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Subject: Re: [RFC][PATCH 04/10] x86/espfix: use kernel-default PTE mask
To:     Nadav Amit <namit@vmware.com>
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From:   Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <f90cef60-1710-1d99-61ee-4527067b115f@linux.intel.com>
Date:   Thu, 22 Feb 2018 13:30:01 -0800
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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.