linux-kernel - Re: [PATCH] x86/nmi: Fix some races in NMI uaccess

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Date:   Tue, 28 Aug 2018 09:50:53 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     X86 ML <x86@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Jann Horn <jannh@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Nadav Amit <nadav.amit@...il.com>
Subject: Re: [PATCH] x86/nmi: Fix some races in NMI uaccess

On Mon, 2018-08-27 at 19:10 -0700, Andy Lutomirski wrote:
> On Mon, Aug 27, 2018 at 6:31 PM, Rik van Riel <riel@...riel.com>
> wrote:
> 
> > What is special about this path wrt nmi_uaccess_ok that is
> > not also true for the need_flush branch right above it?
> > 
> > What am I missing?
> 
> Nothing.  My patch is buggy.  ETOLITTLESLEEP.
> 
> I could drop this part of the patch entirely.  Or I could drop the
> loaded_mm->pgd == __va(read_cr3_pa() check and instead make sure that
> loaded_mm is NULL at any point at which loaded_mm might not match
> CR3.
> The latter will be faster in any (hypothetical) virtualization
> environment where CR3 reads trap.  I don't know if we have any such
> cases where perf works and we care about performance, though.

Moving that loaded_mm = NULL assignment up a few
lines, so it comes before the "if (need_flush)"
test and covers both branches should cover that,
indeed.

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