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

[Steven Price]

On 17/05/2021 17:14, Marc Zyngier wrote:
> On Mon, 17 May 2021 13:32:34 +0100,
> Steven Price <steven.price@arm.com> 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).
> 
> I'm not sure I understand this comment. Of course, execution doesn't
> match tags. But the memory could still have tags associated with
> it. Does this mean such a page would lose its tags is swapped out?

Hmm, I probably should have reread that - the context of the comment is
lost.

I added the comment when changing to pte_access_permitted(), and the
comment on pte_access_permitted() explains a potential gotcha:

 * p??_access_permitted() is true for valid user mappings (PTE_USER
 * bit set, subject to the write permission check). For execute-only
 * mappings, like PROT_EXEC with EPAN (both PTE_USER and PTE_UXN bits
 * not set) must return false. PROT_NONE mappings do not have the
 * PTE_VALID bit set.

So execute-only mappings return false even though that is effectively a
type of user access. However, because MTE checks are not performed by
the PE for instruction fetches this doesn't matter. I'll update the
comment, how about:

/*
 * If the PTE would provide user space access to the tags associated
 * with it then ensure that the MTE tags are synchronised.  Although
 * pte_access_permitted() returns false for exec only mappings, they
 * don't expose tags (instruction fetches don't check tags).
 */

Thanks,

Steve

> Thanks,
> 
>       M.
> 
>> +     */
>> +    if (system_supports_mte() && pte_present(pte) &&
>> +        pte_access_permitted(pte, false) && !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 c88e778c2fa9..a604818c52c1 100644
>> --- a/arch/arm64/kernel/mte.c
>> +++ b/arch/arm64/kernel/mte.c
>> @@ -33,11 +33,15 @@ DEFINE_STATIC_KEY_FALSE(mte_async_mode);
>>  EXPORT_SYMBOL_GPL(mte_async_mode);
>>  #endif
>>  
>> -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, bool 
>> check_swap,
>> +                           bool pte_is_tagged)
>>  {
>>      unsigned long flags;
>>      pte_t old_pte = READ_ONCE(*ptep);
>>  
>> +    if (!is_swap_pte(old_pte) && !pte_is_tagged)
>> +            return;
>> +
>>      spin_lock_irqsave(&tag_sync_lock, flags);
>>  
>>      /* Recheck with the lock held */
>> @@ -53,6 +57,9 @@ static void mte_sync_page_tags(struct page *page, pte_t 
>> *ptep, bool check_swap)
>>              }
>>      }
>>  
>> +    if (!pte_is_tagged)
>> +            goto out;
>> +
>>      page_kasan_tag_reset(page);
>>      /*
>>       * We need smp_wmb() in between setting the flags and clearing the
>> @@ -76,10 +83,15 @@ void mte_sync_tags(pte_t *ptep, pte_t pte)
>>      bool check_swap = nr_pages == 1;
>>      bool pte_is_tagged = pte_tagged(pte);
>>  
>> +    /* Early out if there's nothing to do */
>> +    if (!check_swap && !pte_is_tagged)
>> +            return;
>> +
>>      /* if PG_mte_tagged is set, tags have already been initialised */
>>      for (i = 0; i < nr_pages; i++, page++) {
>>              if (!test_bit(PG_mte_tagged, &page->flags))
>> -                    mte_sync_page_tags(page, ptep, check_swap);
>> +                    mte_sync_page_tags(page, ptep, check_swap,
>> +                                       pte_is_tagged);
>>      }
>>  }
>>  
>> -- 
>> 2.20.1
>>
>>
>