On Wed, Oct 06, 2010 at 11:38:47AM -0300, Marcelo Tosatti wrote:
> 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.
>
Ah, I see now. Thanks! Will fix.
> > > > + 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.
>
Hmm. To zap all cached entires on overflow we need to track them. If we
will track then we can zap them on each slot update and drop "generation"
entirely.
--
Gleb.
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