tty: vt: always invoke vc->vc_sw->con_resize callback

commit ffb324e upstream.

syzbot is reporting OOB write at vga16fb_imageblit() [1], for
resize_screen() from ioctl(VT_RESIZE) returns 0 without checking whether
requested rows/columns fit the amount of memory reserved for the graphical
screen if current mode is KD_GRAPHICS.

----------
  #include <sys/types.h>
  #include <sys/stat.h>
  #include <fcntl.h>
  #include <sys/ioctl.h>
  #include <linux/kd.h>
  #include <linux/vt.h>

  int main(int argc, char *argv[])
  {
        const int fd = open("/dev/char/4:1", O_RDWR);
        struct vt_sizes vt = { 0x4100, 2 };

        ioctl(fd, KDSETMODE, KD_GRAPHICS);
        ioctl(fd, VT_RESIZE, &vt);
        ioctl(fd, KDSETMODE, KD_TEXT);
        return 0;
  }
----------

Allow framebuffer drivers to return -EINVAL, by moving vc->vc_mode !=
KD_GRAPHICS check from resize_screen() to fbcon_resize().

Link: https://syzkaller.appspot.com/bug?extid=1f29e126cf461c4de3b3 [1]
Reported-by: syzbot <syzbot+1f29e126cf461c4de3b3@syzkaller.appspotmail.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Tested-by: syzbot <syzbot+1f29e126cf461c4de3b3@syzkaller.appspotmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/vt/vt.c

@@ -826,7 +826,7 @@ static inline int resize_screen(struct vc_data *vc, int width, int height,
 	/* Resizes the resolution of the display adapater */
 	int err = 0;
 
-	if (vc->vc_mode != KD_GRAPHICS && vc->vc_sw->con_resize)
+	if (vc->vc_sw->con_resize)
 		err = vc->vc_sw->con_resize(vc, width, height, user);
 
 	return err;

drivers/video/console/fbcon.c

@@ -1986,7 +1986,7 @@ static int fbcon_resize(struct vc_data *vc, unsigned int width,
 			return -EINVAL;
 
 		DPRINTK("resize now %ix%i\n", var.xres, var.yres);
-		if (CON_IS_VISIBLE(vc)) {
+		if (CON_IS_VISIBLE(vc) && vc->vc_mode == KD_TEXT) {
 			var.activate = FB_ACTIVATE_NOW |
 				FB_ACTIVATE_FORCE;
 			fb_set_var(info, &var);