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

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Message-ID: <20181025072602.cz3vy2zhzq2px7ik@kshutemo-mobl1>
Date:   Thu, 25 Oct 2018 10:26:03 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        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>,
        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 Wed, Oct 24, 2018 at 11:49:17AM -0700, Andy Lutomirski wrote:
> 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?

Okay, I'll move it into a separate patch in v3.

I'll some more time for comments on v2 before respin.

-- 
 Kirill A. Shutemov