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SubjectRe: [PATCH v4] mm: Add SLUB free list pointer obfuscation
On Tue, 25 Jul 2017, Kees Cook wrote:

> +/*
> + * Returns freelist pointer (ptr). With hardening, this is obfuscated
> + * with an XOR of the address where the pointer is held and a per-cache
> + * random number.
> + */
> +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);
> +#else
> + return ptr;
> +#endif
> +}

Weird function. Why pass both the pointer as well as the address of the
pointer? The address of the pointer would be sufficient I think. Compiler
can optimize the refs on its own. OK ptr_addr is really the obfuscation
value. Maybe a bit confusing to call this ptr_addr and also pass this as
a long. xor_value? If it is a pointer address the it should be void ** or
so.

> static inline void *get_freepointer_safe(struct kmem_cache *s, void *object)
> {
> + unsigned long freepointer_addr;
> void *p;
>
> if (!debug_pagealloc_enabled())
> return get_freepointer(s, object);
>
> - probe_kernel_read(&p, (void **)(object + s->offset), sizeof(p));
> - return p;
> + freepointer_addr = (unsigned long)object + s->offset;

converts the void ** to unsigned long.... which requires another cast in
the following line.

> + probe_kernel_read(&p, (void **)freepointer_addr, sizeof(p));
> + return freelist_ptr(s, p, freepointer_addr);
> }
>
> static inline void set_freepointer(struct kmem_cache *s, void *object, void *fp)
> {
> - *(void **)(object + s->offset) = fp;
> + unsigned long freeptr_addr = (unsigned long)object + s->offset;
> +
> + *(void **)freeptr_addr = freelist_ptr(s, fp, freeptr_addr);
> }
>
> /* Loop over all objects in a slab */
> @@ -3563,6 +3592,9 @@ static int kmem_cache_open(struct kmem_cache *s, unsigned long flags)
> {
> s->flags = kmem_cache_flags(s->size, flags, s->name, s->ctor);
> s->reserved = 0;
> +#ifdef CONFIG_SLAB_FREELIST_HARDENED
> + s->random = get_random_long();
> +#endif
>
> if (need_reserve_slab_rcu && (s->flags & SLAB_TYPESAFE_BY_RCU))
> s->reserved = sizeof(struct rcu_head);
>

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