fbcon: Prevent that screen size is smaller than font size

mainline inclusion
from mainline-v5.19-rc6
commit e64242ca
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I5IQ4M


CVE: CVE-2021-33655

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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: Chen Jun <chenjun102@huawei.com>

Conflicts:
	drivers/video/fbdev/core/fbcon.c
	drivers/video/fbdev/core/fbmem.c
	include/linux/fbcon.h
Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Reviewed-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>