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

[Steven Price]

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 ;) 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);
        }

Steve