Subject: Re: [PATCH v8 28/43] arm64: mte: Reset the page tag in page->flags

From: Catalin Marinas <[email-masked]>
Date: Thu, 5 Nov 2020 15:59:00 +0000

On Thu, Nov 05, 2020 at 12:18:43AM +0100, Andrey Konovalov wrote:
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index 8f99c65837fd..06ba6c923ab7 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -34,6 +34,7 @@ static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
>  			return;
>  	}
>  
> +	page_kasan_tag_reset(page);
>  	mte_clear_page_tags(page_address(page));

I think we need an smp_wmb() between setting the flags and clearing the
actual tags. If another threads reads page->flags and builds a tagged
address out of it (see page_to_virt) there's an address dependency to
the actual memory access. However, on the current thread, we don't
guarantee that the new page->flags are visible before the tags were
updated.

>  }
>  
> diff --git a/arch/arm64/mm/copypage.c b/arch/arm64/mm/copypage.c
> index 70a71f38b6a9..348f4627da08 100644
> --- a/arch/arm64/mm/copypage.c
> +++ b/arch/arm64/mm/copypage.c
> @@ -22,6 +22,7 @@ void copy_highpage(struct page *to, struct page *from)
>  	copy_page(kto, kfrom);
>  
>  	if (system_supports_mte() && test_bit(PG_mte_tagged, &from->flags)) {
> +		page_kasan_tag_reset(to);
>  		set_bit(PG_mte_tagged, &to->flags);
>  		mte_copy_page_tags(kto, kfrom);

Nitpick: move page_kasan_tag_reset() just above mte_copy_page_tags() for
consistency with the other places where PG_mte_tagged is set before or
after the actual tag setting.

>  	}
> diff --git a/arch/arm64/mm/mteswap.c b/arch/arm64/mm/mteswap.c
> index c52c1847079c..0e7eccbe598a 100644
> --- a/arch/arm64/mm/mteswap.c
> +++ b/arch/arm64/mm/mteswap.c
> @@ -53,6 +53,7 @@ bool mte_restore_tags(swp_entry_t entry, struct page *page)
>  	if (!tags)
>  		return false;
>  
> +	page_kasan_tag_reset(page);
>  	mte_restore_page_tags(page_address(page), tags);

There is another mte_restore_page_tags() caller in hibernate.c. That one
doesn't need page_kasan_tag_reset() since the page->flags would have
been already restored but please add a comment in that file why its not
needed.

-- 
Catalin

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