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@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).
> + */
> + 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.
--
Catalin
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