https://bugzilla.mindrot.org/show_bug.cgi?id=3655
https://github.com/openssh/openssh-portable/commit/fe6c6330c1a94c7a537efe9069853ce7a275c50a
https://bugs.gentoo.org/929191

From fe6c6330c1a94c7a537efe9069853ce7a275c50a Mon Sep 17 00:00:00 2001
From: "djm@openbsd.org" <djm@openbsd.org>
Date: Sun, 13 Oct 2024 22:20:06 +0000
Subject: [PATCH] upstream: don't start the ObscureKeystrokeTiming mitigations
 if

there has been traffic on a X11 forwarding channel recently.

Should fix X11 forwarding performance problems when this setting is
enabled. Patch from Antonio Larrosa via bz3655

OpenBSD-Commit-ID: 820284a92eb4592fcd3d181a62c1b86b08a4a7ab
--- a/channels.c
+++ b/channels.c
@@ -5336,3 +5336,22 @@ x11_request_forwarding_with_spoofing(struct ssh *ssh, int client_session_id,
 		fatal_fr(r, "send x11-req");
 	free(new_data);
 }
+
+/*
+ * Returns whether an x11 channel was used recently (less than a second ago)
+ */
+int
+x11_channel_used_recently(struct ssh *ssh) {
+	u_int i;
+	Channel *c;
+	time_t lastused = 0;
+
+	for (i = 0; i < ssh->chanctxt->channels_alloc; i++) {
+		c = ssh->chanctxt->channels[i];
+		if (c == NULL || c->ctype == NULL || c->lastused == 0 ||
+		    strcmp(c->ctype, "x11-connection") != 0)
+			continue;
+		lastused = c->lastused;
+	}
+	return lastused != 0 && monotime() > lastused + 1;
+}
--- a/channels.h
+++ b/channels.h
@@ -382,6 +382,7 @@ int	 x11_connect_display(struct ssh *);
 int	 x11_create_display_inet(struct ssh *, int, int, int, u_int *, int **);
 void	 x11_request_forwarding_with_spoofing(struct ssh *, int,
 	    const char *, const char *, const char *, int);
+int      x11_channel_used_recently(struct ssh *ssh);
 
 /* channel close */
 
--- a/clientloop.c
+++ b/clientloop.c
@@ -659,9 +659,10 @@ obfuscate_keystroke_timing(struct ssh *ssh, struct timespec *timeout,
 	if (just_started)
 		return 1;
 
-	/* Don't arm output fd for poll until the timing interval has elapsed */
+	/* Don't arm output fd for poll until the timing interval has elapsed... */
 	if (timespeccmp(&now, &next_interval, <))
-		return 0;
+		/* ...unless there's x11 communicattion happening */
+		return x11_channel_used_recently(ssh);
 
 	/* Calculate number of intervals missed since the last check */
 	n = (now.tv_sec - next_interval.tv_sec) * 1000LL * 1000 * 1000;