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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 22 Feb 2018 21:59:22 +0000
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Subject: Re: [RFC][PATCH 04/10] x86/espfix: use kernel-default PTE mask
To:     Dave Hansen <dave.hansen@linux.intel.com>
Cc:     Nadav Amit <namit@vmware.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "aarcange@redhat.com" <aarcange@redhat.com>,
        "luto@kernel.org" <luto@kernel.org>,
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        "x86@kernel.org" <x86@kernel.org>
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On Thu, Feb 22, 2018 at 9:30 PM, Dave Hansen
<dave.hansen@linux.intel.com> wrote:
> On 02/22/2018 01:27 PM, Nadav Amit wrote:
>> Dave Hansen <dave.hansen@linux.intel.com> wrote:
>>> From: Dave Hansen <dave.hansen@linux.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.