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<p style="color: #777777;">
<a href="https://gitlab.com/dueno">Daiki Ueno</a>
commented on a
discussion on <a href="https://gitlab.com/gnutls/gnutls/-/merge_requests/1404#note_531527217">lib/accelerated/afalg.c</a>:
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<pre style="margin: 0;">+<span id="LC308" class="line" lang="c"><span class="cm" style="color: #998; font-style: italic;">         * read-only</span></span>
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<pre style="margin: 0;">+<span id="LC309" class="line" lang="c"><span class="cm" style="color: #998; font-style: italic;">         */</span></span>
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<pre style="margin: 0;">+<span id="LC310" class="line" lang="c">    <span class="n" style="color: #333;">iov</span><span class="p">[</span><span class="mi" style="color: #099;">1</span><span class="p">].</span><span class="n" style="color: #333;">iov_base</span> <span class="o" style="font-weight: 600;">=</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">void</span> <span class="o" style="font-weight: 600;">*</span><span class="p">)</span><span class="n" style="color: #333;">encr</span><span class="p">;</span></span>
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<pre style="margin: 0;">+<span id="LC311" class="line" lang="c">    <span class="n" style="color: #333;">iov</span><span class="p">[</span><span class="mi" style="color: #099;">1</span><span class="p">].</span><span class="n" style="color: #333;">iov_len</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">encr_size</span><span class="p">;</span></span>
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<pre style="margin: 0;">+<span id="LC312" class="line" lang="c"></span>
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<pre style="margin: 0;">+<span id="LC313" class="line" lang="c">    <span class="k" style="font-weight: 600;">if</span> <span class="p">(</span><span class="n" style="color: #333;">kcapi_aead_stream_update_last</span><span class="p">(</span><span class="n" style="color: #333;">ctx</span><span class="o" style="font-weight: 600;">-></span><span class="n" style="color: #333;">handle</span><span class="p">,</span> <span class="n" style="color: #333;">iov</span><span class="p">,</span> <span class="mi" style="color: #099;">2</span><span class="p">)</span> <span class="o" style="font-weight: 600;"><</span> <span class="mi" style="color: #099;">0</span><span class="p">)</span> <span class="p">{</span></span>
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<pre style="margin: 0;">+<span id="LC314" class="line" lang="c">            <span class="n" style="color: #333;">gnutls_assert</span><span class="p">();</span></span>
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<pre style="margin: 0;">+<span id="LC315" class="line" lang="c">            <span class="k" style="font-weight: 600;">return</span> <span class="n" style="color: #333;">GNUTLS_E_ENCRYPTION_FAILED</span><span class="p">;</span></span>
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<pre style="margin: 0;">+<span id="LC316" class="line" lang="c">    <span class="p">}</span></span>
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<pre style="margin: 0;">+<span id="LC317" class="line" lang="c"></span>
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<pre style="margin: 0;">+<span id="LC318" class="line" lang="c">    <span class="cm" style="color: #998; font-style: italic;">/* The kernel may set the AAD, avoid modification of auth */</span></span>
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<pre style="margin: 0;">+<span id="LC319" class="line" lang="c">    <span class="n" style="color: #333;">iov</span><span class="p">[</span><span class="mi" style="color: #099;">0</span><span class="p">].</span><span class="n" style="color: #333;">iov_base</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">authtmp</span><span class="p">;</span></span>
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<pre style="margin: 0;">+<span id="LC320" class="line" lang="c"></span>
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<pre style="margin: 0;">+<span id="LC321" class="line" lang="c">    <span class="cm" style="color: #998; font-style: italic;">/* Set PT buffer to be filled by kernel */</span></span>
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<pre style="margin: 0;">+<span id="LC322" class="line" lang="c">    <span class="n" style="color: #333;">iov</span><span class="p">[</span><span class="mi" style="color: #099;">1</span><span class="p">].</span><span class="n" style="color: #333;">iov_base</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">plain</span><span class="p">;</span></span>
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<pre style="margin: 0;">+<span id="LC323" class="line" lang="c">    <span class="n" style="color: #333;">iov</span><span class="p">[</span><span class="mi" style="color: #099;">1</span><span class="p">].</span><span class="n" style="color: #333;">iov_len</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">plain_size</span><span class="p">;</span></span>
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<p dir="auto">The initial motivation of this change is:
<a href="https://gitlab.com/gnutls/gnutls/-/issues/308#note_527421502" data-original="https://gitlab.com/gnutls/gnutls/-/issues/308#note_527421502" data-link="false" data-link-reference="true" data-project="179611" data-issue="7203102" data-reference-type="issue" data-container="body" data-placement="top" title="Add Linux kernel AF_ALG backend" class="gfm gfm-issue has-tooltip">#308 (comment 527421502)</a>
i.e., <code>kcapi_aead_stream_op</code> blocks if the requested output size didn't match.</p>
<p dir="auto">According to the libkcapi documentation, it seems like a legitimate behavior. I agree that it could cause a buffer overflow; maybe we could tighten the length check, and also merge it with the next <code>if</code> block.</p>
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