Re: [PATCH] fix fault_in_multipages_...() on architectures with no-op access_ok()

From: Linus Torvalds
Date: Tue Sep 20 2016 - 16:48:15 EST

On Tue, Sep 20, 2016 at 1:38 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> Not the point. Of course it *would* fail; the problem is that the loop
> that would ping each page is never executed.

You're missing the point.

If "access_ok()" is fine with wrapping, then some otehr system call
that wraps will successfully do a memcpy_from/to_user() with a wrapped
address (and the proper mappings). Which is completely bogus.

So access_ok() should be fixed regardless. An access_ok() that accepts
a wrapping address is a bug. End of story.

And once that bug is fixed, the fault_in_multipages..() issue is moot.
So it shouldn't be an issue.