[PATCH] arm64: mte: Ensure the cleared tags are visible before setting the PTE

[Steven Price]

On 13/05/2022 11:08, Catalin Marinas wrote:
> As an optimisation, only pages mapped with PROT_MTE in user space have
> the MTE tags zeroed. This is done lazily at the set_pte_at() time via
> mte_sync_tags(). However, this function is missing a barrier and another
> CPU may see the PTE updated before the zeroed tags are visible. Add an
> smp_wmb() barrier if the page tags have been updated.
> 
> Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
> Fixes: 34bfeea4a9e9 ("arm64: mte: Clear the tags when a page is mapped in user-space with PROT_MTE")
> Cc: <stable at vger.kernel.org> # 5.10.x
> Reported-by: Vladimir Murzin <vladimir.murzin at arm.com>
> Cc: Will Deacon <will at kernel.org>

Reviewed-by: Steven Price <steven.price at arm.com>

> ---
>  arch/arm64/kernel/mte.c | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index 78b3e0f8e997..07dabd52377d 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -34,18 +34,18 @@ DEFINE_STATIC_KEY_FALSE(mte_async_or_asymm_mode);
>  EXPORT_SYMBOL_GPL(mte_async_or_asymm_mode);
>  #endif
>  
> -static void mte_sync_page_tags(struct page *page, pte_t old_pte,
> +static bool mte_sync_page_tags(struct page *page, pte_t old_pte,
>  			       bool check_swap, bool pte_is_tagged)
>  {
>  	if (check_swap && is_swap_pte(old_pte)) {
>  		swp_entry_t entry = pte_to_swp_entry(old_pte);
>  
>  		if (!non_swap_entry(entry) && mte_restore_tags(entry, page))
> -			return;
> +			return true;
>  	}
>  
>  	if (!pte_is_tagged)
> -		return;
> +		return false;
>  
>  	page_kasan_tag_reset(page);
>  	/*
> @@ -57,6 +57,7 @@ static void mte_sync_page_tags(struct page *page, pte_t old_pte,
>  	 */
>  	smp_wmb();
>  	mte_clear_page_tags(page_address(page));
> +	return true;
>  }
>  
>  void mte_sync_tags(pte_t old_pte, pte_t pte)
> @@ -65,6 +66,7 @@ void mte_sync_tags(pte_t old_pte, pte_t pte)
>  	long i, nr_pages = compound_nr(page);
>  	bool check_swap = nr_pages == 1;
>  	bool pte_is_tagged = pte_tagged(pte);
> +	bool updated = false;
>  
>  	/* Early out if there's nothing to do */
>  	if (!check_swap && !pte_is_tagged)
> @@ -72,10 +74,15 @@ void mte_sync_tags(pte_t old_pte, pte_t pte)
>  
>  	/* if PG_mte_tagged is set, tags have already been initialised */
>  	for (i = 0; i < nr_pages; i++, page++) {
> -		if (!test_and_set_bit(PG_mte_tagged, &page->flags))
> -			mte_sync_page_tags(page, old_pte, check_swap,
> -					   pte_is_tagged);
> +		if (!test_and_set_bit(PG_mte_tagged, &page->flags) &&
> +		    mte_sync_page_tags(page, old_pte, check_swap,
> +				       pte_is_tagged))
> +			updated = true;
>  	}
> +
> +	/* ensure the tags are visible before the PTE is set */
> +	if (updated)
> +		smp_wmb();
>  }
>  
>  int memcmp_pages(struct page *page1, struct page *page2)