linux-kernel - Re: [PATCH 24/30] x86, kaiser: disable native VSYSCALL

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Date:   Thu, 9 Nov 2017 17:04:58 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        moritz.lipp@...k.tugraz.at,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        michael.schwarz@...k.tugraz.at, richard.fellner@...dent.tugraz.at,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 24/30] x86, kaiser: disable native VSYSCALL

On Thu, Nov 9, 2017 at 4:57 PM, Dave Hansen <dave.hansen@...ux.intel.com> wrote:
> On 11/09/2017 04:53 PM, Andy Lutomirski wrote:
>>> The KAISER code attempts to "poison" the user portion of the kernel page
>>> tables.  It detects the entries pages that it wants that it wants to
>>> poison in two ways:
>>>  * Looking for addresses >= PAGE_OFFSET
>>>  * Looking for entries without _PAGE_USER set
>> What do you mean "poison"?
>
> I meant the _PAGE_NX magic that we do in here:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/daveh/x86-kaiser.git/commit/?h=kaiser-414rc7-20171108&id=c4f7d0819170761f092fcf2327b85b082368e73a
>
> to ensure that userspace is unable to run on the kernel PGD.

Aha, I get it.  Why not just drop the _PAGE_USER check?  You could
instead warn if you see a _PAGE_USER page that doesn't have the
correct address for the vsyscall.