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<a href="https://gitlab.com/graingert">Thomas Grainger</a> created a merge request: <a href="https://gitlab.com/gnutls/libtasn1/-/merge_requests/126">!126</a>
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Project:Branches: graingert/libtasn1:fix-asn1-tag-der-overrun to gnutls/libtasn1:master
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Author: Thomas Grainger
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<p dir="auto" style="color: #3a383f; margin: 0px 0px 1rem;" align="initial">Rewrites <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0px; font-weight: inherit; overflow-wrap: break-word; white-space: break-spaces; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: keep-all; padding: 0.125rem 0.25rem;'>_asn1_tag_der()</code> as a single pass that writes the tag number straight to the output, most significant digit first, emitting at most <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; overflow-wrap: break-word; white-space: break-spaces; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: keep-all; padding: 0.125rem 0.25rem;'>ASN1_MAX_TAG_SIZE - 1</code> continuation bytes. The result always fits in <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; overflow-wrap: break-word; white-space: break-spaces; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: keep-all; padding: 0.125rem 0.25rem;'>ASN1_MAX_TAG_SIZE</code>, so the one-byte overrun in the old reverse-copy loop (issue <a href="https://gitlab.com/gnutls/libtasn1/-/work_items/49" title="Potential Buffer Overrun in _asn1_tag_der()" class="gfm gfm-issue" data-original="#49" data-link="false" data-link-reference="false" data-issue="142675697" data-project="919981" data-iid="49" data-namespace-path="gnutls/libtasn1" data-project-path="gnutls/libtasn1" data-issue-type="issue" data-container="body" data-placement="top" data-reference-type="issue">#49</a>) is impossible by construction, as suggested by <a href="https://gitlab.com/jas" title="Simon Josefsson" class="gfm gfm-project_member js-user-link" data-user="47250" data-original="@jas" data-container="body" data-placement="top" data-reference-type="user" style="color: #284779; background-color: #cbe2f9; border-radius: .25rem; padding: 0 2px;">@jas</a> in the issue.</p>
<p dir="auto" style="color: #3a383f; margin: 0px 0px 1rem;" align="initial">The old truncation behaviour for over-long tags is preserved.</p>
<p dir="auto" style="color: #3a383f; margin: 0px 0px 1rem;" align="initial">Adds <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0px; font-weight: inherit; overflow-wrap: break-word; white-space: break-spaces; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: keep-all; padding: 0.125rem 0.25rem;'>tests/Test_tag.c</code> covering:</p>
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<li style="margin-top: 0px; line-height: 1.6em; margin-left: 25px; padding-left: 3px;">positive: <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0px; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 0.125rem 0.25rem;'>[APPLICATION 200000]</code> (needs the full three base-128 tag bytes) encodes to the known-good DER;</li>
<li style="line-height: 1.6em; margin-left: 25px; padding-left: 3px;">negative: <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0px; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 0.125rem 0.25rem;'>[APPLICATION 2097152]</code> (a tag too large to encode) stays within <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 0.125rem 0.25rem;'>ASN1_MAX_TAG_SIZE</code> — before the rewrite this produced a nine-byte tag and AddressSanitizer reports a stack-buffer-overflow in <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; white-space: pre-wrap; overflow-wrap: break-word; word-break: keep-all; padding: 0.125rem 0.25rem;'>_asn1_tag_der</code>.</li>
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<p dir="auto" style="color: #3a383f; margin: 0px;" align="initial">Closes <a href="https://gitlab.com/gnutls/libtasn1/-/work_items/49" title="Potential Buffer Overrun in _asn1_tag_der()" class="gfm gfm-issue" data-original="#49" data-link="false" data-link-reference="false" data-issue="142675697" data-project="919981" data-iid="49" data-namespace-path="gnutls/libtasn1" data-project-path="gnutls/libtasn1" data-issue-type="issue" data-container="body" data-placement="top" data-reference-type="issue" style="margin-top: 0px;">#49</a></p>
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