linux-kernel - Re: [PATCH] kasan, slub: fix more conflicts with CONFIG_SLAB_FREELIST

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Message-ID: <CAAeHK+z=ft93RNx7rvq1QFr3kiOFVzBVACEFN4fL8nbEVOEXKA@mail.gmail.com>
Date:   Thu, 14 Feb 2019 01:27:09 +0100
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Qian Cai <cai@....pw>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Kostya Serebryany <kcc@...gle.com>,
        Evgeniy Stepanov <eugenis@...gle.com>
Subject: Re: [PATCH] kasan, slub: fix more conflicts with CONFIG_SLAB_FREELIST_HARDENED

On Thu, Feb 14, 2019 at 1:25 AM Andrey Konovalov <andreyknvl@...gle.com> wrote:
>
> When CONFIG_KASAN_SW_TAGS is enabled, ptr_addr might be tagged.
> Normally, this doesn't cause any issues, as both set_freepointer()
> and get_freepointer() are called with a pointer with the same tag.
> However, there are some issues with CONFIG_SLUB_DEBUG code. For
> example, when __free_slub() iterates over objects in a cache, it
> passes untagged pointers to check_object(). check_object() in turns
> calls get_freepointer() with an untagged pointer, which causes the
> freepointer to be restored incorrectly.
>
> Add kasan_reset_tag to freelist_ptr(). Also add a detailed comment.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>

Reported-by: Qian Cai <cai@....pw>

> ---
>  mm/slub.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 80da3a40b74d..c80e6699357c 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -249,7 +249,18 @@ static inline void *freelist_ptr(const struct kmem_cache *s, void *ptr,
>                                  unsigned long ptr_addr)
>  {
>  #ifdef CONFIG_SLAB_FREELIST_HARDENED
> -       return (void *)((unsigned long)ptr ^ s->random ^ ptr_addr);
> +       /*
> +        * When CONFIG_KASAN_SW_TAGS is enabled, ptr_addr might be tagged.
> +        * Normally, this doesn't cause any issues, as both set_freepointer()
> +        * and get_freepointer() are called with a pointer with the same tag.
> +        * However, there are some issues with CONFIG_SLUB_DEBUG code. For
> +        * example, when __free_slub() iterates over objects in a cache, it
> +        * passes untagged pointers to check_object(). check_object() in turns
> +        * calls get_freepointer() with an untagged pointer, which causes the
> +        * freepointer to be restored incorrectly.
> +        */
> +       return (void *)((unsigned long)ptr ^ s->random ^
> +                       (unsigned long)kasan_reset_tag((void *)ptr_addr));
>  #else
>         return ptr;
>  #endif
> --
> 2.20.1.791.gb4d0f1c61a-goog
>