linux-kernel - Re: [PATCH v2] x86/nmi: Fix some races in NMI uaccess

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Date:   Wed, 29 Aug 2018 11:49:10 -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 v2] x86/nmi: Fix some races in NMI uaccess

On Wed, 2018-08-29 at 08:36 -0700, Andy Lutomirski wrote:
> On Wed, Aug 29, 2018 at 8:17 AM, Rik van Riel <riel@...riel.com>
> wrote:
> > On Tue, 2018-08-28 at 20:46 -0700, Andy Lutomirski wrote:
> > > On Tue, Aug 28, 2018 at 10:56 AM, Rik van Riel <riel@...riel.com>
> > > wrote:
> > > > On Mon, 27 Aug 2018 16:04:16 -0700
> > > > Andy Lutomirski <luto@...nel.org> wrote:
> > > > 
> > > > > The 0day bot is still chewing on this, but I've tested it a
> > > > > bit
> > > > > locally
> > > > > and it seems to do the right thing.
> > > > 
> > > > Hi Andy,
> > > > 
> > > > the version of the patch below should fix the bug we talked
> > > > about
> > > > in email yesterday.  It should automatically cover kernel
> > > > threads
> > > > in lazy TLB mode, because current->mm will be NULL, while the
> > > > cpu_tlbstate.loaded_mm should never be NULL.
> > > > 
> > > 
> > > That's better than mine.  I tweaked it a bit and added some
> > > debugging,
> > > and I got this:
> > > 
> > > 
> > 
> > 
https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/fixes&id=dd956eba16646fd0b15c3c0741269dfd84452dac
> > > 
> > > I made the loaded_mm handling a little more conservative to make
> > > it
> > > more obvious that switch_mm_irqs_off() is safe regardless of
> > > exactly
> > > when it gets called relative to switching current.
> > 
> > I am not convinced that the dance of writing
> > cpu_tlbstate.loaded_mm twice, with a barrier on
> > each end, is useful or necessary.
> > 
> > At the time switch_mm_irqs_off returns, nmi_uaccess_ok()
> > will still return false, because we have not switched
> > "current" to the task that owns the next mm_struct yet.
> > 
> > We just have to make sure to:
> > 1) Change cpu_tlbstate.loaded_mm before we manipulate
> >    CR3, and
> > 2) Change "current" only once enough of the mm stuff has
> >    been switched, __switch_to seems to get that right.
> > 
> > Between the time switch_mm_irqs_off() sets cpu_tlbstate
> > to the next mm, and __switch_to moves() over current,
> > nmi_uaccess_ok() will return false.
> 
> All true, but I think it stops working as soon as someone starts
> calling switch_mm_irqs_off() for some other reason, such as during
> text_poke().  And that was the original motivation for this patch.

How does calling switch_mm_irqs_off() for text_poke()
change around current->mm and cpu_tlbstate.loaded_mm?

Does current->mm stay the same throughout the entire
text_poke() chain, while cpustate.loaded_mm is the
only thing that is changed out?

If so, then yes the double assignment is indeed
necessary. Good point.

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