[4/4] fbcon: Prevent that screen size is smaller than font size

[4/4] fbcon: Prevent that screen size is smaller than font size

Origin: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit?id=cecb806c766c78e1be62b6b7b1483ef59bbaeabe
Bug-Debian-Security: https://security-tracker.debian.org/tracker/CVE-2021-33655

commit e64242caef18b4a5840b0e7a9bff37abd4f4f933 upstream.

We need to prevent that users configure a screen size which is smaller than the
currently selected font size. Otherwise rendering chars on the screen will
access memory outside the graphics memory region.

This patch adds a new function fbcon_modechange_possible() which
implements this check and which later may be extended with other checks
if necessary.  The new function is called from the FBIOPUT_VSCREENINFO
ioctl handler in fbmem.c, which will return -EINVAL if userspace asked
for a too small screen size.

Signed-off-by: Helge Deller <deller@gmx.de>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: stable@vger.kernel.org # v5.4+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name fbcon-prevent-that-screen-size-is-smaller-than-font-.patch