Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20211001024105.3217339-1-willy@infradead.org>
In-Reply-To: <20211001024105.3217339-1-willy@infradead.org>
From:   Andrey Konovalov <andreyknvl@gmail.com>
Date:   Fri, 1 Oct 2021 15:29:29 +0200
Message-ID: <CA+fCnZfSUxToYKUfHwQT0r3bC9NYZNc2iC3PXv+GciuW0Fm79A@mail.gmail.com>
Subject: Re: [PATCH] kasan: Fix tag for large allocations when using CONFIG_SLAB
To:     "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc:     Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Fri, Oct 1, 2021 at 4:42 AM Matthew Wilcox (Oracle)
<willy@infradead.org> wrote:
>
> If an object is allocated on a tail page of a multi-page slab, kasan
> will get the wrong tagbecause page->s_mem is NULL for tail pages.

Interesting. Is this a known property of tail pages? Why does this
happen? I failed to find this exception in the code.

The tag value won't really be "wrong", just unexpected. But if s_mem
is indeed NULL for tail pages, your fix makes sense.

> I'm not quite sure what the user-visible effect of this might be.

Everything should work, as long as tag values are assigned
consistently based on the object address.

>
> Fixes: 7f94ffbc4c6a ("kasan: add hooks implementation for tag-based mode")
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  mm/kasan/common.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 2baf121fb8c5..41779ad109cd 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -298,7 +298,7 @@ static inline u8 assign_tag(struct kmem_cache *cache,
>         /* For caches that either have a constructor or SLAB_TYPESAFE_BY_RCU: */
>  #ifdef CONFIG_SLAB
>         /* For SLAB assign tags based on the object index in the freelist. */
> -       return (u8)obj_to_index(cache, virt_to_page(object), (void *)object);
> +       return (u8)obj_to_index(cache, virt_to_head_page(object), (void *)object);
>  #else
>         /*
>          * For SLUB assign a random tag during slab creation, otherwise reuse
> --
> 2.32.0
>