From 490bfb28002fc0253010243ac387c756014c06e5 Mon Sep 17 00:00:00 2001 From: Alexander Sosedkin Date: Mon, 23 Mar 2026 15:09:43 +0100 Subject: [PATCH] buffers: switch from end_offset over to frag_length Instead of maintaining an inclusive [start_offset, end_offset] range when reassembling DTLS handshake, track start_offset and a relative frag_length instead. You'd think it'd be a no-op, but it fixes: * 0-length fragments triggering completion if message was 1 byte long * a remotely triggerable underflow and an ensuing heap overrun Reported-by: Joshua Rogers of AISLE Research Team Fixes: #1811 Fixes: CVE-2026-33845 Fixes: GNUTLS-SA-2026-04-29-3 CVSS: 7.5 High CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Signed-off-by: Alexander Sosedkin CVE: CVE-2026-33845 Upstream-Status: Backport [https://gitlab.com/gnutls/gnutls/-/commit/e5b72c53c7d789d19d1d1cd10b275e87d0415413] Signed-off-by: Adarsh Jagadish Kamini --- lib/buffers.c | 50 +++++++++++++++++++++++++----------------------- lib/gnutls_int.h | 4 ++-- 2 files changed, 28 insertions(+), 26 deletions(-) diff --git a/lib/buffers.c b/lib/buffers.c index 09779a8f3..02cc222c4 100644 --- a/lib/buffers.c +++ b/lib/buffers.c @@ -919,10 +919,7 @@ static int parse_handshake_header(gnutls_session_t session, mbuffer_st *bufel, } data_size = _mbuffer_get_udata_size(bufel) - handshake_header_size; - if (frag_size > 0) - hsk->end_offset = hsk->start_offset + frag_size - 1; - else - hsk->end_offset = 0; + hsk->frag_length = frag_size; _gnutls_handshake_log( "HSK[%p]: %s (%u) was received. Length %d[%d], frag offset %d, frag length: %d, sequence: %d\n", @@ -936,9 +933,10 @@ static int parse_handshake_header(gnutls_session_t session, mbuffer_st *bufel, if (hsk->length > 0 && (frag_size > data_size || - (frag_size > 0 && hsk->end_offset >= hsk->length))) { + (frag_size > 0 && hsk->start_offset + frag_size > hsk->length))) { return gnutls_assert_val(GNUTLS_E_UNEXPECTED_PACKET_LENGTH); - } else if (hsk->length == 0 && hsk->end_offset != 0 && + } else if (hsk->length == 0 && + hsk->start_offset + frag_size != hsk->start_offset && hsk->start_offset != 0) return gnutls_assert_val(GNUTLS_E_UNEXPECTED_PACKET_LENGTH); @@ -1002,11 +1000,10 @@ static int merge_handshake_packet(gnutls_session_t session, hsk->data.length = hsk->length; } - if (hsk->length > 0 && hsk->end_offset > 0 && - hsk->end_offset - hsk->start_offset + 1 != hsk->length) { + if (hsk->length > 0 && hsk->frag_length > 0 && + hsk->frag_length != hsk->length) { memmove(&hsk->data.data[hsk->start_offset], - hsk->data.data, - hsk->end_offset - hsk->start_offset + 1); + hsk->data.data, hsk->frag_length); } session->internals.handshake_recv_buffer_size++; @@ -1040,20 +1037,27 @@ static int merge_handshake_packet(gnutls_session_t session, } if (hsk->start_offset < recv_buf[pos].start_offset && - hsk->end_offset + 1 >= recv_buf[pos].start_offset) { + hsk->start_offset + hsk->frag_length >= + recv_buf[pos].start_offset) { memcpy(&recv_buf[pos].data.data[hsk->start_offset], hsk->data.data, hsk->data.length); recv_buf[pos].start_offset = hsk->start_offset; - recv_buf[pos].end_offset = - MIN(hsk->end_offset, recv_buf[pos].end_offset); - } else if (hsk->end_offset > recv_buf[pos].end_offset && - hsk->start_offset <= recv_buf[pos].end_offset + 1) { + recv_buf[pos].frag_length = MIN( + hsk->frag_length, recv_buf[pos].frag_length); + } else if (hsk->start_offset + hsk->frag_length > + recv_buf[pos].start_offset + + recv_buf[pos].frag_length && + hsk->start_offset <= + recv_buf[pos].start_offset + + recv_buf[pos].frag_length) { memcpy(&recv_buf[pos].data.data[hsk->start_offset], hsk->data.data, hsk->data.length); - recv_buf[pos].end_offset = hsk->end_offset; recv_buf[pos].start_offset = MIN( hsk->start_offset, recv_buf[pos].start_offset); + recv_buf[pos].frag_length = hsk->start_offset + + hsk->frag_length - + recv_buf[pos].start_offset; } _gnutls_handshake_buffer_clear(hsk); } @@ -1113,8 +1117,8 @@ static int get_last_packet(gnutls_session_t session, } else if ((recv_buf[LAST_ELEMENT].start_offset == 0 && - recv_buf[LAST_ELEMENT].end_offset == - recv_buf[LAST_ELEMENT].length - 1) || + recv_buf[LAST_ELEMENT].frag_length == + recv_buf[LAST_ELEMENT].length) || recv_buf[LAST_ELEMENT].length == 0) { session->internals.dtls.hsk_read_seq++; _gnutls_handshake_buffer_move(hsk, @@ -1125,8 +1129,9 @@ static int get_last_packet(gnutls_session_t session, /* if we don't have a complete handshake message, but we * have queued data waiting, try again to reconstruct the * handshake packet, using the queued */ - if (recv_buf[LAST_ELEMENT].end_offset != - recv_buf[LAST_ELEMENT].length - 1 && + if ((recv_buf[LAST_ELEMENT].start_offset + + recv_buf[LAST_ELEMENT].frag_length) != + recv_buf[LAST_ELEMENT].length && record_check_unprocessed(session) > 0) return gnutls_assert_val( GNUTLS_E_INT_CHECK_AGAIN); @@ -1313,9 +1318,7 @@ int _gnutls_parse_record_buffered_msgs(gnutls_session_t session) &session->internals.record_buffer, bufel, ret); - data_size = MIN(tmp.length, - tmp.end_offset - - tmp.start_offset + 1); + data_size = MIN(tmp.length, tmp.frag_length); ret = _gnutls_buffer_append_data( &tmp.data, @@ -1331,7 +1334,6 @@ int _gnutls_parse_record_buffered_msgs(gnutls_session_t session) ret = merge_handshake_packet(session, &tmp); if (ret < 0) return gnutls_assert_val(ret); - } while (_mbuffer_get_udata_size(bufel) > 0); prev = bufel; diff --git a/lib/gnutls_int.h b/lib/gnutls_int.h index 54d3c9f67..283f25c07 100644 --- a/lib/gnutls_int.h +++ b/lib/gnutls_int.h @@ -479,10 +479,10 @@ typedef struct { uint16_t sequence; /* indicate whether that message is complete. - * complete means start_offset == 0 and end_offset == length + * complete means start_offset == 0 and frag_length == length */ uint32_t start_offset; - uint32_t end_offset; + uint32_t frag_length; /* used exclusively in DTLS reassembly */ uint8_t header[MAX_HANDSHAKE_HEADER_SIZE]; int header_size;