vtls: initial implementation of rustls backend

This adds a new TLS backend, rustls. It uses the C-to-rustls bindings
from https://github.com/abetterinternet/crustls.

Rustls is at https://github.com/ctz/rustls/.

There is still a fair bit to be done, like sending CloseNotify on
connection shutdown, respecting CAPATH, and properly indicating features
like "supports TLS 1.3 ciphersuites." But it works well enough to make
requests and receive responses.

Blog post for context:
https://www.abetterinternet.org/post/memory-safe-curl/

Closes #6350

12 files changed, 639 insertions, 8 deletions

diff --git a/configure.ac b/configure.ac
index b7aef5ba3..cf4b5278f 100755
--- a/configure.ac
+++ b/configure.ac
@@ -143,7 +143,7 @@ AC_SUBST(PKGADD_VENDOR)
 
 dnl
 dnl initialize all the info variables
-    curl_ssl_msg="no      (--with-{ssl,gnutls,nss,mbedtls,wolfssl,schannel,secure-transport,mesalink,amissl,bearssl} )"
+    curl_ssl_msg="no      (--with-{ssl,gnutls,nss,mbedtls,wolfssl,schannel,secure-transport,mesalink,amissl,bearssl,rustls} )"
     curl_ssh_msg="no      (--with-{libssh,libssh2})"
    curl_zlib_msg="no      (--with-zlib)"
  curl_brotli_msg="no      (--with-brotli)"
@@ -2699,6 +2699,98 @@ if test -z "$ssl_backends" -o "x$OPT_BEARSSL" != xno; then
 fi
 
 dnl ----------------------------------------------------
+dnl check for rustls
+dnl ----------------------------------------------------
+
+OPT_RUSTLS=no
+
+_cppflags=$CPPFLAGS
+_ldflags=$LDFLAGS
+AC_ARG_WITH(rustls,dnl
+AC_HELP_STRING([--with-rustls=PATH],[where to look for rustls, PATH points to the installation root])
+AC_HELP_STRING([--without-rustls], [disable rustls detection]),
+  OPT_RUSTLS=$withval)
+
+if test -z "$ssl_backends" -o "x$OPT_RUSTLS" != xno; then
+  ssl_msg=
+
+  if test X"$OPT_RUSTLS" != Xno; then
+
+    if test "$OPT_RUSTLS" = "yes"; then
+      OPT_RUSTLS=""
+    fi
+
+    if test -z "$OPT_RUSTLS" ; then
+      dnl check for lib first without setting any new path
+
+      AC_CHECK_LIB(crustls, rustls_client_session_read,
+      dnl libcrustls found, set the variable
+       [
+         AC_DEFINE(USE_RUSTLS, 1, [if rustls is enabled])
+         AC_SUBST(USE_RUSTLS, [1])
+         RUSTLS_ENABLED=1
+         USE_RUSTLS="yes"
+         ssl_msg="rustls"
+	 test rustls != "$DEFAULT_SSL_BACKEND" || VALID_DEFAULT_SSL_BACKEND=yes
+        ], [], -lpthread -ldl)
+    fi
+
+    addld=""
+    addlib="-lpthread"
+    addcflags=""
+    bearssllib=""
+
+    if test "x$USE_RUSTLS" != "xyes"; then
+      dnl add the path and test again
+      addld=-L$OPT_RUSTLS/lib$libsuff
+      addcflags=-I$OPT_RUSTLS/include
+      rustlslib=$OPT_RUSTLS/lib$libsuff
+
+      LDFLAGS="$LDFLAGS $addld"
+      if test "$addcflags" != "-I/usr/include"; then
+         CPPFLAGS="$CPPFLAGS $addcflags"
+      fi
+
+      AC_CHECK_LIB(crustls, rustls_client_session_read,
+       [
+       AC_DEFINE(USE_RUSTLS, 1, [if rustls is enabled])
+       AC_SUBST(USE_RUSTLS, [1])
+       RUSTLS_ENABLED=1
+       USE_RUSTLS="yes"
+       ssl_msg="rustls"
+       test rustls != "$DEFAULT_SSL_BACKEND" || VALID_DEFAULT_SSL_BACKEND=yes
+       ],
+       [
+         CPPFLAGS=$_cppflags
+         LDFLAGS=$_ldflags
+       ], -lpthread -ldl)
+    fi
+
+    if test "x$USE_RUSTLS" = "xyes"; then
+      AC_MSG_NOTICE([detected rustls])
+      check_for_ca_bundle=1
+
+      LIBS="-lcrustls -lpthread -ldl $LIBS"
+
+      if test -n "$rustlslib"; then
+        dnl when shared libs were found in a path that the run-time
+        dnl linker doesn't search through, we need to add it to
+        dnl CURL_LIBRARY_PATH to prevent further configure tests to fail
+        dnl due to this
+        if test "x$cross_compiling" != "xyes"; then
+          CURL_LIBRARY_PATH="$CURL_LIBRARY_PATH:$rustlslib"
+          export CURL_LIBRARY_PATH
+          AC_MSG_NOTICE([Added $rustlslib to CURL_LIBRARY_PATH])
+        fi
+      fi
+    fi
+
+  fi dnl rustls not disabled
+
+  test -z "$ssl_msg" || ssl_backends="${ssl_backends:+$ssl_backends, }$ssl_msg"
+fi
+
+dnl ----------------------------------------------------
 dnl NSS. Only check if GnuTLS and OpenSSL are not enabled
 dnl ----------------------------------------------------
 
@@ -2828,7 +2920,8 @@ if test -z "$ssl_backends" -o "x$OPT_NSS" != xno; then
   test -z "$ssl_msg" || ssl_backends="${ssl_backends:+$ssl_backends, }$ssl_msg"
 fi
 
-case "x$OPENSSL_ENABLED$GNUTLS_ENABLED$NSS_ENABLED$MBEDTLS_ENABLED$WOLFSSL_ENABLED$SCHANNEL_ENABLED$SECURETRANSPORT_ENABLED$MESALINK_ENABLED$BEARSSL_ENABLED$AMISSL_ENABLED" in
+case "x$OPENSSL_ENABLED$GNUTLS_ENABLED$NSS_ENABLED$MBEDTLS_ENABLED$WOLFSSL_ENABLED$SCHANNEL_ENABLED$SECURETRANSPORT_ENABLED$MESALINK_ENABLED$BEARSSL_ENABLED$AMISSL_ENABLED$RUSTLS_ENABLED"
+in
 x)
   AC_MSG_WARN([SSL disabled, you will not be able to use HTTPS, FTPS, NTLM and more.])
   AC_MSG_WARN([Use --with-ssl, --with-gnutls, --with-wolfssl, --with-mbedtls, --with-nss, --with-schannel, --with-secure-transport, --with-mesalink, --with-amissl or --with-bearssl to address this.])
diff --git a/docs/FAQ b/docs/FAQ
index 32e8bb86c..d2dfc5f0c 100644
--- a/docs/FAQ
+++ b/docs/FAQ
@@ -417,8 +417,8 @@ FAQ
   curl can be built to use one of the following SSL alternatives: OpenSSL,
   libressl, BoringSSL, GnuTLS, wolfSSL, NSS, mbedTLS, MesaLink, Secure
   Transport (native iOS/OS X), Schannel (native Windows), GSKit (native IBM
-  i), or BearSSL. They all have their pros and cons, and we try to maintain a
-  comparison of them here: https://curl.se/docs/ssl-compared.html
+  i), BearSSL, or Rustls. They all have their pros and cons, and we try to
+  maintain a comparison of them here: https://curl.se/docs/ssl-compared.html
 
   2.4 Does curl support SOCKS (RFC 1928) ?
 
diff --git a/docs/libcurl/symbols-in-versions b/docs/libcurl/symbols-in-versions
index 0b633e41c..66c36c4ce 100644
--- a/docs/libcurl/symbols-in-versions
+++ b/docs/libcurl/symbols-in-versions
@@ -806,6 +806,7 @@ CURLSSLBACKEND_NONE             7.34.0
 CURLSSLBACKEND_NSS              7.34.0
 CURLSSLBACKEND_OPENSSL          7.34.0
 CURLSSLBACKEND_POLARSSL         7.34.0       7.69.0
+CURLSSLBACKEND_RUSTLS           7.76.0
 CURLSSLBACKEND_QSOSSL           7.34.0        -           7.38.1
 CURLSSLBACKEND_SCHANNEL         7.34.0
 CURLSSLBACKEND_SECURETRANSPORT  7.64.1
diff --git a/include/curl/curl.h b/include/curl/curl.h
index 71204ee32..46503178d 100644
--- a/include/curl/curl.h
+++ b/include/curl/curl.h
@@ -155,7 +155,8 @@ typedef enum {
   CURLSSLBACKEND_AXTLS = 10, /* never used since 7.63.0 */
   CURLSSLBACKEND_MBEDTLS = 11,
   CURLSSLBACKEND_MESALINK = 12,
-  CURLSSLBACKEND_BEARSSL = 13
+  CURLSSLBACKEND_BEARSSL = 13,
+  CURLSSLBACKEND_RUSTLS = 14
 } curl_sslbackend;
 
 /* aliases for library clones and renames */
diff --git a/lib/Makefile.inc b/lib/Makefile.inc
index dddb4a491..90ec6aa05 100644
--- a/lib/Makefile.inc
+++ b/lib/Makefile.inc
@@ -50,6 +50,7 @@ LIB_VTLS_CFILES =           \
   vtls/mesalink.c           \
   vtls/nss.c                \
   vtls/openssl.c            \
+  vtls/rustls.c             \
   vtls/schannel.c           \
   vtls/schannel_verify.c    \
   vtls/sectransp.c          \
@@ -66,6 +67,7 @@ LIB_VTLS_HFILES =           \
   vtls/mesalink.h           \
   vtls/nssg.h               \
   vtls/openssl.h            \
+  vtls/rustls.h             \
   vtls/schannel.h           \
   vtls/sectransp.h          \
   vtls/vtls.h               \
diff --git a/lib/curl_setup.h b/lib/curl_setup.h
index 9cef5f7d0..6c9e2d738 100644
--- a/lib/curl_setup.h
+++ b/lib/curl_setup.h
@@ -616,7 +616,7 @@ int netware_init(void);
     defined(USE_MBEDTLS) || \
     defined(USE_WOLFSSL) || defined(USE_SCHANNEL) || \
     defined(USE_SECTRANSP) || defined(USE_GSKIT) || defined(USE_MESALINK) || \
-    defined(USE_BEARSSL)
+    defined(USE_BEARSSL) || defined(USE_RUSTLS)
 #define USE_SSL    /* SSL support has been enabled */
 #endif
 
diff --git a/lib/http.c b/lib/http.c
index 72aa96297..806f639dc 100644
--- a/lib/http.c
+++ b/lib/http.c
@@ -1552,7 +1552,7 @@ static int https_getsock(struct Curl_easy *data,
 {
   (void)data;
   if(conn->handler->flags & PROTOPT_SSL)
-    return Curl_ssl_getsock(conn, socks);
+    return Curl_ssl->getsock(conn, socks);
   return GETSOCK_BLANK;
 }
 #endif /* USE_SSL */
diff --git a/lib/multi.c b/lib/multi.c
index 85707a1af..e0862551d 100644
--- a/lib/multi.c
+++ b/lib/multi.c
@@ -905,7 +905,7 @@ static int waitconnect_getsock(struct connectdata *conn,
 #ifdef USE_SSL
 #ifndef CURL_DISABLE_PROXY
   if(CONNECT_FIRSTSOCKET_PROXY_SSL())
-    return Curl_ssl_getsock(conn, sock);
+    return Curl_ssl->getsock(conn, sock);
 #endif
 #endif
 
diff --git a/lib/vtls/rustls.c b/lib/vtls/rustls.c
new file mode 100644
index 000000000..7e00ce7c0
--- /dev/null
+++ b/lib/vtls/rustls.c
@@ -0,0 +1,496 @@
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) 2020, Jacob Hoffman-Andrews, <github@hoffman-andrews.com>
+ *
+ * This software is licensed as described in the file COPYING, which
+ * you should have received as part of this distribution. The terms
+ * are also available at https://curl.se/docs/copyright.html.
+ *
+ * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+ * copies of the Software, and permit persons to whom the Software is
+ * furnished to do so, under the terms of the COPYING file.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ***************************************************************************/
+#include "curl_setup.h"
+
+#ifdef USE_RUSTLS
+
+#include "curl_printf.h"
+
+#include <errno.h>
+#include <crustls.h>
+
+#include "urldata.h"
+#include "sendf.h"
+#include "vtls.h"
+#include "select.h"
+
+#include "multiif.h"
+
+struct ssl_backend_data
+{
+  const struct rustls_client_config *config;
+  struct rustls_client_session *session;
+  bool data_pending;
+};
+
+/* For a given rustls_result error code, return the best-matching CURLcode. */
+static CURLcode map_error(rustls_result r)
+{
+  if(rustls_result_is_cert_error(r)) {
+    return CURLE_PEER_FAILED_VERIFICATION;
+  }
+  switch(r) {
+    case RUSTLS_RESULT_OK:
+      return CURLE_OK;
+    case RUSTLS_RESULT_NULL_PARAMETER:
+      return CURLE_BAD_FUNCTION_ARGUMENT;
+    default:
+      return CURLE_READ_ERROR;
+  }
+}
+
+static bool
+cr_data_pending(const struct connectdata *conn, int sockindex)
+{
+  struct Curl_easy *data = conn->data;
+  const struct ssl_connect_data *connssl = &conn->ssl[sockindex];
+  struct ssl_backend_data *backend = connssl->backend;
+  infof(data, "rustls_data_pending %d\n", backend->data_pending);
+  return backend->data_pending;
+}
+
+static CURLcode
+cr_connect(struct Curl_easy *data UNUSED_PARAM,
+                    struct connectdata *conn UNUSED_PARAM,
+                    int sockindex UNUSED_PARAM)
+{
+  infof(data, "rustls_connect: unimplemented\n");
+  return CURLE_SSL_CONNECT_ERROR;
+}
+
+/*
+ * On each run:
+ *  - Read a chunk of bytes from the socket into rustls' TLS input buffer.
+ *  - Tell rustls to process any new packets.
+ *  - Read out as many plaintext bytes from rustls as possible, until hitting
+ *    error, EOF, or EAGAIN/EWOULDBLOCK, or plainbuf/plainlen is filled up.
+ *
+ * It's okay to call this function with plainbuf == NULL and plainlen == 0.
+ * In that case, it will copy bytes from the socket into rustls' TLS input
+ * buffer, and process packets, but won't consume bytes from rustls' plaintext
+ * output buffer.
+ */
+static ssize_t
+cr_recv(struct Curl_easy *data, int sockindex,
+            char *plainbuf, size_t plainlen, CURLcode *err)
+{
+  struct connectdata *conn = data->conn;
+  struct ssl_connect_data *const connssl = &conn->ssl[sockindex];
+  struct ssl_backend_data *const backend = connssl->backend;
+  struct rustls_client_session *const session = backend->session;
+  curl_socket_t sockfd = conn->sock[sockindex];
+  /* Per https://www.bearssl.org/api1.html, max TLS record size plus max
+     per-record overhead. */
+  uint8_t tlsbuf[16384 + 325];
+  size_t n = 0;
+  ssize_t tls_bytes_read = 0;
+  size_t tls_bytes_processed = 0;
+  size_t plain_bytes_copied = 0;
+  rustls_result rresult = 0;
+  char errorbuf[255];
+
+  tls_bytes_read = sread(sockfd, tlsbuf, sizeof(tlsbuf));
+  if(tls_bytes_read == 0) {
+    failf(data, "EOF in sread");
+    *err = CURLE_READ_ERROR;
+    return -1;
+  }
+  else if(tls_bytes_read < 0) {
+    if(SOCKERRNO == EAGAIN || SOCKERRNO == EWOULDBLOCK) {
+      infof(data, "sread: EAGAIN or EWOULDBLOCK\n");
+      *err = CURLE_AGAIN;
+      return -1;
+    }
+    failf(data, "reading from socket: %s", strerror(SOCKERRNO));
+    *err = CURLE_READ_ERROR;
+    return -1;
+  }
+
+  /*
+  * Now pull those bytes from the buffer into ClientSession.
+  */
+  DEBUGASSERT(tls_bytes_read > 0);
+  while(tls_bytes_processed < (size_t)tls_bytes_read) {
+    rresult = rustls_client_session_read_tls(session,
+      (uint8_t *)tlsbuf + tls_bytes_processed,
+      tls_bytes_read - tls_bytes_processed,
+      &n);
+    if(rresult != RUSTLS_RESULT_OK) {
+      failf(data, "error in rustls_client_session_read_tls");
+      *err = CURLE_READ_ERROR;
+      return -1;
+    }
+    else if(n == 0) {
+      infof(data, "EOF from rustls_client_session_read_tls\n");
+      break;
+    }
+
+    rresult = rustls_client_session_process_new_packets(session);
+    if(rresult != RUSTLS_RESULT_OK) {
+      rustls_error(rresult, errorbuf, sizeof(errorbuf), &n);
+      failf(data, "%.*s", n, errorbuf);
+      *err = map_error(rresult);
+      return -1;
+    }
+
+    tls_bytes_processed += n;
+    backend->data_pending = TRUE;
+  }
+
+  while(plain_bytes_copied < plainlen) {
+    rresult = rustls_client_session_read(session,
+      (uint8_t *)plainbuf + plain_bytes_copied,
+      plainlen - plain_bytes_copied,
+      &n);
+    if(rresult != RUSTLS_RESULT_OK) {
+      failf(data, "error in rustls_client_session_read");
+      *err = CURLE_READ_ERROR;
+      return -1;
+    }
+    else if(n == 0) {
+      /* rustls returns 0 from client_session_read to mean "all currently
+        available data has been read." If we bring in more ciphertext with
+        read_tls, more plaintext will become available. So don't tell curl
+        this is an EOF. Instead, say "come back later." */
+      infof(data, "EOF from rustls_client_session_read\n");
+      backend->data_pending = FALSE;
+      break;
+    }
+    else {
+      plain_bytes_copied += n;
+    }
+  }
+
+  /* If we wrote out 0 plaintext bytes, it might just mean we haven't yet
+     read a full TLS record. Return CURLE_AGAIN so curl doesn't treat this
+     as EOF. */
+  if(plain_bytes_copied == 0) {
+    *err = CURLE_AGAIN;
+    return -1;
+  }
+
+  return plain_bytes_copied;
+}
+
+/*
+ * On each call:
+ *  - Copy `plainlen` bytes into rustls' plaintext input buffer (if > 0).
+ *  - Fully drain rustls' plaintext output buffer into the socket until
+ *    we get either an error or EAGAIN/EWOULDBLOCK.
+ *
+ * It's okay to call this function with plainbuf == NULL and plainlen == 0.
+ * In that case, it won't read anything into rustls' plaintext input buffer.
+ * It will only drain rustls' plaintext output buffer into the socket.
+ */
+static ssize_t
+cr_send(struct Curl_easy *data, int sockindex,
+            const void *plainbuf, size_t plainlen, CURLcode *err)
+{
+  struct connectdata *conn = data->conn;
+  struct ssl_connect_data *const connssl = &conn->ssl[sockindex];
+  struct ssl_backend_data *const backend = connssl->backend;
+  struct rustls_client_session *const session = backend->session;
+  curl_socket_t sockfd = conn->sock[sockindex];
+  ssize_t n = 0;
+  size_t plainwritten = 0;
+  size_t tlslen = 0;
+  size_t tlswritten = 0;
+  /* Max size of a TLS message, plus some space for TLS framing overhead. */
+  uint8_t tlsbuf[16384 + 325];
+  rustls_result rresult;
+
+  if(plainlen > 0) {
+    rresult = rustls_client_session_write(session,
+        plainbuf, plainlen, &plainwritten);
+    if(rresult != RUSTLS_RESULT_OK) {
+      failf(data, "error in rustls_client_session_write");
+      *err = CURLE_WRITE_ERROR;
+      return -1;
+    }
+    else if(plainwritten == 0) {
+      failf(data, "EOF in rustls_client_session_write");
+      *err = CURLE_WRITE_ERROR;
+      return -1;
+    }
+  }
+
+  while(rustls_client_session_wants_write(session)) {
+    rresult = rustls_client_session_write_tls(
+        session, tlsbuf, sizeof(tlsbuf), &tlslen);
+    if(rresult != RUSTLS_RESULT_OK) {
+      failf(data, "error in rustls_client_session_write_tls");
+      *err = CURLE_WRITE_ERROR;
+      return -1;
+    }
+    else if(tlslen == 0) {
+      failf(data, "EOF in rustls_client_session_write_tls");
+      *err = CURLE_WRITE_ERROR;
+      return -1;
+    }
+
+    tlswritten = 0;
+
+    while(tlswritten < tlslen) {
+      n = swrite(sockfd, tlsbuf + tlswritten, tlslen - tlswritten);
+      if(n < 0) {
+        if(SOCKERRNO == EAGAIN || SOCKERRNO == EWOULDBLOCK) {
+          /* Since recv is called from poll, there should be room to
+            write at least some bytes before hitting EAGAIN. */
+          infof(data, "swrite: EAGAIN after %ld bytes\n", tlswritten);
+          DEBUGASSERT(tlswritten > 0);
+          break;
+        }
+        failf(data, "error in swrite");
+        *err = CURLE_WRITE_ERROR;
+        return -1;
+      }
+      if(n == 0) {
+        failf(data, "EOF in swrite");
+        *err = CURLE_WRITE_ERROR;
+        return -1;
+      }
+      tlswritten += n;
+    }
+
+    DEBUGASSERT(tlswritten <= tlslen);
+  }
+
+  return plainwritten;
+}
+
+static CURLcode
+cr_connect_nonblocking(struct Curl_easy *data, struct connectdata *conn,
+                                int sockindex, bool *done)
+{
+  struct ssl_connect_data *const connssl = &conn->ssl[sockindex];
+  curl_socket_t sockfd = conn->sock[sockindex];
+  struct ssl_backend_data *const backend = connssl->backend;
+  struct rustls_client_session *session = backend->session;
+  struct rustls_client_config_builder *config_builder = NULL;
+  const char *const ssl_cafile = SSL_CONN_CONFIG(CAfile);
+  CURLcode tmperr = CURLE_OK;
+  int result;
+  int what;
+  bool wants_read;
+  bool wants_write;
+  curl_socket_t writefd;
+  curl_socket_t readfd;
+  char errorbuf[256];
+  size_t errorlen;
+
+  if(ssl_connection_none == connssl->state) {
+    config_builder = rustls_client_config_builder_new();
+    if(ssl_cafile) {
+      result = rustls_client_config_builder_load_roots_from_file(
+        config_builder, ssl_cafile);
+      if(result != RUSTLS_RESULT_OK) {
+        failf(data, "failed to load trusted certificates");
+        rustls_client_config_free(
+          rustls_client_config_builder_build(config_builder));
+        return CURLE_SSL_CACERT_BADFILE;
+      }
+    }
+    else {
+      result = rustls_client_config_builder_load_native_roots(config_builder);
+      if(result != RUSTLS_RESULT_OK) {
+        failf(data, "failed to load trusted certificates");
+        rustls_client_config_free(
+          rustls_client_config_builder_build(config_builder));
+        return CURLE_SSL_CACERT_BADFILE;
+      }
+    }
+
+    backend->config = rustls_client_config_builder_build(config_builder);
+    DEBUGASSERT(session == NULL);
+    result = rustls_client_session_new(
+      backend->config, conn->host.name, &session);
+    if(result != RUSTLS_RESULT_OK) {
+      rustls_error(result, errorbuf, sizeof(errorbuf), &errorlen);
+      failf(data, "failed to create client session: %.*s", errorlen, errorbuf);
+      return CURLE_COULDNT_CONNECT;
+    }
+    backend->session = session;
+    connssl->state = ssl_connection_negotiating;
+  }
+
+  /* Read/write data until the handshake is done or the socket would block. */
+  for(;;) {
+    /*
+    * Connection has been established according to rustls. Set send/recv
+    * handlers, and update the state machine.
+    * This check has to come last because is_handshaking starts out false,
+    * then becomes true when we first write data, then becomes false again
+    * once the handshake is done.
+    */
+    if(!rustls_client_session_is_handshaking(session)) {
+      infof(data, "Done handshaking\n");
+      /* Done with the handshake. Set up callbacks to send/receive data. */
+      connssl->state = ssl_connection_complete;
+      conn->recv[sockindex] = cr_recv;
+      conn->send[sockindex] = cr_send;
+      *done = TRUE;
+      return CURLE_OK;
+    }
+
+    wants_read = rustls_client_session_wants_read(session);
+    wants_write = rustls_client_session_wants_write(session);
+    DEBUGASSERT(wants_read || wants_write);
+    writefd = wants_write?sockfd:CURL_SOCKET_BAD;
+    readfd = wants_read?sockfd:CURL_SOCKET_BAD;
+
+    what = Curl_socket_check(readfd, CURL_SOCKET_BAD, writefd, 0);
+    if(what < 0) {
+      /* fatal error */
+      failf(data, "select/poll on SSL socket, errno: %d", SOCKERRNO);
+      return CURLE_SSL_CONNECT_ERROR;
+    }
+    if(0 == what) {
+      infof(data, "Curl_socket_check: %s would block\n",
+          wants_read&&wants_write ?
+            "writing and reading" :
+            wants_write ?
+              "writing" :
+              "reading");
+      *done = FALSE;
+      return CURLE_OK;
+    }
+    /* socket is readable or writable */
+
+    if(wants_write) {
+      infof(data, "ClientSession wants us to write_tls.\n");
+      cr_send(data, sockindex, NULL, 0, &tmperr);
+      if(tmperr == CURLE_AGAIN) {
+        infof(data, "writing would block\n");
+        /* fall through */
+      }
+      else if(tmperr != CURLE_OK) {
+        return tmperr;
+      }
+    }
+
+    if(wants_read) {
+      infof(data, "ClientSession wants us to read_tls.\n");
+
+      cr_recv(data, sockindex, NULL, 0, &tmperr);
+      if(tmperr == CURLE_AGAIN) {
+        infof(data, "reading would block\n");
+        /* fall through */
+      }
+      else if(tmperr != CURLE_OK) {
+        if(tmperr == CURLE_READ_ERROR) {
+          return CURLE_SSL_CONNECT_ERROR;
+        }
+        else {
+          return tmperr;
+        }
+      }
+    }
+  }
+
+  /* We should never fall through the loop. We should return either because
+     the handshake is done or because we can't read/write without blocking. */
+  DEBUGASSERT(false);
+}
+
+/* returns a bitmap of flags for this connection's first socket indicating
+   whether we want to read or write */
+static int
+cr_getsock(struct connectdata *conn, curl_socket_t *socks)
+{
+  struct ssl_connect_data *const connssl = &conn->ssl[FIRSTSOCKET];
+  curl_socket_t sockfd = conn->sock[FIRSTSOCKET];
+  struct ssl_backend_data *const backend = connssl->backend;
+  struct rustls_client_session *session = backend->session;
+
+  if(rustls_client_session_wants_write(session)) {
+    socks[0] = sockfd;
+    return GETSOCK_WRITESOCK(0);
+  }
+  if(rustls_client_session_wants_read(session)) {
+    socks[0] = sockfd;
+    return GETSOCK_READSOCK(0);
+  }
+
+  return GETSOCK_BLANK;
+}
+
+static void *
+cr_get_internals(struct ssl_connect_data *connssl,
+                          CURLINFO info UNUSED_PARAM)
+{
+  struct ssl_backend_data *backend = connssl->backend;
+  return &backend->session;
+}
+
+static void
+cr_close(struct Curl_easy *data, struct connectdata *conn,
+                  int sockindex)
+{
+  struct ssl_connect_data *connssl = &conn->ssl[sockindex];
+  struct ssl_backend_data *backend = connssl->backend;
+  CURLcode tmperr = CURLE_OK;
+  ssize_t n = 0;
+
+  if(backend->session) {
+    rustls_client_session_send_close_notify(backend->session);
+    n = cr_send(data, sockindex, NULL, 0, &tmperr);
+    if(n < 0) {
+      failf(data, "error sending close notify: %d", tmperr);
+    }
+
+    rustls_client_session_free(backend->session);
+    backend->session = NULL;
+  }
+  if(backend->config) {
+    rustls_client_config_free(backend->config);
+    backend->config = NULL;
+  }
+}
+
+const struct Curl_ssl Curl_ssl_rustls = {
+  { CURLSSLBACKEND_RUSTLS, "rustls" },
+  SSLSUPP_TLS13_CIPHERSUITES,      /* supports */
+  sizeof(struct ssl_backend_data),
+
+  Curl_none_init,                  /* init */
+  Curl_none_cleanup,               /* cleanup */
+  rustls_version,                  /* version */
+  Curl_none_check_cxn,             /* check_cxn */
+  Curl_none_shutdown,              /* shutdown */
+  cr_data_pending,                 /* data_pending */
+  Curl_none_random,                /* random */
+  Curl_none_cert_status_request,   /* cert_status_request */
+  cr_connect,                      /* connect */
+  cr_connect_nonblocking,          /* connect_nonblocking */
+  cr_getsock,                      /* cr_getsock */
+  cr_get_internals,                /* get_internals */
+  cr_close,                        /* close_one */
+  Curl_none_close_all,             /* close_all */
+  Curl_none_session_free,          /* session_free */
+  Curl_none_set_engine,            /* set_engine */
+  Curl_none_set_engine_default,    /* set_engine_default */
+  Curl_none_engines_list,          /* engines_list */
+  Curl_none_false_start,           /* false_start */
+  NULL                             /* sha256sum */
+};
+
+#endif /* USE_RUSTLS */
diff --git a/lib/vtls/rustls.h b/lib/vtls/rustls.h
new file mode 100644
index 000000000..06effac4d
--- /dev/null
+++ b/lib/vtls/rustls.h
@@ -0,0 +1,32 @@
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) 2020, Jacob Hoffman-Andrews, <github@hoffman-andrews.com>
+ *
+ * This software is licensed as described in the file COPYING, which
+ * you should have received as part of this distribution. The terms
+ * are also available at https://curl.se/docs/copyright.html.
+ *
+ * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+ * copies of the Software, and permit persons to whom the Software is
+ * furnished to do so, under the terms of the COPYING file.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ***************************************************************************/
+#ifndef HEADER_CURL_RUSTLS_H
+#define HEADER_CURL_RUSTLS_H
+
+#include "curl_setup.h"
+
+#ifdef USE_RUSTLS
+
+extern const struct Curl_ssl Curl_ssl_rustls;
+
+#endif /* USE_RUSTLS */
+#endif /* HEADER_CURL_RUSTLS_H */
diff --git a/lib/vtls/vtls.c b/lib/vtls/vtls.c
index 5a1240938..6a0069237 100644
--- a/lib/vtls/vtls.c
+++ b/lib/vtls/vtls.c
@@ -1222,6 +1222,8 @@ const struct Curl_ssl *Curl_ssl =
   &Curl_ssl_mbedtls;
 #elif defined(USE_NSS)
   &Curl_ssl_nss;
+#elif defined(USE_RUSTLS)
+  &Curl_ssl_rustls;
 #elif defined(USE_OPENSSL)
   &Curl_ssl_openssl;
 #elif defined(USE_SCHANNEL)
@@ -1265,6 +1267,9 @@ static const struct Curl_ssl *available_backends[] = {
 #if defined(USE_BEARSSL)
   &Curl_ssl_bearssl,
 #endif
+#if defined(USE_RUSTLS)
+  &Curl_ssl_rustls,
+#endif
   NULL
 };
 
diff --git a/lib/vtls/vtls.h b/lib/vtls/vtls.h
index 250a8b99f..9de8a80c1 100644
--- a/lib/vtls/vtls.h
+++ b/lib/vtls/vtls.h
@@ -116,6 +116,7 @@ bool Curl_ssl_tls13_ciphersuites(void);
 #include "mbedtls.h"        /* mbedTLS versions */
 #include "mesalink.h"       /* MesaLink versions */
 #include "bearssl.h"        /* BearSSL versions */
+#include "rustls.h"         /* rustls versions */
 
 #ifndef MAX_PINNED_PUBKEY_SIZE
 #define MAX_PINNED_PUBKEY_SIZE 1048576 /* 1MB */