Re: [PATCH v11 2/6] arm64: kvm: Introduce MTE VM feature

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

On 28/04/2021 18:07, Catalin Marinas wrote:
> On Fri, Apr 16, 2021 at 04:43:05PM +0100, Steven Price wrote:
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index 77cb2d28f2a4..5f8e165ea053 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -879,6 +879,26 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>   	if (vma_pagesize == PAGE_SIZE && !force_pte)
>>   		vma_pagesize = transparent_hugepage_adjust(memslot, hva,
>>   							   &pfn, &fault_ipa);
>> +
>> +	if (fault_status != FSC_PERM && kvm_has_mte(kvm) && !device &&
>> +	    pfn_valid(pfn)) {
> 
> In the current implementation, device == !pfn_valid(), so we could skip
> the latter check.

Thanks, I'll drop that check.

>> +		/*
>> +		 * VM will be able to see the page's tags, so we must ensure
>> +		 * they have been initialised. if PG_mte_tagged is set, tags
>> +		 * have already been initialised.
>> +		 */
>> +		unsigned long i, nr_pages = vma_pagesize >> PAGE_SHIFT;
>> +		struct page *page = pfn_to_online_page(pfn);
>> +
>> +		if (!page)
>> +			return -EFAULT;
> 
> I think that's fine, though maybe adding a comment that otherwise it
> would be mapped at stage 2 as Normal Cacheable and we cannot guarantee
> that the memory supports MTE tags.

That's what I intended by "be able to see the page's tags", but I'll 
reword to be explicit about it being Normal Cacheable.

>> +
>> +		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));
>> +		}
>> +	}
>> +
>>   	if (writable)
>>   		prot |= KVM_PGTABLE_PROT_W;
> 
> I probably asked already but is the only way to map a standard RAM page
> (not device) in stage 2 via the fault handler? One case I had in mind
> was something like get_user_pages() but it looks like that one doesn't
> call set_pte_at_notify(). There are a few other places where
> set_pte_at_notify() is called and these may happen before we got a
> chance to fault on stage 2, effectively populating the entry (IIUC). If
> that's an issue, we could move the above loop and check closer to the
> actual pte setting like kvm_pgtable_stage2_map().

The only call sites of kvm_pgtable_stage2_map() are in mmu.c:

  * kvm_phys_addr_ioremap() - maps as device in stage 2

  * user_mem_abort() - handled above

  * kvm_set_spte_handler() - ultimately called from the .change_pte() 
callback of the MMU notifier

So the last one is potentially a problem. It's called via the MMU 
notifiers in the case of set_pte_at_notify(). The users of that are:

  * uprobe_write_opcode(): Allocates a new page and performs a 
copy_highpage() to copy the data to the new page (which with MTE 
includes the tags and will copy across the PG_mte_tagged flag).

  * write_protect_page() (KSM): Changes the permissions on the PTE but 
it's still the same page, so nothing to do regarding MTE.

  * replace_page() (KSM): If the page has MTE tags then the MTE version 
of memcmp_pages() will return false, so the only caller 
(try_to_merge_one_page()) will never call this on a page with tags.

  * wp_page_copy(): This one is more interesting - if we go down the 
cow_user_page() path with an old page then everything is safe (tags are 
copied over). The is_zero_pfn() case worries me a bit - a new page is 
allocated, but I can't instantly see anything to zero out the tags (and 
set PG_mte_tagged).

  * migrate_vma_insert_page(): I think migration should be safe as the 
tags should be copied.

So wp_page_copy() looks suspicious.

kvm_pgtable_stage2_map() looks like it could be a good place for the 
checks, it looks like it should work and is probably a more obvious 
place for the checks.

> While the set_pte_at() race on the page flags is somewhat clearer, we
> may still have a race here with the VMM's set_pte_at() if the page is
> mapped as tagged. KVM has its own mmu_lock but it wouldn't be held when
> handling the VMM page tables (well, not always, see below).
> 
> gfn_to_pfn_prot() ends up calling get_user_pages*(). At least the slow
> path (hva_to_pfn_slow()) ends up with FOLL_TOUCH in gup and the VMM pte
> would be set, tags cleared (if PROT_MTE) before the stage 2 pte. I'm not
> sure whether get_user_page_fast_only() does the same.
> 
> The race with an mprotect(PROT_MTE) in the VMM is fine I think as the
> KVM mmu notifier is invoked before set_pte_at() and racing with another
> user_mem_abort() is serialised by the KVM mmu_lock. The subsequent
> set_pte_at() would see the PG_mte_tagged set either by the current CPU
> or by the one it was racing with.
> 

Given the changes to set_pte_at() which means that tags are restored 
from swap even if !PROT_MTE, the only race I can see remaining is the 
creation of new PROT_MTE mappings. As you mention an attempt to change 
mappings in the VMM memory space should involve a mmu notifier call 
which I think serialises this. So the remaining issue is doing this in a 
separate address space.

So I guess the potential problem is:

  * allocate memory MAP_SHARED but !PROT_MTE
  * fork()
  * VM causes a fault in parent address space
  * child does a mprotect(PROT_MTE)

With the last two potentially racing. Sadly I can't see a good way of 
handling that.

Steve