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

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Date:   Tue, 20 Sep 2016 13:48:10 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Heiko Carstens <heiko.carstens@...ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Jan Stancek <jstancek@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, Greg Ungerer <gerg@...inux.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fix fault_in_multipages_...() on architectures with no-op access_ok()

On Tue, Sep 20, 2016 at 1:38 PM, Al Viro <viro@...iv.linux.org.uk> 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.

             Linus