fix PTY allocation on Cygwin, broken by sshd split

Cygwin doesn't support FD passing and so used to disable post-auth
privilege separation entirely because privsep requires PTY allocation
to happen in the privileged monitor process with the PTY file
descriptors being passed back to the unprivileged process.

This brings back a minimal version of the previous special treatment
for Cygwin (and any other platform that sets DISABLE_FD_PASSING):
privilege separation remains enabled, but PTY allocation happens in
the post-auth user process rather than the monitor.

This either requires PTY allocation to not need privilege to begin
with (this appears to be the case on Cygwin), or the post-auth
privsep process retain privilege (other platforms that set the
DISABLE_FD_PASSING option).

Keeping privileges here is bad, but the non-Cygwin systems that set
DISABLE_FD_PASSING are so deeply legacy that this is likely to be the
least of their problems.

session.c

@@ -103,6 +103,17 @@
 #include <selinux/selinux.h>
 #endif
 
+/*
+ * Hack for systems that do not support FD passing: allocate PTYs directly
+ * without calling into the monitor. This requires either the post-auth
+ * privsep process retain root privileges (see the comment in
+ * sshd-session:privsep_postauth) or that PTY allocation doesn't require
+ * privileges to begin with (e.g. Cygwin).
+ */
+#ifdef DISABLE_FD_PASSING
+#define mm_pty_allocate pty_allocate
+#endif
+
 #define IS_INTERNAL_SFTP(c) \
 	(!strncmp(c, INTERNAL_SFTP_NAME, sizeof(INTERNAL_SFTP_NAME) - 1) && \
 	 (c[sizeof(INTERNAL_SFTP_NAME) - 1] == '\0' || \

sshd-session.c

@@ -379,6 +379,21 @@ privsep_preauth(struct ssh *ssh)
 static void
 privsep_postauth(struct ssh *ssh, Authctxt *authctxt)
 {
+	int skip_privdrop = 0;
+
+	/*
+	 * Hack for systems that don't support FD passing: retain privileges
+	 * in the post-auth privsep process so it can allocate PTYs directly.
+	 * This is basically equivalent to what we did <= 9.7, which was to
+	 * disable post-auth privsep entriely.
+	 * Cygwin doesn't need to drop privs here although it doesn't support
+	 * fd passing, as AFAIK PTY allocation on this platform doesn't require
+	 * special privileges to begin with.
+	 */
+#if defined(DISABLE_FD_PASSING) && !defined(HAVE_CYGWIN)
+	skip_privdrop = 1;
+#endif
+
 	/* New socket pair */
 	monitor_reinit(pmonitor);
 
@@ -406,7 +421,8 @@ privsep_postauth(struct ssh *ssh, Authctxt *authctxt)
 	reseed_prngs();
 
 	/* Drop privileges */
-	do_setusercontext(authctxt->pw);
+	if (!skip_privdrop)
+		do_setusercontext(authctxt->pw);
 
 	/* It is safe now to apply the key state */
 	monitor_apply_keystate(ssh, pmonitor);