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

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Date:   Thu, 22 Feb 2018 21:27:31 +0000
From:   Nadav Amit <namit@...are.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
CC:     "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

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?