Bug workaround to delay sending our SSH greeting.

Ian Jackson recently tried to use the recipe in the psusan manpage for
talking to UML, and found that the connection was not successfully set
up, because at some point during startup, UML read the SSH greeting
(ok, the bare-ssh-connection greeting) from its input fd and threw it
away. So by the time psusan was run by the guest init process, the
greeting wasn't there to be read.

Ian's report: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=991958

I was also able to reproduce this locally, which makes me wonder why I
_didn't_ notice it when I originally wrote that part of the psusan man
page. It worked for me before, honest! But now it doesn't.

Anyway. The ssh verstring module already has a mode switch to decide
whether we ought to send our greeting before or after waiting for the
other side's greeting (because that decision varies between client and
server, and between SSH-1 and SSH-2). So it's easy to implement an
override that forces it to 'wait for the server greeting first'.

I've added this as yet another bug workaround flag. But unlike all the
others, it can't be autodetected from the server's version string,
because, of course, we have to act on it _before_ seeing the server's
greeting and version string! So it's a manual-only flag.

However, I've mentioned it in the UML section of the psusan man page,
since that's the place where I _know_ people are likely to need to use
this flag.

config.c

@@ -735,6 +735,37 @@ static void sshbug_handler(union control *ctrl, dlgparam *dlg,
     }
 }
 
+static void sshbug_handler_manual_only(union control *ctrl, dlgparam *dlg,
+                                       void *data, int event)
+{
+    /*
+     * This is just like sshbug_handler, except that there's no 'Auto'
+     * option. Used for bug workaround flags that can't be
+     * autodetected, and have to be manually enabled if they're to be
+     * used at all.
+     */
+    Conf *conf = (Conf *)data;
+    if (event == EVENT_REFRESH) {
+        int oldconf = conf_get_int(conf, ctrl->listbox.context.i);
+        dlg_update_start(ctrl, dlg);
+        dlg_listbox_clear(ctrl, dlg);
+        dlg_listbox_addwithid(ctrl, dlg, "Off", FORCE_OFF);
+        dlg_listbox_addwithid(ctrl, dlg, "On", FORCE_ON);
+        switch (oldconf) {
+          case FORCE_OFF: dlg_listbox_select(ctrl, dlg, 0); break;
+          case FORCE_ON:  dlg_listbox_select(ctrl, dlg, 1); break;
+        }
+        dlg_update_done(ctrl, dlg);
+    } else if (event == EVENT_SELCHANGE) {
+        int i = dlg_listbox_index(ctrl, dlg);
+        if (i < 0)
+            i = FORCE_OFF;
+        else
+            i = dlg_listbox_getid(ctrl, dlg, i);
+        conf_set_int(conf, ctrl->listbox.context.i, i);
+    }
+}
+
 struct sessionsaver_data {
     union control *editbox, *listbox, *loadbutton, *savebutton, *delbutton;
     union control *okbutton, *cancelbutton;
@@ -3049,6 +3080,13 @@ void setup_config_box(struct controlbox *b, bool midsession,
                           HELPCTX(ssh_bugs_maxpkt2),
                           sshbug_handler, I(CONF_sshbug_maxpkt2));
 
+            s = ctrl_getset(b, "Connection/SSH/Bugs", "manual",
+                            "Manually enabled workarounds");
+            ctrl_droplist(s, "Discards data sent before its greeting", 'd', 20,
+                          HELPCTX(ssh_bugs_dropstart),
+                          sshbug_handler_manual_only,
+                          I(CONF_sshbug_dropstart));
+
             ctrl_settitle(b, "Connection/SSH/More bugs",
                           "Further workarounds for SSH server bugs");
 

doc/config.but

@@ -3206,7 +3206,10 @@ three states:
 \b \q{On}: PuTTY will assume the server \e{does} have the bug.
 
 \b \q{Auto}: PuTTY will use the server's version number announcement
-to try to guess whether or not the server has the bug.
+to try to guess whether or not the server has the bug. (This option is
+not available for bugs that \e{cannot} be detected from the server
+version, e.g. because they must be acted on before the server version
+is known.)
 
 \S{config-ssh-bug-ignore2} \q{Chokes on SSH-2 \i{ignore message}s}
 
@@ -3299,6 +3302,29 @@ send an over-sized packet.  If this bug is enabled when talking to a
 correct server, the session will work correctly, but download
 performance will be less than it could be.
 
+\S{config-ssh-bug-dropstart} \q{Discards data sent before its greeting}
+
+Just occasionally, an SSH connection can be established over some
+channel that will accidentally discard outgoing data very early in the
+connection.
+
+This is not typically seen as a bug in an actual SSH server, but it
+can sometimes occur in situations involving a complicated proxy
+process. An example is
+\W{https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=991958}{Debian
+bug #991958}, in which a connection going over the console of a User
+Mode Linux kernel can lose outgoing data before the kernel has fully
+booted.
+
+You can work around this problem by manually enabling this bug flag,
+which will cause PuTTY to wait to send its initial SSH greeting until
+after it sees the greeting from the server.
+
+Note that this bug flag can never be automatically detected, since
+auto-detection relies on the version string in the server's greeting,
+and PuTTY has to decide whether to expect this bug \e{before} it sees
+the server's greeting. So this is a manual workaround only.
+
 \S{config-ssh-bug-sig} \q{Requires padding on SSH-2 \i{RSA} \i{signatures}}
 
 Versions below 3.3 of \i{OpenSSH} require SSH-2 RSA signatures to be

doc/man-psusan.but

@@ -199,10 +199,18 @@ And the setup script \cw{uml-psusan.sh} might look like this:
 \c exec /home/simon/src/putty/misc/psusan
 
 Now set up a PuTTY saved session as in the Docker example above, using
-that \cw{linux} command as the local proxy command, and you'll have a
-PuTTY session that starts up a clean UML instance when you run it, and
-(if you enabled connection sharing) further instances of the same
-session will connect to the same instance again.
+that \cw{linux} command as the local proxy command. You may also find
+that you have to enable the bug workaround that indicates that the
+server \q{Discards data sent before its greeting}, because otherwise
+PuTTY's outgoing protocol greeting can be accidentally lost during UML
+startup. (See
+\W{https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=991958}{Debian
+bug #991958}.)
+
+Once you've done that, you'll have a PuTTY session that starts up a
+clean UML instance when you run it, and (if you enabled connection
+sharing) further instances of the same session will connect to the
+same instance again.
 
 \S2{psusan-manpage-examples-wsl} Windows Subsystem for Linux
 

putty.h

@@ -1647,6 +1647,7 @@ NORETURN void cleanup_exit(int);
     X(INT, NONE, sshbug_oldgex2) \
     X(INT, NONE, sshbug_winadj) \
     X(INT, NONE, sshbug_chanreq) \
+    X(INT, NONE, sshbug_dropstart) \
     /*                                                                \
      * ssh_simple means that we promise never to open any channel     \
      * other than the main one, which means it can safely use a very  \

settings.c

@@ -769,6 +769,7 @@ void save_open_settings(settings_w *sesskey, Conf *conf)
     write_setting_i(sesskey, "BugOldGex2", 2-conf_get_int(conf, CONF_sshbug_oldgex2));
     write_setting_i(sesskey, "BugWinadj", 2-conf_get_int(conf, CONF_sshbug_winadj));
     write_setting_i(sesskey, "BugChanReq", 2-conf_get_int(conf, CONF_sshbug_chanreq));
+    write_setting_i(sesskey, "BugDropStart", 2-conf_get_int(conf, CONF_sshbug_dropstart));
     write_setting_b(sesskey, "StampUtmp", conf_get_bool(conf, CONF_stamp_utmp));
     write_setting_b(sesskey, "LoginShell", conf_get_bool(conf, CONF_login_shell));
     write_setting_b(sesskey, "ScrollbarOnLeft", conf_get_bool(conf, CONF_scrollbar_on_left));
@@ -1244,6 +1245,7 @@ void load_open_settings(settings_r *sesskey, Conf *conf)
     i = gppi_raw(sesskey, "BugOldGex2", 0); conf_set_int(conf, CONF_sshbug_oldgex2, 2-i);
     i = gppi_raw(sesskey, "BugWinadj", 0); conf_set_int(conf, CONF_sshbug_winadj, 2-i);
     i = gppi_raw(sesskey, "BugChanReq", 0); conf_set_int(conf, CONF_sshbug_chanreq, 2-i);
+    i = gppi_raw(sesskey, "BugDropStart", 0); conf_set_int(conf, CONF_sshbug_dropstart, 2-i);
     conf_set_bool(conf, CONF_ssh_simple, false);
     gppb(sesskey, "StampUtmp", true, conf, CONF_stamp_utmp);
     gppb(sesskey, "LoginShell", true, conf, CONF_login_shell);

ssh/verstring.c

@@ -104,6 +104,14 @@ BinaryPacketProtocol *ssh_verstring_new(
      */
     s->send_early = server_mode || !ssh_version_includes_v1(protoversion);
 
+    /*
+     * Override: we don't send our version string early if the server
+     * has a bug that will make it discard it. See for example
+     * https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=991958
+     */
+    if (conf_get_int(s->conf, CONF_sshbug_dropstart) == FORCE_ON)
+        s->send_early = false;
+
     s->bpp.vt = &ssh_verstring_vtable;
     ssh_bpp_common_setup(&s->bpp);
     return &s->bpp;

windows/help.h

@@ -162,6 +162,7 @@
 #define WINHELP_CTX_ssh_bugs_winadj "config-ssh-bug-winadj"
 #define WINHELP_CTX_ssh_bugs_chanreq "config-ssh-bug-chanreq"
 #define WINHELP_CTX_ssh_bugs_oldgex2 "config-ssh-bug-oldgex2"
+#define WINHELP_CTX_ssh_bugs_dropstart "config-ssh-bug-dropstart"
 #define WINHELP_CTX_serial_line "config-serial-line"
 #define WINHELP_CTX_serial_speed "config-serial-speed"
 #define WINHELP_CTX_serial_databits "config-serial-databits"