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

From: Catalin Marinas <[email-masked]>
Date: Wed, 19 May 2021 19:06:10 +0100

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 <[email-masked]>
> ---
>  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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