Re: [PATCH v4 2/2] arm64: kvm: Introduce MTE VCPU feature

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

On 19/11/2020 16:24, Catalin Marinas wrote:
> On Thu, Nov 19, 2020 at 12:45:52PM +0000, Steven Price wrote:
>> On 18/11/2020 17:05, Andrew Jones wrote:
>>> On Wed, Nov 18, 2020 at 04:50:01PM +0000, Catalin Marinas wrote:
>>>> On Wed, Nov 18, 2020 at 04:01:20PM +0000, Steven Price wrote:
>>>>> On 17/11/2020 16:07, Catalin Marinas wrote:
>>>>>> On Mon, Oct 26, 2020 at 03:57:27PM +0000, Steven Price wrote:
>>>>>>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>>>>>>> index 19aacc7d64de..38fe25310ca1 100644
>>>>>>> --- a/arch/arm64/kvm/mmu.c
>>>>>>> +++ b/arch/arm64/kvm/mmu.c
>>>>>>> @@ -862,6 +862,26 @@ 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);
>>>>>>> +
>>>>>>> +	/*
>>>>>>> +	 * The otherwise redundant test for system_supports_mte() allows the
>>>>>>> +	 * code to be compiled out when CONFIG_ARM64_MTE is not present.
>>>>>>> +	 */
>>>>>>> +	if (system_supports_mte() && kvm->arch.mte_enabled && pfn_valid(pfn)) {
>>>>>>> +		/*
>>>>>>> +		 * VM will be able to see the page's tags, so we must ensure
>>>>>>> +		 * they have been initialised.
>>>>>>> +		 */
>>>>>>> +		struct page *page = pfn_to_page(pfn);
>>>>>>> +		long i, nr_pages = compound_nr(page);
>>>>>>> +
>>>>>>> +		/* if PG_mte_tagged is set, tags have already been initialised */
>>>>>>> +		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 this page was swapped out and mapped back in, where does the
>>>>>> restoring from swap happen?
>>>>>
>>>>> Restoring from swap happens above this in the call to gfn_to_pfn_prot()
>>>>
>>>> Looking at the call chain, gfn_to_pfn_prot() ends up with
>>>> get_user_pages() using the current->mm (the VMM) and that does a
>>>> set_pte_at(), presumably restoring the tags. Does this mean that all
>>>> memory mapped by the VMM in user space should have PROT_MTE set?
>>>> Otherwise we don't take the mte_sync_tags() path in set_pte_at() and no
>>>> tags restored from swap (we do save them since when they were mapped,
>>>> PG_mte_tagged was set).
>>>>
>>>> So I think the code above should be similar to mte_sync_tags(), even
>>>> calling a common function, but I'm not sure where to get the swap pte
>>>> from.
>>
>> You're right - the code is broken as it stands. I've just been able to
>> reproduce the loss of tags due to swap.
>>
>> The problem is that we also don't have a suitable pte to do the restore from
>> swap from. So either set_pte_at() would have to unconditionally check for
>> MTE tags for all previous swap entries as you suggest below. I had a quick
>> go at testing this and hit issues with the idle task getting killed during
>> boot - I fear there are some fun issues regarding initialisation order here.
> 
> My attempt here but not fully tested (just booted, no swap support):

Ah, very similar to what I had, just without the silly mistake... ;)

I just did a quick test with this and it seems to work. I obviously 
should have looked harder before giving up on this approach.

Thanks!

Steve

> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index b35833259f08..27d7fd336a16 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -304,7 +304,7 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
>   		__sync_icache_dcache(pte);
>   
>   	if (system_supports_mte() &&
> -	    pte_present(pte) && pte_tagged(pte) && !pte_special(pte))
> +	    pte_present(pte) && pte_valid_user(pte) && !pte_special(pte))
>   		mte_sync_tags(ptep, pte);
>   
>   	__check_racy_pte_update(mm, ptep, pte);
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index 52a0638ed967..bbd6c56d33d9 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -20,18 +20,24 @@
>   #include <asm/ptrace.h>
>   #include <asm/sysreg.h>
>   
> -static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
> +static void mte_sync_page_tags(struct page *page, pte_t *ptep, pte_t pte,
> +			       bool check_swap)
>   {
>   	pte_t old_pte = READ_ONCE(*ptep);
>   
>   	if (check_swap && is_swap_pte(old_pte)) {
>   		swp_entry_t entry = pte_to_swp_entry(old_pte);
>   
> -		if (!non_swap_entry(entry) && mte_restore_tags(entry, page))
> +		if (!non_swap_entry(entry) && mte_restore_tags(entry, page)) {
> +			set_bit(PG_mte_tagged, &page->flags);
>   			return;
> +		}
>   	}
>   
> -	mte_clear_page_tags(page_address(page));
> +	if (pte_tagged(pte)) {
> +		mte_clear_page_tags(page_address(page));
> +		set_bit(PG_mte_tagged, &page->flags);
> +	}
>   }
>   
>   void mte_sync_tags(pte_t *ptep, pte_t pte)
> @@ -42,8 +48,8 @@ void mte_sync_tags(pte_t *ptep, pte_t pte)
>   
>   	/* if PG_mte_tagged is set, tags have already been initialised */
>   	for (i = 0; i < nr_pages; i++, page++) {
> -		if (!test_and_set_bit(PG_mte_tagged, &page->flags))
> -			mte_sync_page_tags(page, ptep, check_swap);
> +		if (!test_bit(PG_mte_tagged, &page->flags))
> +			mte_sync_page_tags(page, ptep, pte, check_swap);
>   	}
>   }
>