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<a href="https://gitlab.com/tatsuhiro-t">Tatsuhiro Tsujikawa</a> created an issue: <a href="https://gitlab.com/gnutls/gnutls/-/issues/1319">#1319</a>
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<a id="user-content-description-of-problem" class="anchor" href="#description-of-problem" aria-hidden="true"></a>Description of problem:</h2>
<p dir="auto">When received data is too short to contain a message header, they are temporarily buffered in somewhere and when the next segment is arrived, they are gathered into one buffer (linearized). I'm specifically looking at this code:
<a href="https://gitlab.com/gnutls/gnutls/-/blob/7f3ea9acc2a2da1d697d423514dfa9c3b56acb15/lib/buffers.c#L1259">https://gitlab.com/gnutls/gnutls/-/blob/7f3ea9acc2a2da1d697d423514dfa9c3b56acb15/lib/buffers.c#L1259</a></p>
<p dir="auto">The problem is, when the buffers are linearized, mbuffer_st type is cleared. Next time we hit <a href="https://gitlab.com/gnutls/gnutls/-/blob/7f3ea9acc2a2da1d697d423514dfa9c3b56acb15/lib/buffers.c#L1217">https://gitlab.com/gnutls/gnutls/-/blob/7f3ea9acc2a2da1d697d423514dfa9c3b56acb15/lib/buffers.c#L1217</a>, the check fails, and the endpoint sends fatal alert with unexpected message.</p>
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<a id="user-content-version-of-gnutls-used" class="anchor" href="#version-of-gnutls-used" aria-hidden="true"></a>Version of gnutls used:</h2>
<p dir="auto">3.7.3</p>
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<a id="user-content-distributor-of-gnutls-eg-ubuntu-fedora-rhel" class="anchor" href="#distributor-of-gnutls-eg-ubuntu-fedora-rhel" aria-hidden="true"></a>Distributor of gnutls (e.g., Ubuntu, Fedora, RHEL)</h2>
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<a id="user-content-how-reproducible" class="anchor" href="#how-reproducible" aria-hidden="true"></a>How reproducible:</h2>
<p dir="auto">Steps to Reproduce:</p>
<p dir="auto">I think feeding data ends in the middle of a message header to gnutls_session_t via gnutls_handshake_write would trigger this bug.
For myself, I discovered it when testing client certificate authentication as QUIC client.</p>
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<a id="user-content-actual-results" class="anchor" href="#actual-results" aria-hidden="true"></a>Actual results:</h2>
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<a id="user-content-expected-results" class="anchor" href="#expected-results" aria-hidden="true"></a>Expected results:</h2>
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