Introduce and use dtls12_handshake_msg.

Add struct dtls12_handshake_msg and various related functions, which
allow for the construction of DTLS handshake messages and associated
fragments.

Use this on the DTLS write path for sending handshake message fragments.
This means that we no longer modify the init buffer, which also fixes a
bug where the message callback is called with a corrupted handshake
message when multiple fragments have been sent.

We also now correctly track fragment offsets when sending a handshake
message that results in multiple calls to dtls1_do_write_handshake_message().

This is the first step towards further untangling of the write path in
the legacy TLS stack.

ok kenjiro@ tb@

6 files changed, 363 insertions, 77 deletions

diff --git a/src/lib/libssl/Makefile b/src/lib/libssl/Makefile
index 7e423b0b43..0d5934508a 100644
--- a/src/lib/libssl/Makefile
+++ b/src/lib/libssl/Makefile
@@ -1,4 +1,4 @@
-# $OpenBSD: Makefile,v 1.86 2026/04/03 07:26:20 jsing Exp $
+# $OpenBSD: Makefile,v 1.87 2026/05/16 08:20:41 jsing Exp $
 
 .include <bsd.own.mk>
 .ifndef NOMAN
@@ -43,6 +43,7 @@ SRCS= \
         d1_lib.c \
         d1_pkt.c \
         d1_srtp.c \
+        dtls12_handshake_msg.c \
         pqueue.c \
         s3_cbc.c \
         s3_lib.c \
diff --git a/src/lib/libssl/d1_both.c b/src/lib/libssl/d1_both.c
index de915fa4b2..2af307f594 100644
--- a/src/lib/libssl/d1_both.c
+++ b/src/lib/libssl/d1_both.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: d1_both.c,v 1.93 2026/05/06 15:06:35 jsing Exp $ */
+/* $OpenBSD: d1_both.c,v 1.94 2026/05/16 08:20:41 jsing Exp $ */
 /*
  * DTLS implementation written by Nagendra Modadugu
  * (nagendra@cs.stanford.edu) for the OpenSSL project 2005.
@@ -205,12 +205,48 @@ dtls1_hm_fragment_free(hm_fragment *frag)
 }
 
 static int
+dtls12_create_handshake_msg(SSL *s)
+{
+        CBB cbb;
+
+        OPENSSL_assert(s->init_off == 0);
+        OPENSSL_assert(s->init_num == (int)s->d1->w_msg_hdr.msg_len +
+            DTLS1_HM_HEADER_LENGTH);
+
+        /* Skip over the existing header. */
+        s->init_off += DTLS1_HM_HEADER_LENGTH;
+        s->init_num -= DTLS1_HM_HEADER_LENGTH;
+
+        if (s->d1->hs_msg != NULL)
+                goto err;
+
+        if ((s->d1->hs_msg = dtls12_handshake_msg_new()) == NULL)
+                goto err;
+        if (!dtls12_handshake_msg_start(s->d1->hs_msg, &cbb,
+            s->d1->w_msg_hdr.type, s->d1->w_msg_hdr.seq))
+                goto err;
+        if (!CBB_add_bytes(&cbb, &s->init_buf->data[s->init_off],
+            s->init_num))
+                goto err;
+        if (!dtls12_handshake_msg_finish(s->d1->hs_msg))
+                goto err;
+
+        return 1;
+
+ err:
+        dtls12_handshake_msg_free(s->d1->hs_msg);
+        s->d1->hs_msg = NULL;
+
+        return 0;
+}
+
+static int
 dtls1_do_write_handshake_message(SSL *s)
 {
-        int ret;
-        int curr_mtu;
-        unsigned int len, frag_off;
+        int curr_mtu, written;
         size_t overhead;
+        CBS cbs;
+        int ret;
 
         /* AHA!  Figure out the MTU, and stick to the right size */
         if (s->d1->mtu < dtls1_min_mtu() &&
@@ -234,15 +270,15 @@ dtls1_do_write_handshake_message(SSL *s)
         OPENSSL_assert(s->d1->mtu >= dtls1_min_mtu());
         /* should have something reasonable now */
 
-        if (s->init_off == 0)
-                OPENSSL_assert(s->init_num ==
-                    (int)s->d1->w_msg_hdr.msg_len + DTLS1_HM_HEADER_LENGTH);
+        if (s->d1->hs_msg == NULL) {
+                if (!dtls12_create_handshake_msg(s))
+                        return -1;
+        }
 
         if (!tls12_record_layer_write_overhead(s->rl, &overhead))
                 return -1;
 
-        frag_off = 0;
-        while (s->init_num > 0) {
+        do {
                 curr_mtu = s->d1->mtu - BIO_wpending(SSL_get_wbio(s)) -
                     DTLS1_RT_HEADER_LENGTH - overhead;
 
@@ -255,35 +291,14 @@ dtls1_do_write_handshake_message(SSL *s)
                             overhead;
                 }
 
-                if (s->init_num > curr_mtu)
-                        len = curr_mtu;
-                else
-                        len = s->init_num;
-
-                if (s->init_off != 0) {
-                        OPENSSL_assert(s->init_off > DTLS1_HM_HEADER_LENGTH);
-                        s->init_off -= DTLS1_HM_HEADER_LENGTH;
-                        s->init_num += DTLS1_HM_HEADER_LENGTH;
-
-                        if (s->init_num > curr_mtu)
-                                len = curr_mtu;
-                        else
-                                len = s->init_num;
-                }
-
-                OPENSSL_assert(len >= DTLS1_HM_HEADER_LENGTH);
-
-                s->d1->w_msg_hdr.frag_off = frag_off;
-                s->d1->w_msg_hdr.frag_len = len - DTLS1_HM_HEADER_LENGTH;
+                OPENSSL_assert(curr_mtu >= DTLS1_HM_HEADER_LENGTH);
 
-                if (!dtls1_write_message_header(&s->d1->w_msg_hdr,
-                    s->d1->w_msg_hdr.frag_off, s->d1->w_msg_hdr.frag_len,
-                    (unsigned char *)&s->init_buf->data[s->init_off]))
+                if (!dtls12_handshake_msg_fragment_build(s->d1->hs_msg,
+                    curr_mtu - DTLS1_HM_HEADER_LENGTH, &cbs))
                         return -1;
 
-                ret = dtls1_write_bytes(s, SSL3_RT_HANDSHAKE,
-                    &s->init_buf->data[s->init_off], len);
-                if (ret < 0) {
+                if ((written = dtls1_write_bytes(s, SSL3_RT_HANDSHAKE,
+                    CBS_data(&cbs), CBS_len(&cbs))) < 0) {
                         /*
                          * Might need to update MTU here, but we don't know
                          * which previous packet caused the failure -- so
@@ -293,7 +308,7 @@ dtls1_do_write_handshake_message(SSL *s)
                          */
                         if (BIO_ctrl(SSL_get_wbio(s),
                             BIO_CTRL_DGRAM_MTU_EXCEEDED, 0, NULL) <= 0)
-                                return (-1);
+                                return -1;
 
                         s->d1->mtu = BIO_ctrl(SSL_get_wbio(s),
                             BIO_CTRL_DGRAM_QUERY_MTU, 0, NULL);
@@ -306,49 +321,34 @@ dtls1_do_write_handshake_message(SSL *s)
                  * handshake message got sent.  but why would
                  * this happen?
                  */
-                OPENSSL_assert(len == (unsigned int)ret);
+                OPENSSL_assert(CBS_len(&cbs) == (size_t)written);
 
-                if (!s->d1->retransmitting) {
-                        /*
-                         * Should not be done for 'Hello Request's,
-                         * but in that case we'll ignore the result
-                         * anyway
-                         */
-                        unsigned char *p = (unsigned char *)&s->init_buf->data[s->init_off];
-                        const struct hm_header_st *msg_hdr = &s->d1->w_msg_hdr;
-                        int xlen;
-
-                        if (frag_off == 0) {
-                                /*
-                                 * Reconstruct message header is if it
-                                 * is being sent in single fragment
-                                 */
-                                if (!dtls1_write_message_header(msg_hdr,
-                                    0, msg_hdr->msg_len, p))
-                                        return (-1);
-                                xlen = ret;
-                        } else {
-                                p += DTLS1_HM_HEADER_LENGTH;
-                                xlen = ret - DTLS1_HM_HEADER_LENGTH;
-                        }
-
-                        tls1_transcript_record(s, p, xlen);
-                }
+                if (!dtls12_handshake_msg_fragment_next(s->d1->hs_msg))
+                        return -1;
 
-                if (ret == s->init_num) {
-                        ssl_msg_callback(s, 1, SSL3_RT_HANDSHAKE,
-                            s->init_buf->data, s->init_off + s->init_num);
+        } while (dtls12_handshake_msg_fragment_pending(s->d1->hs_msg));
 
-                        s->init_off = 0;
-                        s->init_num = 0;
+        dtls12_handshake_msg_data(s->d1->hs_msg, &cbs);
 
-                        return (1);
-                }
-                s->init_off += ret;
-                s->init_num -= ret;
-                frag_off += (ret -= DTLS1_HM_HEADER_LENGTH);
+        if (!s->d1->retransmitting) {
+                /*
+                 * The TLS transcript is based on each handshake message being
+                 * sent as a single fragment - see RFC 6347 section 4.2.6. This
+                 * should not be called for a HelloRequest, however the result
+                 * will be ignored.
+                 */
+                tls1_transcript_record(s, CBS_data(&cbs), CBS_len(&cbs));
         }
-        return (0);
+
+        ssl_msg_callback(s, 1, SSL3_RT_HANDSHAKE, CBS_data(&cbs), CBS_len(&cbs));
+
+        dtls12_handshake_msg_free(s->d1->hs_msg);
+        s->d1->hs_msg = NULL;
+
+        s->init_off = 0;
+        s->init_num = 0;
+
+        return 1;
 }
 
 static int
diff --git a/src/lib/libssl/d1_lib.c b/src/lib/libssl/d1_lib.c
index 69db8a0df4..fa546b381c 100644
--- a/src/lib/libssl/d1_lib.c
+++ b/src/lib/libssl/d1_lib.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: d1_lib.c,v 1.65 2024/07/23 14:40:53 jsing Exp $ */
+/* $OpenBSD: d1_lib.c,v 1.66 2026/05/16 08:20:41 jsing Exp $ */
 /*
  * DTLS implementation written by Nagendra Modadugu
  * (nagendra@cs.stanford.edu) for the OpenSSL project 2005.
@@ -179,6 +179,8 @@ dtls1_free(SSL *s)
         pqueue_free(s->d1->sent_messages);
         pqueue_free(s->d1->buffered_app_data.q);
 
+        dtls12_handshake_msg_free(s->d1->hs_msg);
+
         freezero(s->d1, sizeof(*s->d1));
         s->d1 = NULL;
 }
@@ -199,6 +201,9 @@ dtls1_clear(SSL *s)
                 buffered_app_data = s->d1->buffered_app_data.q;
                 mtu = s->d1->mtu;
 
+                dtls12_handshake_msg_free(s->d1->hs_msg);
+                s->d1->hs_msg = NULL;
+
                 dtls1_clear_queues(s);
 
                 memset(s->d1, 0, sizeof(*s->d1));
diff --git a/src/lib/libssl/dtls12_handshake_msg.c b/src/lib/libssl/dtls12_handshake_msg.c
new file mode 100644
index 0000000000..7874aa8598
--- /dev/null
+++ b/src/lib/libssl/dtls12_handshake_msg.c
@@ -0,0 +1,231 @@
+/* $OpenBSD: dtls12_handshake_msg.c,v 1.1 2026/05/16 08:20:41 jsing Exp $ */
+/*
+ * Copyright (c) 2026 Joel Sing <jsing@openbsd.org>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "bytestring.h"
+#include "dtls12_internal.h"
+
+#define DTLS12_HANDSHAKE_MSG_HEADER_LEN                 12
+#define DTLS12_HANDSHAKE_MSG_INITIAL_LEN                256
+
+#define DTLS12_HANDSHAKE_MSG_FRAGMENT_LENGTH_OFFSET     9
+
+struct dtls12_handshake_msg {
+        uint8_t msg_type;
+        uint32_t msg_len;
+        uint16_t msg_seq;
+
+        uint32_t fragment_offset;
+        uint32_t fragment_len;
+        uint8_t *fragment_data;
+        size_t fragment_data_len;
+        int fragment_pending;
+
+        uint8_t *data;
+        size_t data_len;
+
+        CBS cbs;
+        CBB cbb;
+};
+
+struct dtls12_handshake_msg *
+dtls12_handshake_msg_new(void)
+{
+        struct dtls12_handshake_msg *msg = NULL;
+
+        if ((msg = calloc(1, sizeof(struct dtls12_handshake_msg))) == NULL)
+                return NULL;
+
+        return msg;
+}
+
+void
+dtls12_handshake_msg_free(struct dtls12_handshake_msg *msg)
+{
+        if (msg == NULL)
+                return;
+
+        CBB_cleanup(&msg->cbb);
+
+        freezero(msg->data, msg->data_len);
+        freezero(msg->fragment_data, msg->fragment_data_len);
+
+        freezero(msg, sizeof(struct dtls12_handshake_msg));
+}
+
+void
+dtls12_handshake_msg_data(struct dtls12_handshake_msg *msg, CBS *cbs)
+{
+        CBS_init(cbs, msg->data, msg->data_len);
+}
+
+uint8_t
+dtls12_handshake_msg_type(struct dtls12_handshake_msg *msg)
+{
+        return msg->msg_type;
+}
+
+int
+dtls12_handshake_msg_content(struct dtls12_handshake_msg *msg, CBS *cbs)
+{
+        dtls12_handshake_msg_data(msg, cbs);
+
+        return CBS_skip(cbs, DTLS12_HANDSHAKE_MSG_HEADER_LEN);
+}
+
+int
+dtls12_handshake_msg_start(struct dtls12_handshake_msg *msg, CBB *body,
+    uint8_t msg_type, size_t msg_seq)
+{
+        msg->msg_type = msg_type;
+        msg->msg_seq = msg_seq;
+
+        msg->msg_len = 0;
+        msg->fragment_offset = 0;
+
+        if (!CBB_init(&msg->cbb, DTLS12_HANDSHAKE_MSG_INITIAL_LEN))
+                return 0;
+        if (!CBB_add_u8(&msg->cbb, msg->msg_type))
+                return 0;
+        if (!CBB_add_u24(&msg->cbb, msg->msg_len))
+                return 0;
+        if (!CBB_add_u16(&msg->cbb, msg->msg_seq))
+                return 0;
+        if (!CBB_add_u24(&msg->cbb, msg->fragment_offset))
+                return 0;
+        if (!CBB_add_u24_length_prefixed(&msg->cbb, body))
+                return 0;
+
+        return 1;
+}
+
+int
+dtls12_handshake_msg_finish(struct dtls12_handshake_msg *msg)
+{
+        CBS cbs;
+
+        if (!CBB_finish(&msg->cbb, &msg->data, &msg->data_len))
+                return 0;
+
+        /* Update message length to match fragment length. */
+        CBS_init(&cbs, msg->data, msg->data_len);
+        if (!CBS_skip(&cbs, DTLS12_HANDSHAKE_MSG_FRAGMENT_LENGTH_OFFSET))
+                return 0;
+        if (!CBS_get_u24(&cbs, &msg->msg_len))
+                return 0;
+
+        if (!CBB_init_fixed(&msg->cbb, msg->data, msg->data_len))
+                return 0;
+        if (!CBB_add_u8(&msg->cbb, msg->msg_type))
+                return 0;
+        if (!CBB_add_u24(&msg->cbb, msg->msg_len))
+                return 0;
+        if (!CBB_finish(&msg->cbb, NULL, NULL))
+                return 0;
+
+        dtls12_handshake_msg_fragment_reset(msg);
+
+        return 1;
+}
+
+int
+dtls12_handshake_msg_fragment_reset(struct dtls12_handshake_msg *msg)
+{
+        freezero(msg->fragment_data, msg->fragment_data_len);
+        msg->fragment_data = NULL;
+        msg->fragment_data_len = 0;
+
+        msg->fragment_offset = 0;
+        msg->fragment_pending = 1;
+
+        return dtls12_handshake_msg_content(msg, &msg->cbs);
+}
+
+int
+dtls12_handshake_msg_fragment_build(struct dtls12_handshake_msg *msg,
+    size_t max_fragment_len, CBS *cbs)
+{
+        CBB body;
+
+        CBS_init(cbs, NULL, 0);
+
+        if (msg->fragment_offset > msg->msg_len)
+                return 0;
+        if (msg->msg_len - msg->fragment_offset > CBS_len(&msg->cbs))
+                return 0;
+
+        freezero(msg->fragment_data, msg->fragment_data_len);
+        msg->fragment_data = NULL;
+        msg->fragment_data_len = 0;
+
+        if ((msg->fragment_len = CBS_len(&msg->cbs)) > max_fragment_len)
+                msg->fragment_len = max_fragment_len;
+
+        /* Build the fragment. */
+        if (!CBB_init(&msg->cbb, DTLS12_HANDSHAKE_MSG_INITIAL_LEN))
+                goto err;
+        if (!CBB_add_u8(&msg->cbb, msg->msg_type))
+                goto err;
+        if (!CBB_add_u24(&msg->cbb, msg->msg_len))
+                goto err;
+        if (!CBB_add_u16(&msg->cbb, msg->msg_seq))
+                goto err;
+        if (!CBB_add_u24(&msg->cbb, msg->fragment_offset))
+                goto err;
+        if (!CBB_add_u24_length_prefixed(&msg->cbb, &body))
+                goto err;
+        if (!CBB_add_bytes(&body, CBS_data(&msg->cbs), msg->fragment_len))
+                goto err;
+        if (!CBB_finish(&msg->cbb, &msg->fragment_data, &msg->fragment_data_len))
+                goto err;
+
+        CBS_init(cbs, msg->fragment_data, msg->fragment_data_len);
+
+        return 1;
+
+ err:
+        CBB_cleanup(&msg->cbb);
+
+        return 0;
+}
+
+int
+dtls12_handshake_msg_fragment_next(struct dtls12_handshake_msg *msg)
+{
+        if (msg->fragment_offset > msg->msg_len)
+                return 0;
+        if (msg->msg_len - msg->fragment_offset < msg->fragment_len)
+                return 0;
+
+        if (!CBS_skip(&msg->cbs, msg->fragment_len))
+                return 0;
+
+        msg->fragment_offset += msg->fragment_len;
+
+        msg->fragment_pending = (CBS_len(&msg->cbs) > 0);
+
+        return 1;
+}
+
+int
+dtls12_handshake_msg_fragment_pending(struct dtls12_handshake_msg *msg)
+{
+        return msg->fragment_pending;
+}
diff --git a/src/lib/libssl/dtls12_internal.h b/src/lib/libssl/dtls12_internal.h
new file mode 100644
index 0000000000..f3fa9b7490
--- /dev/null
+++ b/src/lib/libssl/dtls12_internal.h
@@ -0,0 +1,46 @@
+/* $OpenBSD: dtls12_internal.h,v 1.1 2026/05/16 08:20:41 jsing Exp $ */
+/*
+ * Copyright (c) 2026 Joel Sing <jsing@openbsd.org>
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+ * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+ * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#ifndef HEADER_DTLS12_INTERNAL_H
+#define HEADER_DTLS12_INTERNAL_H
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "bytestring.h"
+
+__BEGIN_HIDDEN_DECLS
+
+struct dtls12_handshake_msg;
+
+struct dtls12_handshake_msg *dtls12_handshake_msg_new(void);
+void dtls12_handshake_msg_free(struct dtls12_handshake_msg *msg);
+void dtls12_handshake_msg_data(struct dtls12_handshake_msg *msg, CBS *cbs);
+uint8_t dtls12_handshake_msg_type(struct dtls12_handshake_msg *msg);
+int dtls12_handshake_msg_content(struct dtls12_handshake_msg *msg, CBS *cbs);
+int dtls12_handshake_msg_start(struct dtls12_handshake_msg *msg, CBB *body,
+    uint8_t msg_type, size_t msg_seq);
+int dtls12_handshake_msg_finish(struct dtls12_handshake_msg *msg);
+int dtls12_handshake_msg_fragment_build(struct dtls12_handshake_msg *msg,
+    size_t max_fragment_len, CBS *cbs);
+int dtls12_handshake_msg_fragment_next(struct dtls12_handshake_msg *msg);
+int dtls12_handshake_msg_fragment_pending(struct dtls12_handshake_msg *msg);
+int dtls12_handshake_msg_fragment_reset(struct dtls12_handshake_msg *msg);
+
+__END_HIDDEN_DECLS
+
+#endif
diff --git a/src/lib/libssl/dtls_local.h b/src/lib/libssl/dtls_local.h
index 5a85bba607..8691663eb3 100644
--- a/src/lib/libssl/dtls_local.h
+++ b/src/lib/libssl/dtls_local.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: dtls_local.h,v 1.4 2026/05/06 15:06:35 jsing Exp $ */
+/* $OpenBSD: dtls_local.h,v 1.5 2026/05/16 08:20:41 jsing Exp $ */
 /*
  * DTLS implementation written by Nagendra Modadugu
  * (nagendra@cs.stanford.edu) for the OpenSSL project 2005.
@@ -65,6 +65,7 @@
 #include <openssl/dtls1.h>
 
 #include "ssl_local.h"
+#include "dtls12_internal.h"
 #include "tls_content.h"
 
 __BEGIN_HIDDEN_DECLS
@@ -179,6 +180,8 @@ struct dtls1_state_st {
 
         unsigned int retransmitting;
         unsigned int change_cipher_spec_ok;
+
+        struct dtls12_handshake_msg *hs_msg;
 };
 
 int dtls1_do_write(SSL *s, int type);