not a blacklist; this means there is a large but finite
list of instructions the validator allows, not a small list of
instructions the validator rejects):</li>
</ul>
<blockquote>
<div><ul class="small-gap">
<li>any privileged instructions</li>
<li><code>mov</code> to/from segment registers</li>
<li><code>int</code></li>
<li><code>pusha</code>/<code>popa</code> (not dangerous but not needed for GCC)</li>
</ul>
</div></blockquote>
<ul class="small-gap">
<li>There must be space for at least 32 bytes after the text segment and
before the next segment in ELF (towards higher addresses) that ends
strictly at a 64K boundary (a minimum page size for untrusted
code). This space will be padded with HLT instructions as part of
the validation process, along with the optional 64K page.</li>
<li>Neither instructions nor <em>pseudo-instructions</em> are permitted to span
a 32-byte boundary.</li>
<li>The ELF entry address must be 32-byte aligned.</li>
<li>Direct <code>CALL</code>/<code>JUMP</code> targets:</li>
</ul>
<blockquote>
<div><ul class="small-gap">
<li>must point to a valid instruction boundary</li>
<li>must not point into a <em>pseudo-instruction</em></li>
<li>must not point between a <em>restricted register</em> (see below for
definition) producer instruction and its corresponding restricted
register consumer instruction.</li>
</ul>
</div></blockquote>
<ul class="small-gap">
<li><code>CALL</code> instructions must be 5 bytes before a 32-byte boundary, so
that the return address will be 32-byte aligned.</li>
<li>Indirect call targets must be 32-byte aligned. Instead of indirect
<code>CALL</code>/<code>JMP</code> x, use <code>nacljmp</code> and <code>naclcall</code> (see below for
definitions of these <em>pseudo-instructions</em>)</li>
<li>All instructions that <strong>read</strong> or <strong>write</strong> from/to memory must use
one of the four registers <code>RZP</code>, <code>RIP</code>, <code>RBP</code> or <code>RSP</code> as a
base, restricted (see below) register index (multiplied by 0, 1, 2,
4 or 8) and constant displacement (optional).</li>
</ul>
<blockquote>
<div><ul class="small-gap">
<li><p class="first">Exception to this rule: string instructions are allowed if used in
following sequences (the sequences should not cross <em>bundle</em>
boundaries; segment overrides are disallowed):</p>
<pre>
mov %edi, %edi
lea (%rZP,%rdi),%rdi
[rep] stos ; other string instructions can be used here
</pre>
<p>Note: this is identical to the <em>pseudo-instruction</em>: <code>[rep] stos
%?ax, %nacl:(%rdi),%rZP</code></p>
</li>
</ul>
</div></blockquote>
<ul class="small-gap">
<li>An operand of a command is said to be a <strong>restricted register</strong> iff
it is a register that is the target of a 32-bit move in the
immediately-preceding command in the same <em>bundle</em> (consider the
previous command as additional sandboxing prefix):</li>
</ul>
<blockquote>
<div><pre>
; any 32-bit register can be used here; the first operand is
; unrestricted but often is the same register
mov ..., %eXX
</pre>
</div></blockquote>
<ul class="small-gap">
<li>Instructions capable of changing <code>%RBP</code> and <code>%RSP</code> are
forbidden, except the instruction sequences in the whitelist below,
which must not cross <em>bundle</em> boundaries:</li>
</ul>
<blockquote>
<div><pre>
mov %rbp, %rsp
mov %rsp, %rbp
mov ..., %ebp
; restoration of %RBP from memory, register or stack - keeps the
; invariant intact
add %rZP, %rbp
mov ..., %esp
; restoration of %RSP from memory, register or stack - keeps the
; invariant intact
add %rZP, %rsp
lea xxx(%rbp), %esp
add %rZP, %rsp ; restoration of %RSP from %RBP with adjust
sub ..., %esp
add %rZP, %rsp ; stack space allocation
add ..., %esp
add %rZP, %rsp ; stack space deallocation
and $XX, %rsp ; alignment; XX must be between -128 and -1
pushq ...
popq ... ; except pop %RSP, pop %RBP
</pre>
</div></blockquote>
<h2 id="list-of-pseudo-instructions">List of Pseudo-instructions</h2>
<p>Pseudo-instructions were introduced to let the compiler maintain the
invariants without needing to know the code alignment rules. The
assembler guarantees 32-bit alignment for all <em>pseudo-instructions</em> in
the table below. In addition, to the pseudo-instructions, one
pseudo-operand prefix is introduced: <code>%nacl</code>. Presence of the
<code>%nacl</code> operand prefix ensures that:</p>
<ul class="small-gap">
<li>The instruction <code>"%mov %eXX, %eXX"</code> is added immediately before the
actual command using prefix <code>%nacl</code> (where <code>%eXX</code> is a 32-bit
part of the index register of the actual command, for example: in
operand <code>%nacl:(,%r11)</code>, the notation <code>%eXX</code> is referring to
<code>%r11d</code>)</li>
<li>The resulting sequence of two instructions does not cross the
<em>bundle</em> boundary.</li>
</ul>
<p>For example, the instruction:</p>
<pre>
mov %eax,%nacl:(%r15,%rdi,2)
</pre>
<p>is translated by the assembler to:</p>
<pre>
mov %edi,%edi
mov %eax,(%r15,%rdi,2)
</pre>
<p>The complete list of introduced <em>pseudo-instructions</em> is as follows:</p>
<table border=1>
<tbody>
<tr>
<td>Pseudo-instruction</td>
<td>Is translated to<br/>
</td>
</tr>
<tr>
<td>[rep] cmps %nacl:(%rsi),%nacl:(%rdi),%rZP<br/>
<i>(sandboxed cmps)</i><br/>
</td>
<td>mov %esi,%esi<br/>
lea (%rZP,%rsi,1),%rsi<br/>
mov %edi,%edi<br/>
lea (%rZP,%rdi,1),%rdi<br/>
[rep] cmps (%rsi),(%rdi)<i><br/>
</i>
</td>
</tr>
<tr>
<td>[rep] movs %nacl:(%rsi),%nacl:(%rdi),%rZP<br/>
<i>(sandboxed movs)</i><br/>
</td>
<td>mov %esi,%esi<br/>
lea (%rZP,%rsi,1),%rsi<br/>
mov %edi,%edi<br/>
lea (%rZP,%rdi,1),%rdi<br/>
[rep] movs (%rsi),(%rdi)<i><br/>
</i>
</td>
</tr>
<tr>
<td>naclasp ...,%rZP<br/>
<i>(sandboxed stack increment)</i></td>
<td>add ...,%esp<br/>
add %rZP,%rsp</td>
</tr>
<tr>
<td>naclcall %eXX,%rZP<br/>
<i>(sandboxed indirect call)</i></td>
<td>and $-32, %eXX<br/>
add %rZP, %rXX<br/>
call *%rXX<br/>
<i>Note: the assembler ensures all calls (including
naclcall) will end at the bundle boundary.</i></td>
</tr>
<tr>
<td>nacljmp %eXX,%rZP<br/>
<i>(sandboxed indirect jump)</i></td>
<td>and $-32,%eXX<br/>
add %rZP,%rXX<br/>
jmp *%rXX<br/>
</td>
</tr>
<tr>
<td>naclrestbp ...,%rZP<br/>
<i>(sandboxed %ebp/rbp restore)</i></td>
<td>mov ...,%ebp<br/>
add %rZP,%rbp</td>
</tr>
<tr>
<td>naclrestsp ...,%rZP
<i>(sandboxed %esp/rsp restore)</i></td>
<td>mov ...,%esp<br/>
add %rZP,%rsp</td>
</tr>
<tr>
<td>naclrestsp_noflags ...,%rZP
<i>(sandboxed %esp/rsp restore)</i></td>
<td>mov ...,%esp<br/>
lea (%rsp,%rZP,1),%rsp</td>
</tr>
<tr>
<td>naclspadj $N,%rZP<br/>
<i>(sandboxed %esp/rsp restore from %rbp; incudes $N offset)</i></td>
<td>lea N(%rbp),%esp<br/>
add %rZP,%rsp</td>
</tr>
<tr>
<td>naclssp ...,%rZP<br/>
<i>(sandboxed stack decrement)</i></td>
<td>sub ...,%esp<br/>
add %rZP,%rsp</td>
</tr>
<tr>
<td>[rep] scas %nacl:(%rdi),%?ax,%rZP<br/>
<i>(sandboxed stos)</i></td>
<td>mov %edi,%edi<br/>
lea (%rZP,%rdi,1),%rdi<br/>
[rep] scas (%rdi),%?ax<br/>
</td>
</tr>
<tr>
<td>[rep] stos %?ax,%nacl:(%rdi),%rZP<br/>
<i>(sandboxed stos)</i></td>
<td>mov %edi,%edi<br/>
lea (%rZP,%rdi,1),%rdi<br/>
[rep] stos %?ax,(%rdi)<br/>
</td>
</tr>
</tbody>
</table></section>
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