[PATCH 5.15 162/230] fbcon: Prevent that screen size is smaller than font size

Greg Kroah-Hartman
2022-07-11T09:56:54Z

From: Helge Deller <deller@gmx.de>

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>
---
drivers/video/fbdev/core/fbcon.c | 28 ++++++++++++++++++++++++++++
drivers/video/fbdev/core/fbmem.c | 4 +++-
include/linux/fbcon.h | 4 ++++
3 files changed, 35 insertions(+), 1 deletion(-)

--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -2747,6 +2747,34 @@ void fbcon_update_vcs(struct fb_info *in
}
EXPORT_SYMBOL(fbcon_update_vcs);

+/* let fbcon check if it supports a new screen resolution */
+int fbcon_modechange_possible(struct fb_info *info, struct fb_var_screeninfo *var)
+{
+ struct fbcon_ops *ops = info->fbcon_par;
+ struct vc_data *vc;
+ unsigned int i;
+
+ WARN_CONSOLE_UNLOCKED();
+
+ if (!ops)
+ return 0;
+
+ /* prevent setting a screen size which is smaller than font size */
+ for (i = first_fb_vc; i <= last_fb_vc; i++) {
+ vc = vc_cons[i].d;
+ if (!vc || vc->vc_mode != KD_TEXT ||
+ registered_fb[con2fb_map[i]] != info)
+ continue;
+
+ if (vc->vc_font.width > FBCON_SWAP(var->rotate, var->xres, var->yres) ||
+ vc->vc_font.height > FBCON_SWAP(var->rotate, var->yres, var->xres))
+ return -EINVAL;
+ }
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(fbcon_modechange_possible);
+
int fbcon_mode_deleted(struct fb_info *info,
struct fb_videomode *mode)
{
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -1120,7 +1120,9 @@ static long do_fb_ioctl(struct fb_info *
return -EFAULT;
console_lock();
lock_fb_info(info);
- ret = fb_set_var(info, &var);
+ ret = fbcon_modechange_possible(info, &var);
+ if (!ret)
+ ret = fb_set_var(info, &var);
if (!ret)
fbcon_update_vcs(info, var.activate & FB_ACTIVATE_ALL);
unlock_fb_info(info);
--- a/include/linux/fbcon.h
+++ b/include/linux/fbcon.h
@@ -15,6 +15,8 @@ void fbcon_new_modelist(struct fb_info *
void fbcon_get_requirement(struct fb_info *info,
struct fb_blit_caps *caps);
void fbcon_fb_blanked(struct fb_info *info, int blank);
+int fbcon_modechange_possible(struct fb_info *info,
+ struct fb_var_screeninfo *var);
void fbcon_update_vcs(struct fb_info *info, bool all);
void fbcon_remap_all(struct fb_info *info);
int fbcon_set_con2fb_map_ioctl(void __user *argp);
@@ -33,6 +35,8 @@ static inline void fbcon_new_modelist(st
static inline void fbcon_get_requirement(struct fb_info *info,
struct fb_blit_caps *caps) {}
static inline void fbcon_fb_blanked(struct fb_info *info, int blank) {}
+static inline int fbcon_modechange_possible(struct fb_info *info,
+ struct fb_var_screeninfo *var) { return 0; }
static inline void fbcon_update_vcs(struct fb_info *info, bool all) {}
static inline void fbcon_remap_all(struct fb_info *info) {}
static inline int fbcon_set_con2fb_map_ioctl(void __user *argp) { return 0; }