Re: [PATCH] kasan: slab: always reset the tag in get_freepointer_safe()

[Alexander Potapenko]

On Fri, May 14, 2021 at 9:22 AM Alexander Potapenko <glider@xxxxxxxxxx> wrote:
>
> With CONFIG_DEBUG_PAGEALLOC enabled, the kernel should also untag the
> object pointer, as done in get_freepointer().
>
> Failing to do so reportedly leads to SLUB freelist corruptions that
> manifest as boot-time crashes.
>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Marco Elver <elver@xxxxxxxxxx>
> Cc: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> Cc: Andrey Konovalov <andreyknvl@xxxxxxxxx>
> Cc: Elliot Berman <eberman@xxxxxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
> ---
>  mm/slub.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index feda53ae62ba..9a4f59e5b0c2 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -301,6 +301,7 @@ static inline void *get_freepointer_safe(struct kmem_cache *s, void *object)
>         if (!debug_pagealloc_enabled_static())
>                 return get_freepointer(s, object);
>
> +       object = kasan_reset_tag(object);
>         freepointer_addr = (unsigned long)object + s->offset;
>         copy_from_kernel_nofault(&p, (void **)freepointer_addr, sizeof(p));
>         return freelist_ptr(s, p, freepointer_addr);
> --
> 2.31.1.751.gd2f1c929bd-goog
>


-- 
Alexander Potapenko
Software Engineer

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