linux-kernel - Re: l1tf: Kernel suggests I throw away third of my memory. I'd rather not

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Message-ID: <20181017113226.GA5176@amd>
Date:   Wed, 17 Oct 2018 13:32:26 +0200
From:   Pavel Machek <pavel@....cz>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     vbabka@...e.cz, hpa@...or.com, torvalds@...ux-foundation.org,
        ak@...ux.intel.com, dave.hansen@...el.com,
        kernel list <linux-kernel@...r.kernel.org>, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de
Subject: Re: l1tf: Kernel suggests I throw away third of my memory. I'd
 rather not

Hi!

> > 6a012288 suggests I throw away 1GB on RAM. On 3GB system.. that is not
> > going to be pleasant.
> > 
> > l1tf.html says:
> > 
> > # The Linux kernel contains a mitigation for this attack vector, PTE
> > # inversion, which is permanently enabled and has no performance
> > # impact.
> > 
> > I don't believe it has "no" performance impact, but I guess it is lost
> > in the noise.
> 
> Please prove otherwise. I would be more than surprised if inverting pfn
> part of the pte is noticeable. But I can be wrong of course.

I'm not saying its noticeable. I'm saying that inversion takes few
clock cycles (including a branch?) and that neither caches nor RAM is
free.

"no noticeable performance impact" I'd agree with :).

> > #  The kernel ensures that the address bits of PTEs, which are
> > # not marked present, never point to cacheable physical memory space.
> > 
> > # A system with an up to date kernel is protected against attacks from
> > # malicious user space applications.
> > 
> > These are not true.
> > 
> > cat /sys/devices/system/cpu/vulnerabilities/l1tf
> > Vulnerable
> > uname -a
> > Linux amd 4.19.0-rc8-next-20181017autobisect1539371050 #189 SMP Wed
> > Oct 17 12:04:23 CEST 2018 i686 GNU/Linux
> 
> This is a result of you having memory above MAX_PFN/2 right?

Yes.

> > Now question is... can we do better? Kernel stores information about
> > swapped-out pages there, right? That sounds like a cool hack, but
> > maybe it is time to get rid of that hack?
> 
> Patches are welcome.

Cooperation will be needed if you want to see patches. As
in... answering the questions above.

> > As a workaround, can I simply do swapoff -a to be safe for now?
> 
> Well, that depends. Do you care about PROT_NONE attacks as well? If not
> then no-swap would help you. But even then no-swap is rather theoretical
> attack on a physical host unless you allow an arbitrary swapout to a
> malicious user (e.g. allow a user controlled memcg hard limit that would
> cause excessive local swapouts).

PROT_NONE attack.. aha, so kernel stores not only information about
swapped-out pages but also about file-backed pages that are currently
not present? Hmm. That makes it more complex :-(. 

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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