linux-kernel - Re: [RFC][PATCH 04/10] x86/espfix: use kernel-default PTE mask

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Message-ID: <CALCETrU_o5Mk5YTdN0_+2QGfEV5rQ7-MJC7_7dyjHhq+CdD3Uw@mail.gmail.com>
Date:   Thu, 22 Feb 2018 21:59:22 +0000
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Nadav Amit <namit@...are.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "aarcange@...hat.com" <aarcange@...hat.com>,
        "luto@...nel.org" <luto@...nel.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "keescook@...gle.com" <keescook@...gle.com>,
        "hughd@...gle.com" <hughd@...gle.com>,
        "jgross@...e.com" <jgross@...e.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [RFC][PATCH 04/10] x86/espfix: use kernel-default PTE mask

On Thu, Feb 22, 2018 at 9:30 PM, Dave Hansen
<dave.hansen@...ux.intel.com> wrote:
> On 02/22/2018 01:27 PM, Nadav Amit wrote:
>> Dave Hansen <dave.hansen@...ux.intel.com> wrote:
>>> From: Dave Hansen <dave.hansen@...ux.intel.com>
>>> In creating its page tables, the espfix code masks its PGTABLE_PROT
>>> value with the supported mask: __supported_pte_mask.  This ensures
>>> that unsupported bits are not set in the final PTE.  But, it also
>>> sets _PAGE_GLOBAL which we do not want for PTE.  Use
>>> __default_kernel_pte_mask instead which clears _PAGE_GLOBAL for PTI.
>>
>> Can you please explain what is your concern? Exposing more gadgets for
>> speculative ROP attacks?
>>
>> Or is it a general rule of not exposing any kernel code &data more than
>> absolutely necessary?
>
> I think it's good practice to just expose only the *minimal* amount of
> data necessary.  It's easier to audit and less likely to expose things
> accidentall.

But espfix64 is geniunely global.  I'm confused.