fbmem: pull fbcon_update_vcs() out of fb_set_var()

syzbot is reporting OOB read bug in vc_do_resize() [1] caused by memcpy()
based on outdated old_{rows,row_size} values, for resize_screen() can
recurse into vc_do_resize() which changes vc->vc_{cols,rows} that outdates
old_{rows,row_size} values which were saved before calling resize_screen().

Daniel Vetter explained that resize_screen() should not recurse into
fbcon_update_vcs() path due to FBINFO_MISC_USEREVENT being still set
when calling resize_screen().

Instead of masking FBINFO_MISC_USEREVENT before calling fbcon_update_vcs(),
we can remove FBINFO_MISC_USEREVENT by calling fbcon_update_vcs() only if
fb_set_var() returned 0. This change assumes that it is harmless to call
fbcon_update_vcs() when fb_set_var() returned 0 without reaching
fb_notifier_call_chain().

[1] https://syzkaller.appspot.com/bug?id=c70c88cfd16dcf6e1d3c7f0ab8648b3144b5b25e

Reported-and-tested-by: syzbot <[e-mail]>
Suggested-by: Daniel Vetter <[e-mail]>
Signed-off-by: Tetsuo Handa <[e-mail]>
Reported-by: kernel test robot <[e-mail]> for missing #include
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/video/fbdev/core/fbmem.c

···
 int
 fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
 {
-	int flags = info->flags;
 	int ret = 0;
 	u32 activate;
 	struct fb_var_screeninfo old_var;
···
 	event.data = &mode;
 	fb_notifier_call_chain(FB_EVENT_MODE_CHANGE, &event);
 
-	if (flags & FBINFO_MISC_USEREVENT)
-		fbcon_update_vcs(info, activate & FB_ACTIVATE_ALL);
-
 	return 0;
 }
 EXPORT_SYMBOL(fb_set_var);
···
 			return -EFAULT;
 		console_lock();
 		lock_fb_info(info);
-		info->flags |= FBINFO_MISC_USEREVENT;
 		ret = fb_set_var(info, &var);
-		info->flags &= ~FBINFO_MISC_USEREVENT;
+		if (!ret)
+			fbcon_update_vcs(info, var.activate & FB_ACTIVATE_ALL);
 		unlock_fb_info(info);
 		console_unlock();
 		if (!ret && copy_to_user(argp, &var, sizeof(var)))

drivers/video/fbdev/core/fbsysfs.c

···
 
 	var->activate |= FB_ACTIVATE_FORCE;
 	console_lock();
-	fb_info->flags |= FBINFO_MISC_USEREVENT;
 	err = fb_set_var(fb_info, var);
-	fb_info->flags &= ~FBINFO_MISC_USEREVENT;
+	if (!err)
+		fbcon_update_vcs(fb_info, var->activate & FB_ACTIVATE_ALL);
 	console_unlock();
 	if (err)
 		return err;

drivers/video/fbdev/ps3fb.c

···
 #include <linux/freezer.h>
 #include <linux/uaccess.h>
 #include <linux/fb.h>
+#include <linux/fbcon.h>
 #include <linux/init.h>
 
 #include <asm/cell-regs.h>
···
 				var = info->var;
 				fb_videomode_to_var(&var, vmode);
 				console_lock();
-				info->flags |= FBINFO_MISC_USEREVENT;
 				/* Force, in case only special bits changed */
 				var.activate |= FB_ACTIVATE_FORCE;
 				par->new_mode_id = val;
 				retval = fb_set_var(info, &var);
-				info->flags &= ~FBINFO_MISC_USEREVENT;
+				if (!retval)
+					fbcon_update_vcs(info, var.activate & FB_ACTIVATE_ALL);
 				console_unlock();
 			}
 			break;

include/linux/fb.h

···
 #define FBINFO_HWACCEL_YPAN		0x2000 /* optional */
 #define FBINFO_HWACCEL_YWRAP		0x4000 /* optional */
 
-#define FBINFO_MISC_USEREVENT          0x10000 /* event request
-						  from userspace */
 #define FBINFO_MISC_TILEBLITTING       0x20000 /* use tile blitting */
 
 /* A driver may set this flag to indicate that it does want a set_par to be