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?
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
>
>
--
Without deviation from the norm, progress is not possible.
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