Re: [PATCH] kasan, slub: fix more conflicts with CONFIG_SLAB_FREELIST_HARDENED

[Andrey Konovalov <andreyknvl@google.com>]

On Thu, Feb 14, 2019 at 1:25 AM Andrey Konovalov <andreyknvl@google.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@google.com>

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