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

[Andy Lutomirski]

On Thu, Nov 9, 2017 at 4:57 PM, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> 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.