fbcon: Prevent that screen size is smaller than font size

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+

diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index a335325..5632870 100644 (file)
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -2736,6 +2736,34 @@ void fbcon_update_vcs(struct fb_info *info, bool all)
 }
 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)
 {

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index ee63b5b..7dc6848 100644 (file)
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -1107,7 +1107,9 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
                        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);

diff --git a/include/linux/fbcon.h b/include/linux/fbcon.h
index ff5596d..2382dec 100644 (file)
--- a/include/linux/fbcon.h
+++ b/include/linux/fbcon.h
@@ -15,6 +15,8 @@ void fbcon_new_modelist(struct fb_info *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(struct fb_info *info) {}
 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; }