linux-kernel - Re: [PATCH 05/23] x86, kaiser: unmap kernel from userspace page tables (core patch)

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Date:   Sun, 26 Nov 2017 08:24:37 -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>,
        richard.fellner@...dent.tugraz.at, moritz.lipp@...k.tugraz.at,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        michael.schwarz@...k.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 05/23] x86, kaiser: unmap kernel from userspace page
 tables (core patch)

On 11/26/2017 08:10 AM, Andy Lutomirski wrote:
>> As a side benefit, this shouldn't have magical interactions with the
>> vsyscall page any more.
>>
>> Are there cases that this would get wrong?
>>
> Quick ping: did this get lost?

It does drop a warning that the other version of the code has, but
that's pretty minor.

Basically, we need two checks:

	pgd_userspace_access() (aka _PAGE_USER) and
	pgdp_maps_userspace()

The original code does pgd_userspace_access() in a top-level if and then
the pgdp_maps_userspace() checks at the second level.  I think you are
basically suggesting that we flip that.

Logically, I'm sure we can make it work.  It's just a matter of needing
to look at other things first.

BTW, this comment is, I think incorrect:

>   if (pgdp_maps_userspace(pgdp)) {
...
>   } else {
>     /*
>      * We can get here due to vmalloc, a vmalloc fault, memory
> hot-add, or initial setup
>      * of kernelmode page tables.  Regardless of which particular code
> path we're in,
>      * these mappings should not be automatically propagated to the
> usermode tables.
>      */

Since we pre-populated the entire kernel area's PGDs, I don't think
we'll ever have a valid reason to be doing a set_pgd() again on the
kernel area.