Re: [PATCH v12 3/8] arm64: mte: Sync tags for pages where PTE is untagged

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

On 20/05/2021 13:25, Catalin Marinas wrote:
> On Thu, May 20, 2021 at 12:55:21PM +0100, Steven Price wrote:
>> On 19/05/2021 19:06, Catalin Marinas wrote:
>>> On Mon, May 17, 2021 at 01:32:34PM +0100, Steven Price wrote:
>>>> A KVM guest could store tags in a page even if the VMM hasn't mapped
>>>> the page with PROT_MTE. So when restoring pages from swap we will
>>>> need to check to see if there are any saved tags even if !pte_tagged().
>>>>
>>>> However don't check pages for which pte_access_permitted() returns false
>>>> as these will not have been swapped out.
>>>>
>>>> Signed-off-by: Steven Price <steven.price@arm.com>
>>>> ---
>>>>  arch/arm64/include/asm/pgtable.h |  9 +++++++--
>>>>  arch/arm64/kernel/mte.c          | 16 ++++++++++++++--
>>>>  2 files changed, 21 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>>>> index 0b10204e72fc..275178a810c1 100644
>>>> --- a/arch/arm64/include/asm/pgtable.h
>>>> +++ b/arch/arm64/include/asm/pgtable.h
>>>> @@ -314,8 +314,13 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
>>>>  	if (pte_present(pte) && pte_user_exec(pte) && !pte_special(pte))
>>>>  		__sync_icache_dcache(pte);
>>>>  
>>>> -	if (system_supports_mte() &&
>>>> -	    pte_present(pte) && pte_tagged(pte) && !pte_special(pte))
>>>> +	/*
>>>> +	 * If the PTE would provide user space access to the tags associated
>>>> +	 * with it then ensure that the MTE tags are synchronised.  Exec-only
>>>> +	 * mappings don't expose tags (instruction fetches don't check tags).
>>>> +	 */
>>>> +	if (system_supports_mte() && pte_present(pte) &&
>>>> +	    pte_access_permitted(pte, false) && !pte_special(pte))
>>>>  		mte_sync_tags(ptep, pte);
>>>
>>> Looking at the mte_sync_page_tags() logic, we bail out early if it's the
>>> old pte is not a swap one and the new pte is not tagged. So we only need
>>> to call mte_sync_tags() if it's a tagged new pte or the old one is swap.
>>> What about changing the set_pte_at() test to:
>>>
>>> 	if (system_supports_mte() && pte_present(pte) && !pte_special(pte) &&
>>> 	    (pte_tagged(pte) || is_swap_pte(READ_ONCE(*ptep))))
>>> 		mte_sync_tags(ptep, pte);
>>>
>>> We can even change mte_sync_tags() to take the old pte directly:
>>>
>>> 	if (system_supports_mte() && pte_present(pte) && !pte_special(pte)) {
>>> 		pte_t old_pte = READ_ONCE(*ptep);
>>> 		if (pte_tagged(pte) || is_swap_pte(old_pte))
>>> 			mte_sync_tags(old_pte, pte);
>>> 	}
>>>
>>> It would save a function call in most cases where the page is not
>>> tagged.
>>
>> Yes that looks like a good optimisation - although you've missed the
>> pte_access_permitted() part of the check ;)
> 
> I was actually wondering if we could remove it. I don't think it buys us
> much as we have a pte_present() check already, so we know it is pointing
> to a valid page. Currently we'd only get a tagged pte on user mappings,
> same with swap entries.

Actually the other way round makes more sense surely?
pte_access_permitted() is true if both PTE_VALID & PTE_USER are set.
pte_present() is true if *either* PTE_VALID or PTE_PROT_NONE are set. So
the pte_present() is actually redundant.

> When vmalloc kasan_hw will be added, I think we have a set_pte_at() with
> a tagged pte but init_mm and high address (we might as well add a
> warning if addr > TASK_SIZE_64 on the mte_sync_tags path so that we
> don't forget).

While we might not yet have tagged kernel pages - I'm not sure there's
much point weakening the check to have to then check addr as well in the
future.

>> The problem I hit is one of include dependencies:
>>
>> is_swap_pte() is defined (as a static inline) in
>> include/linux/swapops.h. However the definition depends on
>> pte_none()/pte_present() which are defined in pgtable.h - so there's a
>> circular dependency.
>>
>> Open coding is_swap_pte() in set_pte_at() works, but it's a bit ugly.
>> Any ideas on how to improve on the below?
>>
>> 	if (system_supports_mte() && pte_present(pte) &&
>> 	    pte_access_permitted(pte, false) && !pte_special(pte)) {
>> 		pte_t old_pte = READ_ONCE(*ptep);
>> 		/*
>> 		 * We only need to synchronise if the new PTE has tags enabled
>> 		 * or if swapping in (in which case another mapping may have
>> 		 * set tags in the past even if this PTE isn't tagged).
>> 		 * (!pte_none() && !pte_present()) is an open coded version of
>> 		 * is_swap_pte()
>> 		 */
>> 		if (pte_tagged(pte) || (!pte_none(pte) && !pte_present(pte)))
>> 			mte_sync_tags(old_pte, pte);
>> 	}
> 
> That's why I avoided testing my suggestion ;). I think we should just
> add !pte_none() in there with a comment that it may be a swap pte and
> use the is_swap_pte() again on the mte_sync_tags() path. We already have
> the pte_present() check.

Well of course I didn't test the above beyond building - and I've
screwed up because the open coded is_swap_pte() should have been called
on old_pte not pte!

So the pte_present() check above (which I've just removed...) is for the
*new* PTE. So I think we need to keep both here.

Steve

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