On 17/05/2021 17:14, Marc Zyngier wrote:
> On Mon, 17 May 2021 13:32:34 +0100,
> Steven Price <steven.price@xxxxxxx> 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@xxxxxxx>
>> ---
>> 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
>>
>>
>
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