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<a href="https://gitlab.com/garycplin">Gary Lin</a> created an issue: <a href="https://gitlab.com/gnutls/libtasn1/-/issues/49">#49</a>
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<a id="user-content-description-of-problem" class="anchor" href="#description-of-problem" aria-hidden="true" style="margin-top: 0; float: left; margin-left: -20px; text-decoration: none; outline: none;"></a>Description of problem:</h2>
<p dir="auto" style="color: #333238; margin: 0 0 16px;" align="initial">When merging libtasn1 into grub2, a potential buffer overrun issue was spotted by coverity:</p>
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<pre class="code highlight js-syntax-highlight language-plaintext" lang="plaintext" v-pre="true" style='display: block; font-size: 14px; color: #333238; line-height: 1.6em; overflow-x: auto; border-radius: 4px; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: #fbfafd; margin: 0 0 16px; padding: 12px; border: 1px solid #dcdcde;'><code style='font-size: inherit; color: inherit; word-wrap: normal; word-break: keep-all; background-color: inherit; border-radius: 4px; white-space: pre; margin-top: 0; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; overflow-wrap: normal; padding: unset;'><span id="LC1" class="line" lang="plaintext" style="margin-top: 0;">*** CID 435762: Memory - corruptions (OVERRUN)</span>
<span id="LC2" class="line" lang="plaintext">________________________________________________________________________________________________________</span>
<span id="LC3" class="line" lang="plaintext">/grub-core/lib/libtasn1/lib/coding.c: 152 in _asn1_tag_der()</span>
<span id="LC4" class="line" lang="plaintext">146 if (k > ASN1_MAX_TAG_SIZE - 1)</span>
<span id="LC5" class="line" lang="plaintext">147 break; /* will not encode larger tags */</span>
<span id="LC6" class="line" lang="plaintext">148 }</span>
<span id="LC7" class="line" lang="plaintext">149 *ans_len = k + 1;</span>
<span id="LC8" class="line" lang="plaintext">150 while (k--)</span>
<span id="LC9" class="line" lang="plaintext">151 ans[*ans_len - 1 - k] = temp[k] + 128;</span>
<span id="LC10" class="line" lang="plaintext">>>> CID 435762: Memory - corruptions (OVERRUN)</span>
<span id="LC11" class="line" lang="plaintext">>>> Overrunning array of 4 bytes at byte offset 4 by dereferencing pointer "ans + (*ans_len - 1)".</span>
<span id="LC12" class="line" lang="plaintext">152 ans[*ans_len - 1] -= 128;</span>
<span id="LC13" class="line" lang="plaintext">153 }</span>
<span id="LC14" class="line" lang="plaintext">154 }</span>
<span id="LC15" class="line" lang="plaintext">155 </span>
<span id="LC16" class="line" lang="plaintext">156 /**</span>
<span id="LC17" class="line" lang="plaintext">157 * asn1_octet_der:</span></code></pre>
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<p dir="auto" style="color: #333238; margin: 0 0 16px;" align="initial">Here is the code snippet:</p>
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<pre class="code highlight js-syntax-highlight language-plaintext" lang="plaintext" v-pre="true" style='display: block; font-size: 14px; color: #333238; line-height: 1.6em; overflow-x: auto; border-radius: 4px; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: #fbfafd; margin: 0 0 16px; padding: 12px; border: 1px solid #dcdcde;'><code style='font-size: inherit; color: inherit; word-wrap: normal; word-break: keep-all; background-color: inherit; border-radius: 4px; white-space: pre; margin-top: 0; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; overflow-wrap: normal; padding: unset;'><span id="LC1" class="line" lang="plaintext" style="margin-top: 0;"> k = 0;</span>
<span id="LC2" class="line" lang="plaintext"> while (tag_value != 0)</span>
<span id="LC3" class="line" lang="plaintext"> {</span>
<span id="LC4" class="line" lang="plaintext"> temp[k++] = tag_value & 0x7F;</span>
<span id="LC5" class="line" lang="plaintext"> tag_value >>= 7;</span>
<span id="LC6" class="line" lang="plaintext"></span>
<span id="LC7" class="line" lang="plaintext"> if (k > ASN1_MAX_TAG_SIZE - 1)</span>
<span id="LC8" class="line" lang="plaintext"> break; /* will not encode larger tags */</span>
<span id="LC9" class="line" lang="plaintext"> }</span>
<span id="LC10" class="line" lang="plaintext"> *ans_len = k + 1;</span>
<span id="LC11" class="line" lang="plaintext"> while (k--)</span>
<span id="LC12" class="line" lang="plaintext"> ans[*ans_len - 1 - k] = temp[k] + 128;</span></code></pre>
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<p dir="auto" style="color: #333238; margin: 0 0 16px;" align="initial">In the first while loop, <code style='font-size: 90%; color: #1f1e24; word-wrap: break-word; background-color: #ececef; border-radius: 4px; margin-top: 0; font-weight: inherit; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>k</code> may become <code style='font-size: 90%; color: #1f1e24; word-wrap: break-word; background-color: #ececef; border-radius: 4px; font-weight: inherit; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>ASN1_MAX_TAG_SIZE</code>, i.e.<code style='font-size: 90%; color: #1f1e24; word-wrap: break-word; background-color: #ececef; border-radius: 4px; font-weight: inherit; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>4</code>, and trigger <code style='font-size: 90%; color: #1f1e24; word-wrap: break-word; background-color: #ececef; border-radius: 4px; font-weight: inherit; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>break</code>. Then, in the second while loop, the iteration will be like this:</p>
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<pre class="code highlight js-syntax-highlight language-plaintext" lang="plaintext" v-pre="true" style='display: block; font-size: 14px; color: #333238; line-height: 1.6em; overflow-x: auto; border-radius: 4px; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: #fbfafd; margin: 0 0 16px; padding: 12px; border: 1px solid #dcdcde;'><code style='font-size: inherit; color: inherit; word-wrap: normal; word-break: keep-all; background-color: inherit; border-radius: 4px; white-space: pre; margin-top: 0; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; overflow-wrap: normal; padding: unset;'><span id="LC1" class="line" lang="plaintext" style="margin-top: 0;">*ans_len - 1 - k: 1, k: 3</span>
<span id="LC2" class="line" lang="plaintext">*ans_len - 1 - k: 2, k: 2</span>
<span id="LC3" class="line" lang="plaintext">*ans_len - 1 - k: 3, k: 1</span>
<span id="LC4" class="line" lang="plaintext">*ans_len - 1 - k: 4, k: 0</span></code></pre>
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<p dir="auto" style="color: #333238; margin: 0 0 16px;" align="initial">The code may access ans[4] which excesses the boundary of the array. Maybe the if statement should be <code style='font-size: 90%; color: #1f1e24; word-wrap: break-word; background-color: #ececef; border-radius: 4px; margin-top: 0; font-weight: inherit; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>k >= ASN1_MAX_TAG_SIZE - 1</code> to cap <code style='font-size: 90%; color: #1f1e24; word-wrap: break-word; background-color: #ececef; border-radius: 4px; font-weight: inherit; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>k</code> below <code style='font-size: 90%; color: #1f1e24; word-wrap: break-word; background-color: #ececef; border-radius: 4px; font-weight: inherit; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 2px 4px;'>ASN1_MAX_TAG_SIZE</code>.</p>
<p dir="auto" style="color: #333238; margin: 0 0 16px;" align="initial">The full report is available in grub-devel mailing list:</p>
<p dir="auto" style="color: #333238; margin: 0 0 16px;" align="initial"><a href="https://lists.gnu.org/archive/html/grub-devel/2024-02/txtKIuUb5lf3O.txt" rel="nofollow noreferrer noopener" target="_blank" style="margin-top: 0;">https://lists.gnu.org/archive/html/grub-devel/2024-02/txtKIuUb5lf3O.txt</a></p>
<h2 dir="auto" style="font-size: 1.5em; font-weight: 600; padding-bottom: .3em; border-bottom-width: 1px; border-bottom-color: #bfbfc3; border-bottom-style: solid; color: #333238; margin: 24px 0 16px;" align="initial">
<a id="user-content-version-of-libtasn1-used" class="anchor" href="#version-of-libtasn1-used" aria-hidden="true" style="margin-top: 0; float: left; margin-left: -20px; text-decoration: none; outline: none;"></a>Version of libtasn1 used:</h2>
<p dir="auto" style="color: #333238; margin: 0 0 16px;" align="initial">4.19</p>
<h2 dir="auto" style="font-size: 1.5em; font-weight: 600; padding-bottom: .3em; border-bottom-width: 1px; border-bottom-color: #bfbfc3; border-bottom-style: solid; color: #333238; margin: 24px 0 16px;" align="initial">
<a id="user-content-distributor-of-libtasn1-eg-ubuntu-fedora-rhel" class="anchor" href="#distributor-of-libtasn1-eg-ubuntu-fedora-rhel" aria-hidden="true" style="margin-top: 0; float: left; margin-left: -20px; text-decoration: none; outline: none;"></a>Distributor of libtasn1 (e.g., Ubuntu, Fedora, RHEL)</h2>
<p dir="auto" style="color: #333238; margin: 0 0 16px;" align="initial">Upstream official tarball</p>
<h2 dir="auto" style="font-size: 1.5em; font-weight: 600; padding-bottom: .3em; border-bottom-width: 1px; border-bottom-color: #bfbfc3; border-bottom-style: solid; color: #333238; margin: 24px 0 16px;" align="initial">
<a id="user-content-how-reproducible" class="anchor" href="#how-reproducible" aria-hidden="true" style="margin-top: 0; float: left; margin-left: -20px; text-decoration: none; outline: none;"></a>How reproducible:</h2>
<p dir="auto" style="color: #333238; margin: 0 0 16px;" align="initial">Steps to Reproduce:</p>
<ul dir="auto" style="text-align: initial; list-style-type: disc; margin: 0 0 16px; padding: 0;">
<li style="margin-top: 0; line-height: 1.6em; margin-left: 25px; padding-left: 3px;">one</li>
<li style="line-height: 1.6em; margin-left: 25px; padding-left: 3px;">two</li>
<li style="line-height: 1.6em; margin-left: 25px; padding-left: 3px;">three</li>
</ul>
<h2 dir="auto" style="font-size: 1.5em; font-weight: 600; padding-bottom: .3em; border-bottom-width: 1px; border-bottom-color: #bfbfc3; border-bottom-style: solid; color: #333238; margin: 24px 0 16px;" align="initial">
<a id="user-content-actual-results" class="anchor" href="#actual-results" aria-hidden="true" style="margin-top: 0; float: left; margin-left: -20px; text-decoration: none; outline: none;"></a>Actual results:</h2>
<h2 dir="auto" style="font-size: 1.5em; font-weight: 600; padding-bottom: .3em; border-bottom-width: 1px; border-bottom-color: #bfbfc3; border-bottom-style: solid; color: #333238; margin: 24px 0 0;" align="initial">
<a id="user-content-expected-results" class="anchor" href="#expected-results" aria-hidden="true" style="margin-top: 0; float: left; margin-left: -20px; text-decoration: none; outline: none;"></a>Expected results:</h2>
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