Re: [Bugme-new] [Bug 33502] New: Caught 64-bit read from uninitialized memory in __alloc_skb

Eric Dumazet
2011-05-10T18:05:58Z

Le mardi 10 mai 2011 à 12:43 -0500, Christoph Lameter a écrit :
> Draft for a patch
>
>
> Subject: slub: Make CONFIG_PAGE_ALLOC work with new fastpath
>
> Fastpath can do a speculative access to a page that CONFIG_PAGE_ALLOC may have
> marked as invalid to retrieve the pointer to the next free object.
>
> Probe that address before dereferencing the pointer to the page.
> All of that needs to occur with interrupts disabled since an interrupt
> could cause the page status to change (as pointed out by Eric).
>
> Signed-off-by: Christoph Lameter <cl@linux.com>
> ---
> mm/slub.c | 23 ++++++++++++++++++++++-
> 1 file changed, 22 insertions(+), 1 deletion(-)
>
> Index: linux-2.6/mm/slub.c
> ===================================================================
> --- linux-2.6.orig/mm/slub.c 2011-05-10 12:35:30.000000000 -0500
> +++ linux-2.6/mm/slub.c 2011-05-10 12:38:53.000000000 -0500
> @@ -261,6 +261,27 @@ static inline void *get_freepointer(stru
> return *(void **)(object + s->offset);
> }
>
> +static inline void *get_freepointer_safe(struct kmem_cache *s, void *object)
> +{
> + void *p;
> +
> +#ifdef CONFIG_PAGE_ALLOC
> + unsigned long flags;
> +
> + local_irq_save(flags);
> +
> + if (probe_kernel_address(object))
> + p = NULL; /* Invalid */
> + else
> + p = get_freepointer(s, object);
> +
> + local_irq_restore(flags);
> +#else
> + p = get_freepointer(s, object);
> +#endif
> + return p;
> +}
> +
> static inline void set_freepointer(struct kmem_cache *s, void *object, void *fp)
> {
> *(void **)(object + s->offset) = fp;
> @@ -1933,7 +1954,7 @@ redo:
> if (unlikely(!irqsafe_cpu_cmpxchg_double(
> s->cpu_slab->freelist, s->cpu_slab->tid,
> object, tid,
> - get_freepointer(s, object), next_tid(tid)))) {
> + get_freepointer_safe(s, object), next_tid(tid)))) {
>
> note_cmpxchg_failure("slab_alloc", s, tid);
> goto redo;

Really this wont work Stephen

You have to disable IRQ _before_ even fetching 'object'

Or else, you can have an IRQ, allocate this object, pass to another cpu.

This other cpu can free the object and unmap page right after you did
the probe_kernel_address(object) (successfully), and before your cpu :

p = get_freepointer(s, object); << BUG >>

I really dont understand your motivation to keep the buggy commit.