vgacon: disallow console operations when in KD_GRAPHICS mode

Reported by James Pearson as:

	 boot to run level 3

	 if not root, then make sure /dev/console is writeable

	 login and type:

	 setterm -blank 0

	 start X

	 type into an xterm:

	 while true; do echo "" > /dev/console; usleep 100000; done

	 while the above loop is running switch to the text console and back
	 again (Ctrl-Alt-F1 then Ctrl-Alt-F7)

	 ... and the screen will be shifting (and wrapping) to the left.

This problem stems from continuously writing text to the system console (which
is in KD_TEXT mode) while the foreground console is in KD_GRAPHICS
mode. Somewhere along the way, console printing got confused and omitted the
KD_GRAPHICS/KD_TEXT test.  Thus, vgacon attempted to scroll the screen of X,
which causes X to shift.

Fix by disallowing vgacon to touch the hardware when the vc is in KD_GRAPHICS
mode. A definitive fix entails a full audit of the console code.
Signed-off-by: NAntonino Daplas <adaplas@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/video/console/vgacon.c

@@ -660,6 +660,9 @@ static void vgacon_set_cursor_size(int xpos, int from, int to)
 
 static void vgacon_cursor(struct vc_data *c, int mode)
 {
+	if (c->vc_mode != KD_TEXT)
+		return;
+
 	vgacon_restore_screen(c);
 
 	switch (mode) {
@@ -1318,7 +1321,7 @@ static int vgacon_scroll(struct vc_data *c, int t, int b, int dir,
 	unsigned long oldo;
 	unsigned int delta;
 
-	if (t || b != c->vc_rows || vga_is_gfx)
+	if (t || b != c->vc_rows || vga_is_gfx || c->vc_mode != KD_TEXT)
 		return 0;
 
 	if (!vga_hardscroll_enabled || lines >= c->vc_rows / 2)