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

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Date:   Thu, 9 Nov 2017 17:22:33 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     "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 11/09/2017 05:04 PM, Andy Lutomirski wrote:
> 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.

The _PAGE_USER check helps us with kernel things that want to create
mappings below PAGE_OFFSET.  The EFI code was the prime user for this.
Without this, we poison the EFI mappings and the EFI calls die.

I think there might have also been a case for the secondary CPU bringup
that needed hacking if we didn't do this.