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  <title>Standard library hardening</title>
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<header id="title-block-header">
<h1 class="title" style="text-align:center">Standard library
hardening</h1>
<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P3471R0</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2024-10-15</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project:</td>
    <td>Programming Language C++</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Audience:</td>
    <td>
      Library Evolution, SG23<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Konstantin Varlamov<br>&lt;<a href="mailto:varconst@apple.com" class="email">varconst@apple.com</a>&gt;<br>
      Louis Dionne<br>&lt;<a href="mailto:ldionne@apple.com" class="email">ldionne@apple.com</a>&gt;<br>
    </td>
  </tr>
</table>
</header>
<div style="clear:both">
<h1 data-number="1" id="introduction"><span class="header-section-number">1</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p>This paper proposes introducing <em>standard library hardening</em>
into the C++ Standard. Hardening allows turning some instances of
undefined behavior in the standard library into a guaranteed termination
of the program. This proposal is based on our experience implementing <a href="https://libcxx.llvm.org/Hardening.html">hardening in
libc++</a>.</p>
<h1 data-number="2" id="motivation"><span class="header-section-number">2</span> Motivation<a href="#motivation" class="self-link"></a></h1>
<p>There has been significantly increased attention to safety and
security in C++ over the last few years, as exemplified by the
well-known <a href="https://www.whitehouse.gov/wp-content/uploads/2024/02/Final-ONCD-Technical-Report.pdf">White
House report</a> and numerous recent security-related proposals.</p>
<p>While it is important to explore ways to make <em>new</em> code
safer, we believe that the highest priority to deliver immediate
real-world value should be to make <em>existing</em> code safer with
minimal or no effort on behalf of users. Indeed, the amount of existing
security-critical C++ code is so large that rewriting it or modifying it
is both economically unviable and dangerous given the risk of
introducing new issues.</p>
<p>There have been a few proposals accepted recently that eliminate some
cases of undefined behavior in the core language. The standard library
also contains many instances of undefined behavior, some of which is a
direct source of security vulnerabilities; addressing those is often
trivial, can be done with low overhead and almost no work on behalf of
users.</p>
<p>In fact, at the moment all three major library implementations have
some notion of a hardened or debug mode. This clearly shows interest,
both from users and from implementors, in having a safer mode for the
standard library. However, we believe these efforts would be vastly more
useful if they were standardized and provided portable, cross-platform
guarantees to users; as it stands, implementations differ in levels of
coverage, performance guarantees and ways to enable the safer mode.</p>
<p>Finally, leaving security of the library to be a pure vendor
extension fails to position ISO C++ as providing a credible solution for
code bases with formal security requirements. We believe that formally
requiring the basic safety guarantees that most implementations already
provide in one way or another could make a significant difference from
the point of view of anyone writing or following safety and security
coding standards and guidelines.</p>
<h1 data-number="3" id="design-overview"><span class="header-section-number">3</span> Design overview<a href="#design-overview" class="self-link"></a></h1>
<p>At a high level, this proposal consists of two parts:</p>
<ul>
<li>Annotate <em>some</em> existing library preconditions as
<em>hardened</em>.</li>
<li>Provide a way for the user to enable a <em>hardened mode</em>. When
the hardened mode is enabled, all hardened preconditions are checked at
runtime and violating them is guaranteed to terminate the program in an
implementation-defined way.</li>
</ul>
<p>There are a few important aspects to the proposed design:</p>
<ul>
<li>We only propose to formally check (some of the) existing function
preconditions, not to add any new preconditions. As a drive-by, we also
make a few existing preconditions more explicit in the specification
(e.g. <code class="sourceCode cpp">vector<span class="op">::</span><span class="kw">operator</span><span class="op">[]</span></code>
only has implicit preconditions via iterator validity).</li>
<li>Hardening deals exclusively with preconditions that must be checked
at runtime. Thus, performance overhead is an important constraint on the
solution.</li>
<li>Hardening is intended to avoid requiring changes to an
implementation’s ABI. In particular, the intent is to make it possible
for implementations to allow mixing hardened and unhardened code in the
same program, if they wish to do so.</li>
<li>Hardening needs to be lightweight enough to be used in production
environments. In our experience, a debug-only approach is not sufficient
to really move the needle security-wise. Thus, the goal is to identify
the minimal set of checks that would deliver the most value to users
while requiring minimal overhead, with a particular focus on memory
safety as a major source of security vulnerabilities. Checking
<em>all</em> preconditions in the library is an explicit non-goal, and
would be impossible for many semantic requirements anyway.</li>
</ul>
<p>To reiterate the last point, an important design principle is that
hardening needs to be lightweight enough for production use by a wide
variety of real-world programs. In our experience in libc++, a small set
of checks that is widely used delivers far more value than a more
extensive set of checks that is only enabled by select few users.
Thankfully, many of the most valuable checks, such as checking for
out-of-bounds access in standard containers, also happen to be
relatively cheap.</p>
<h1 data-number="4" id="hardened-preconditions"><span class="header-section-number">4</span> Hardened preconditions<a href="#hardened-preconditions" class="self-link"></a></h1>
<p>To specify hardening in the Standard, this proposal introduces the
notion of a <em>hardened precondition</em>. A hardened precondition is a
precondition that the implementation is required to check if hardening
is enabled, and violating which reliably terminates the program. Thus,
adding hardening to the library largely consists of turning some of the
existing preconditions into hardened preconditions in the specification.
For example:</p>
<p><span class="marginalizedparent"><a class="marginalized">(24.7.2.2.6)</a></span>
Element access <span>24.7.2.2.6
<a href="https://wg21.link/span.elem">[span.elem]</a></span></p>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="kw">constexpr</span> reference <span class="kw">operator</span><span class="op">[](</span>size_type idx<span class="op">)</span> <span class="kw">const</span>;</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized">1</a></span>
<span class="add" style="color: #006e28"><ins>Hardened</ins></span>
Preconditions: <code class="sourceCode cpp">idx <span class="op">&lt;</span> size<span class="op">()</span></code>
is <code class="sourceCode cpp"><span class="kw">true</span></code>.</p>
<p>In the initial proposal, we would prefer to focus on hardened
preconditions that prevent out-of-bounds memory access, i.e., compromise
the memory safety of the program. These would be some of the most
valuable for the user since they help prevent potential security
vulnerabilities; many of them are also relatively cheap to implement.
More hardened preconditions can potentially be added in the future, but
the intent is for their number to be limited to keep hardening viable
for production use. Specifically, the proposal is to add hardened
preconditions to:</p>
<ul>
<li>Accessors of sequence containers,
<code class="sourceCode cpp">std<span class="op">::</span>span</code>,
<code class="sourceCode cpp">std<span class="op">::</span>mdspan</code>,
<code class="sourceCode cpp">std<span class="op">::</span>string</code>,
<code class="sourceCode cpp">std<span class="op">::</span>string_view</code>
and other similar classes that might attempt to access non-existent
elements
(e.g. <code class="sourceCode cpp">back<span class="op">()</span></code>
on an empty container or <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">[]</span></code>
with an invalid index).</li>
<li>Modifiers of sequence containers and string classes that assume the
container to be non-empty
(e.g. <code class="sourceCode cpp">pop_back<span class="op">()</span></code>).</li>
<li>Accessors of <code class="sourceCode cpp">optional</code> and
<code class="sourceCode cpp">expected</code> that expect the object to
be non-empty.</li>
</ul>
<p>In our experience, hardening all of these operations is trivial to
implement and provides significant security value.</p>
<h1 data-number="5" id="enabling-hardening"><span class="header-section-number">5</span> Enabling hardening<a href="#enabling-hardening" class="self-link"></a></h1>
<p>Implementations are required to provide a hardened mode. However, the
way this hardening mode is enabled is implementation-defined, and so are
details like whether the mode can be selected on a per-TU basis or needs
to be the same program-wide. For example, a valid implementation would
be to provide a compiler flag like <em>-flibrary-hardening</em> or to
provide a library macro. If this proposal gets accepted, we expect
implementations to converge on a portable mechanism.</p>
<p>If a hardening check fails, the program is terminated in an
implementation-defined way. We don’t propose allowing this behavior to
be customized by the user. Calling <code class="sourceCode cpp">std<span class="op">::</span>abort<span class="op">()</span></code>
or <code class="sourceCode cpp"><span class="fu">__builtin_trap</span><span class="op">()</span></code>
would be a valid implementation; if Contracts are available, using
contract assertions would also be a valid implementation strategy.</p>
<h1 data-number="6" id="relationship-to-contracts-profiles-and-erroneous-behavior"><span class="header-section-number">6</span> Relationship to Contracts,
Profiles, and Erroneous Behavior<a href="#relationship-to-contracts-profiles-and-erroneous-behavior" class="self-link"></a></h1>
<p>Hardening has some overlap with the recently-adopted Erroneous
Behavior, as well as two other in-progress proposals: Contracts and
Profiles.</p>
<h2 data-number="6.1" id="contracts"><span class="header-section-number">6.1</span> Contracts<a href="#contracts" class="self-link"></a></h2>
<p>Contracts is a new language feature that allows expressing
preconditions and much more, with a lot of flexibility on what happens
when an assertion is not satisfied. Using contract assertions is one
valid implementation strategy for hardening, but the intent is to allow
for other implementation strategies as well. As such, this proposal is
independent from contracts.</p>
<p>Note that the mechanism for enabling the hardened mode would need to
be different from the one for choosing the evaluation semantic – the
hardened mode affects only hardened preconditions in the library, while
the evaluation semantic affects any contract assertions in the
program.</p>
<h2 data-number="6.2" id="profiles"><span class="header-section-number">6.2</span> Profiles<a href="#profiles" class="self-link"></a></h2>
<p>The Profiles proposal introduces a framework to specify sets of
safety guarantees (such as a type safety profile or an invalidation
profile). If profiles become a part of the Standard in the future,
hardening can most likely be formulated as an additional profile; this
would formalize how hardening is turned on and off. However, specifying
hardened preconditions standardizes existing practice and delivers value
today without requiring a new language feature.</p>
<p>While the current proposal does leave a bit more than we’d like to be
implementation-defined, we don’t expect this to significantly hinder
portability since we expect implementers to converge on how to enable
hardening.</p>
<h2 data-number="6.3" id="erroneous-behavior"><span class="header-section-number">6.3</span> Erroneous Behavior<a href="#erroneous-behavior" class="self-link"></a></h2>
<p>While Erroneous Behavior is a way to clearly mark some code as
incorrect in the specification, it does not provide any termination
guarantees. Implementations are only encouraged, but not required, to
diagnose EB. Since a major aim of this proposal is to provide portable
guarantees for the library, it is not sufficient to simply turn certain
library precondition violations into EB.</p>
<!--
# Proposed wording approach

In [`intro.compliance`](https://eel.is/c++draft/intro.compliance), add a new paragraph that states:

> An implementation can be hardened or unhardened. A hardened implementation is one in which violating a hardened precondition results in program termination and does not result in undefined behavior. In an unhardened implementation violating a hardened precondition is equivalent to violating an unhardened precondition. A conforming implementation shall provide a hardened implementation.

In [`structure.specifications`](http://eel.is/c++draft/description#structure.specifications-3.3), add a new element called “Hardened Preconditions” (the exact name is subject to discussion) which says:

> Hardened preconditions: the conditions that the function assumes to hold whenever it is called. In a hardened implementation, violation of any hardened preconditions results in program termination in an implementation-defined way; otherwise, the violation results in undefined behavior.

Then, change a few places such as [`sequence.reqmts`](http://eel.is/c++draft/sequence.reqmts#117) to either add Hardened Preconditions, or turn existing “Preconditions” into “Hardened Preconditions” (for example, [`span.elem`](http://eel.is/c++draft/views.span#span.elem-1)). Note, however, that adding Hardened Preconditions would only be made to make existing implicit preconditions explicit; the scope of this proposal is only to add checking to existing preconditions, whether they are specified explicitly or implicitly, not to add any actual new preconditions.
-->
<h1 data-number="7" id="suggested-polls"><span class="header-section-number">7</span> Suggested polls<a href="#suggested-polls" class="self-link"></a></h1>
<ul>
<li>Are we interested in standardizing some notion of hardening for the
Standard Library?</li>
<li>Are we interested in forcing a conforming implementation to provide
a hardened mode (as opposed to permitting that it provides one)?</li>
</ul>
</div>
</div>
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