linux-kernel - Re: [PATCH 2/2] x86/ldt: Unmap PTEs for the slow before freeing LDT

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Date:   Wed, 24 Oct 2018 11:49:17 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] x86/ldt: Unmap PTEs for the slow before freeing LDT

On Tue, Oct 23, 2018 at 9:32 AM Kirill A. Shutemov
<kirill.shutemov@...ux.intel.com> wrote:
>
> modify_ldt(2) leaves old LDT mapped after we switch over to the new one.
> Memory for the old LDT gets freed and the pages can be re-used.
>
> Leaving the mapping in place can have security implications. The mapping
> is present in userspace copy of page tables and Meltdown-like attack can
> read these freed and possibly reused pages.

Code looks okay.  But:

> -       /*
> -        * Did we already have the top level entry allocated?  We can't
> -        * use pgd_none() for this because it doens't do anything on
> -        * 4-level page table kernels.
> -        */
> -       pgd = pgd_offset(mm, LDT_BASE_ADDR);

This looks like an unrelated cleanup.  Can it be its own patch?