Messages in this thread | | | Date | Wed, 17 Oct 2018 13:32:26 +0200 | From | Pavel Machek <> | Subject | Re: l1tf: Kernel suggests I throw away third of my memory. I'd rather not |
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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 [unhandled content-type:application/pgp-signature] | |