linux-kernel - Re: [PATCH 00/23] KAISER: unmap most of the kernel from userspace page tables

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Message-ID: <d6400e78-4ca5-9837-59bd-fe2e2f8f9f68@intel.com>
Date:   Tue, 31 Oct 2017 17:21:56 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 00/23] KAISER: unmap most of the kernel from userspace
 page tables

On 10/31/2017 04:44 PM, Dave Hansen wrote:
>>      That seems insane. Why isn't only tyhe top level shadowed, and
>> then lower levels are shared between the shadowed and the "kernel"
>> page tables?
> There are obviously two PGDs.  The userspace half of the PGD is an exact
> copy so all the lower levels are shared.  You can see this bit in the
> memcpy that we do in clone_pgd_range().

This is wrong.

The userspace copying is done via the code we add to native_set_pgd().
Whenever we set the kernel PGD, we also make sure to make a
corresponding entry in the user/shadow PGD.

The memcpy() that I was talking about does the kernel portion of the PGD.