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 08:29:12 -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>,
        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 Sun, Nov 26, 2017 at 8:24 AM, Dave Hansen
<dave.hansen@...ux.intel.com> wrote:
> 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.

Right, forgot about that.  So it's just initial setup, then.