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.
*/
Thanks,
Steve
_______________________________________________
kvmarm mailing list
kvmarm@xxxxxxxxxxxxxxxxxxxxx
https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
