fbcon: always restore the old font data in fbcon_do_set_font()

Commit a5a92303 (fbdev: fbcon: Properly revert changes when
vc_resize() failed) started restoring old font data upon failure (of
vc_resize()). But it performs so only for user fonts. It means that the
"system"/internal fonts are not restored at all. So in result, the very
first call to fbcon_do_set_font() performs no restore at all upon
failing vc_resize().

This can be reproduced by Syzkaller to crash the system on the next
invocation of font_get(). It's rather hard to hit the allocation failure
in vc_resize() on the first font_set(), but not impossible. Esp. if
fault injection is used to aid the execution/failure. It was
demonstrated by Sirius:
  BUG: unable to handle page fault for address: fffffffffffffff8
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD cb7b067 P4D cb7b067 PUD cb7d067 PMD 0
  Oops: 0000 [#1] PREEMPT SMP KASAN
  CPU: 1 PID: 8007 Comm: poc Not tainted 6.7.0-g9d1694dc #20
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
  RIP: 0010:fbcon_get_font+0x229/0x800 drivers/video/fbdev/core/fbcon.c:2286
  Call Trace:
   <TASK>
   con_font_get drivers/tty/vt/vt.c:4558 [inline]
   con_font_op+0x1fc/0xf20 drivers/tty/vt/vt.c:4673
   vt_k_ioctl drivers/tty/vt/vt_ioctl.c:474 [inline]
   vt_ioctl+0x632/0x2ec0 drivers/tty/vt/vt_ioctl.c:752
   tty_ioctl+0x6f8/0x1570 drivers/tty/tty_io.c:2803
   vfs_ioctl fs/ioctl.c:51 [inline]
  ...

So restore the font data in any case, not only for user fonts. Note the
later 'if' is now protected by 'old_userfont' and not 'old_data' as the
latter is always set now. (And it is supposed to be non-NULL. Otherwise
we would see the bug above again.)
Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Fixes: a5a92303 ("fbdev: fbcon: Properly revert changes when vc_resize() failed")
Reported-and-tested-by: Ubisectech Sirius <bugreport@ubisectech.com>
Cc: Ubisectech Sirius <bugreport@ubisectech.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-fbdev@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20240208114411.14604-1-jirislaby@kernel.org

drivers/video/fbdev/core/fbcon.c

@@ -2399,11 +2399,9 @@ static int fbcon_do_set_font(struct vc_data *vc, int w, int h, int charcount,
 	struct fbcon_ops *ops = info->fbcon_par;
 	struct fbcon_display *p = &fb_display[vc->vc_num];
 	int resize, ret, old_userfont, old_width, old_height, old_charcount;
-	char *old_data = NULL;
+	u8 *old_data = vc->vc_font.data;
 
 	resize = (w != vc->vc_font.width) || (h != vc->vc_font.height);
-	if (p->userfont)
-		old_data = vc->vc_font.data;
 	vc->vc_font.data = (void *)(p->fontdata = data);
 	old_userfont = p->userfont;
 	if ((p->userfont = userfont))
@@ -2437,13 +2435,13 @@ static int fbcon_do_set_font(struct vc_data *vc, int w, int h, int charcount,
 		update_screen(vc);
 	}
 
-	if (old_data && (--REFCOUNT(old_data) == 0))
+	if (old_userfont && (--REFCOUNT(old_data) == 0))
 		kfree(old_data - FONT_EXTRA_WORDS * sizeof(int));
 	return 0;
 
 err_out:
 	p->fontdata = old_data;
-	vc->vc_font.data = (void *)old_data;
+	vc->vc_font.data = old_data;
 
 	if (userfont) {
 		p->userfont = old_userfont;