From: Rik van Riel <riel@surriel.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: X86 ML <x86@kernel.org>, Borislav Petkov <bp@alien8.de>,
Jann Horn <jannh@google.com>, LKML <linux-kernel@vger.kernel.org>,
stable <stable@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Nadav Amit <nadav.amit@gmail.com>
Subject: Re: [PATCH v2] x86/nmi: Fix some races in NMI uaccess
Date: Wed, 29 Aug 2018 11:49:10 -0400 [thread overview]
Message-ID: <041532109326b7bd3e67087808fee9d3289f9682.camel@surriel.com> (raw)
In-Reply-To: <CALCETrXjH175OLPRCwA8=Stbq1KmJwwvkZ36Y74=hF_i8GFWCw@mail.gmail.com>
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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@surriel.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@surriel.com>
> > > wrote:
> > > > On Mon, 27 Aug 2018 16:04:16 -0700
> > > > Andy Lutomirski <luto@kernel.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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next prev parent reply other threads:[~2018-08-29 15:49 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-27 23:04 [PATCH] x86/nmi: Fix some races in NMI uaccess Andy Lutomirski
2018-08-27 23:12 ` Jann Horn
2018-08-27 23:25 ` Andy Lutomirski
2018-08-27 23:34 ` Jann Horn
2018-08-28 1:31 ` Rik van Riel
2018-08-28 2:10 ` Andy Lutomirski
2018-08-28 13:50 ` Rik van Riel
2018-08-28 17:56 ` [PATCH v2] " Rik van Riel
2018-08-29 3:46 ` Andy Lutomirski
2018-08-29 15:17 ` Rik van Riel
2018-08-29 15:36 ` Andy Lutomirski
2018-08-29 15:49 ` Rik van Riel [this message]
2018-08-29 16:14 ` Andy Lutomirski
2018-08-29 15:47 Andy Lutomirski
2018-08-29 16:06 ` Rik van Riel
2018-08-29 18:56 ` Nadav Amit
2018-08-30 13:36 ` Thomas Gleixner
2018-08-30 14:21 ` Andy Lutomirski
2018-08-30 14:27 ` Thomas Gleixner
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