On Thu, Jun 03, 2021 at 05:00:31PM +0100, Catalin Marinas wrote:
> On Mon, May 24, 2021 at 11:45:09AM +0100, Steven Price wrote:
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index c5d1f3c87dbd..226035cf7d6c 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -822,6 +822,42 @@ transparent_hugepage_adjust(struct kvm_memory_slot *memslot,
> > return PAGE_SIZE;
> > }
> >
> > +static int sanitise_mte_tags(struct kvm *kvm, kvm_pfn_t pfn,
> > + unsigned long size)
> > +{
> > + if (kvm_has_mte(kvm)) {
> > + /*
> > + * The page will be mapped in stage 2 as Normal Cacheable, so
> > + * the VM will be able to see the page's tags and therefore
> > + * they must be initialised first. If PG_mte_tagged is set,
> > + * tags have already been initialised.
> > + * pfn_to_online_page() is used to reject ZONE_DEVICE pages
> > + * that may not support tags.
> > + */
> > + unsigned long i, nr_pages = size >> PAGE_SHIFT;
> > + struct page *page = pfn_to_online_page(pfn);
> > +
> > + if (!page)
> > + return -EFAULT;
> > +
> > + for (i = 0; i < nr_pages; i++, page++) {
> > + /*
> > + * There is a potential (but very unlikely) race
> > + * between two VMs which are sharing a physical page
> > + * entering this at the same time. However by splitting
> > + * the test/set the only risk is tags being overwritten
> > + * by the mte_clear_page_tags() call.
> > + */
>
> And I think the real risk here is when the page is writable by at least
> one of the VMs sharing the page. This excludes KSM, so it only leaves
> the MAP_SHARED mappings.
>
> > + if (!test_bit(PG_mte_tagged, &page->flags)) {
> > + mte_clear_page_tags(page_address(page));
> > + set_bit(PG_mte_tagged, &page->flags);
> > + }
> > + }
>
> If we want to cover this race (I'd say in a separate patch), we can call
> mte_sync_page_tags(page, __pte(0), false, true) directly (hopefully I
> got the arguments right). We can avoid the big lock in most cases if
> kvm_arch_prepare_memory_region() sets a VM_MTE_RESET (tag clear etc.)
> and __alloc_zeroed_user_highpage() clears the tags on allocation (as we
> do for VM_MTE but the new flag would not affect the stage 1 VMM page
> attributes).
Another idea: if VM_SHARED is found for any vma within a region in
kvm_arch_prepare_memory_region(), we either prevent the enabling of MTE
for the guest or reject the memory slot if MTE was already enabled.
An alternative here would be to clear VM_MTE_ALLOWED so that any
subsequent mprotect(PROT_MTE) in the VMM would fail in
arch_validate_flags(). MTE would still be allowed in the guest but in
the VMM for the guest memory regions. We can probably do this
irrespective of VM_SHARED. Of course, the VMM can still mmap() the
memory initially with PROT_MTE but that's not an issue IIRC, only the
concurrent mprotect().
--
Catalin
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