video: fbdev: vesafb: Fix a use-after-free due early fb_info cleanup

[ Upstream commit acde400 ]

Commit b3c9a92 ("fbdev: vesafb: Cleanup fb_info in .fb_destroy rather
than .remove") fixed a use-after-free error due the vesafb driver freeing
the fb_info in the .remove handler instead of doing it in .fb_destroy.

This can happen if the .fb_destroy callback is executed after the .remove
callback, since the former tries to access a pointer freed by the latter.

But that change didn't take into account that another possible scenario is
that .fb_destroy is called before the .remove callback. For example, if no
process has the fbdev chardev opened by the time the driver is removed.

If that's the case, fb_info will be freed when unregister_framebuffer() is
called, making the fb_info pointer accessed in vesafb_remove() after that
to no longer be valid.

To prevent that, move the expression containing the info->par to happen
before the unregister_framebuffer() function call.

Fixes: b3c9a92 ("fbdev: vesafb: Cleanup fb_info in .fb_destroy rather than .remove")
Reported-by: Pascal Ernster <dri-devel@hardfalcon.net>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Tested-by: Pascal Ernster <dri-devel@hardfalcon.net>
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/video/fbdev/vesafb.c

@@ -490,11 +490,12 @@ static int vesafb_remove(struct platform_device *pdev)
 {
 	struct fb_info *info = platform_get_drvdata(pdev);
 
-	/* vesafb_destroy takes care of info cleanup */
-	unregister_framebuffer(info);
 	if (((struct vesafb_par *)(info->par))->region)
 		release_region(0x3c0, 32);
 
+	/* vesafb_destroy takes care of info cleanup */
+	unregister_framebuffer(info);
+
 	return 0;
 }