Date:   Fri, 10 Dec 2021 17:48:46 +0000
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     andrey.konovalov@linux.dev
Cc:     Marco Elver <elver@google.com>,
        Alexander Potapenko <glider@google.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Peter Collingbourne <pcc@google.com>,
        Andrey Konovalov <andreyknvl@gmail.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        kasan-dev@googlegroups.com,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        Will Deacon <will@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        linux-arm-kernel@lists.infradead.org,
        Evgenii Stepanov <eugenis@google.com>,
        linux-kernel@vger.kernel.org,
        Andrey Konovalov <andreyknvl@google.com>
Subject: Re: [PATCH v2 08/34] kasan: only apply __GFP_ZEROTAGS when memory is
 zeroed
Message-ID: <YbOS/jskofqqOc0y@arm.com>
References: <cover.1638825394.git.andreyknvl@google.com>
 <cca947c05c4881cf5b7548614909f1625f47be61.1638825394.git.andreyknvl@google.com>
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On Mon, Dec 06, 2021 at 10:43:45PM +0100, andrey.konovalov@linux.dev wrote:
> From: Andrey Konovalov <andreyknvl@google.com>
> 
> __GFP_ZEROTAGS should only be effective if memory is being zeroed.
> Currently, hardware tag-based KASAN violates this requirement.
> 
> Fix by including an initialization check along with checking for
> __GFP_ZEROTAGS.
> 
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> Reviewed-by: Alexander Potapenko <glider@google.com>
> ---
>  mm/kasan/hw_tags.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
> index 0b8225add2e4..c643740b8599 100644
> --- a/mm/kasan/hw_tags.c
> +++ b/mm/kasan/hw_tags.c
> @@ -199,11 +199,12 @@ void kasan_alloc_pages(struct page *page, unsigned int order, gfp_t flags)
>  	 * page_alloc.c.
>  	 */
>  	bool init = !want_init_on_free() && want_init_on_alloc(flags);
> +	bool init_tags = init && (flags & __GFP_ZEROTAGS);
>  
>  	if (flags & __GFP_SKIP_KASAN_POISON)
>  		SetPageSkipKASanPoison(page);
>  
> -	if (flags & __GFP_ZEROTAGS) {
> +	if (init_tags) {

You can probably leave this unchanged but add a WARN_ON_ONCE() if !init.
AFAICT there's only a single place where __GFP_ZEROTAGS is passed.

-- 
Catalin