Re: [PATCH v12 4/8] arm64: kvm: Introduce MTE VM feature

[Steven Price <steven.price@arm.com>]

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@arm.com
> 
> 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