On Thu, May 20, 2021 at 04:05:46PM +0100, Steven Price wrote:
> On 20/05/2021 12:54, Catalin Marinas wrote:
> > On Mon, May 17, 2021 at 01:32:35PM +0100, Steven Price wrote:
> >> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> >> index c5d1f3c87dbd..8660f6a03f51 100644
> >> --- a/arch/arm64/kvm/mmu.c
> >> +++ b/arch/arm64/kvm/mmu.c
> >> @@ -822,6 +822,31 @@ transparent_hugepage_adjust(struct kvm_memory_slot *memslot,
> >> return PAGE_SIZE;
> >> }
> >>
> >> +static int sanitise_mte_tags(struct kvm *kvm, unsigned long size,
> >> + kvm_pfn_t pfn)
> >> +{
> >> + 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.
> >> + */
> >> + unsigned long i, nr_pages = size >> PAGE_SHIFT;
> >> + struct page *page = pfn_to_online_page(pfn);
> >> +
> >> + if (!page)
> >> + return -EFAULT;
> >
> > IIRC we ended up with pfn_to_online_page() to reject ZONE_DEVICE pages
> > that may be mapped into a guest and we have no idea whether they support
> > MTE. It may be worth adding a comment, otherwise, as Marc said, the page
> > wouldn't disappear.
>
> I'll add a comment.
>
> >> +
> >> + for (i = 0; i < nr_pages; i++, page++) {
> >> + if (!test_and_set_bit(PG_mte_tagged, &page->flags))
> >> + mte_clear_page_tags(page_address(page));
> >
> > We started the page->flags thread and ended up fixing it for the host
> > set_pte_at() as per the first patch:
> >
> > https://lore.kernel.org/r/c3293d47-a5f2-ea4a-6730-f5cae26d8a7e@xxxxxxx
> >
> > Now, can we have a race between the stage 2 kvm_set_spte_gfn() and a
> > stage 1 set_pte_at()? Only the latter takes a lock. Or between two
> > kvm_set_spte_gfn() in different VMs? I think in the above thread we
> > concluded that there's only a problem if the page is shared between
> > multiple VMMs (MAP_SHARED). How realistic is this and what's the
> > workaround?
> >
> > Either way, I think it's worth adding a comment here on the race on
> > page->flags as it looks strange that here it's just a test_and_set_bit()
> > while set_pte_at() uses a spinlock.
> >
>
> Very good point! I should have thought about that. I think splitting the
> test_and_set_bit() in two (as with the cache flush) is sufficient. While
> there technically still is a race which could lead to user space tags
> being clobbered:
>
> a) It's very odd for a VMM to be doing an mprotect() after the fact to
> add PROT_MTE, or to be sharing the memory with another process which
> sets PROT_MTE.
>
> b) The window for the race is incredibly small and the VMM (generally)
> needs to be robust against the guest changing tags anyway.
>
> But I'll add a comment here as well:
>
> /*
> * There is a potential race between sanitising the
> * flags here and user space using mprotect() to add
> * PROT_MTE to access the tags, however by splitting
> * the test/set the only risk is user space tags
> * being overwritten by the mte_clear_page_tags() call.
> */
I think (well, I haven't re-checked), an mprotect() in the VMM ends up
calling set_pte_at_notify() which would call kvm_set_spte_gfn() and that
will map the page in the guest. So the problem only appears between
different VMMs sharing the same page. In principle they can be
MAP_PRIVATE but they'd be CoW so the race wouldn't matter. So it's left
with MAP_SHARED between multiple VMMs.
I think we should just state that this is unsafe and they can delete
each-others tags. If we are really worried, we can export that lock you
added in mte.c.
--
Catalin
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