linux-kernel - Re: [PATCH v6 04/12] Add memory slot versioning and use it to provide fast guest write interface

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Date:	Wed, 6 Oct 2010 11:38:47 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	kvm@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, avi@...hat.com, mingo@...e.hu,
	a.p.zijlstra@...llo.nl, tglx@...utronix.de, hpa@...or.com,
	riel@...hat.com, cl@...ux-foundation.org
Subject: Re: [PATCH v6 04/12] Add memory slot versioning and use it to
 provide fast guest write interface

On Wed, Oct 06, 2010 at 01:14:17PM +0200, Gleb Natapov wrote:
> > > +int kvm_gfn_to_hva_cache_init(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
> > > +			      gpa_t gpa)
> > > +{
> > > +	struct kvm_memslots *slots = kvm_memslots(kvm);
> > > +	int offset = offset_in_page(gpa);
> > > +	gfn_t gfn = gpa >> PAGE_SHIFT;
> > > +
> > > +	ghc->gpa = gpa;
> > > +	ghc->generation = slots->generation;

kvm->memslots can change here.

> > > +	ghc->memslot = gfn_to_memslot(kvm, gfn);
> > > +	ghc->hva = gfn_to_hva(kvm, gfn);

And if so, gfn_to_memslot / gfn_to_hva will use new memslots pointer.

Should dereference all values from one copy of kvm->memslots pointer.
 
> > > +	if (!kvm_is_error_hva(ghc->hva))
> > > +		ghc->hva += offset;
> > > +	else
> > > +		return -EFAULT;
> > > +
> > > +	return 0;
> > > +}
> > 
> > Should use a unique kvm_memslots structure for the cache entry, since it
> > can change in between (use gfn_to_hva_memslot, etc on "slots" pointer).
> > 
> I do not understand what do you mean here. kvm_memslots structure itself
> is not cached only various translation that use it are cached. Translation
> result are never used if kvm_memslots was changed.

> > Also should zap any cached entries on overflow, otherwise malicious
> > userspace could make use of stale slots:
> > 
> There is only one cached entry at each given time. User who wants to
> write into guest memory often defines gfn_to_hva_cache variable
> somewhere. Init it with kvm_gfn_to_hva_cache_init() and then calls
> kvm_write_guest_cached() on it. If there was no slot changes in between
> cached translation are used. Otherwise cache is recalculated.

Malicious userspace can cause entry to be cached, ioctl
SET_USER_MEMORY_REGION 2^32 times, generation number will match,
mark_page_dirty_in_slot will be called with pointer to freed memory.

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