Re: [PATCH] mm/slab_common.c: fix possible spectre-v1 in kmalloc_slab()

Dianzhang Chen
2019-05-29T16:40:08Z

It's come from `192+1`.

The more code fragment is:

if (size <= 192) {

if (!size)

return ZERO_SIZE_PTR;

size = array_index_nospec(size, 193);

index = size_index[size_index_elem(size)];

}

Sine array_index_nospec(index, size) can clamp the index within the
range of [0, size), so in order to make the `size<=192`, need to clamp
the index in the range of [0, 192+1) .

On Thu, May 30, 2019 at 12:25 AM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Wed 29-05-19 20:37:28, Dianzhang Chen wrote:
> [...]
> > @@ -1056,6 +1057,7 @@ struct kmem_cache *kmalloc_slab(size_t size, gfp_t flags)
> > if (!size)
> > return ZERO_SIZE_PTR;
> >
> > + size = array_index_nospec(size, 193);
> > index = size_index[size_index_elem(size)];
>
> What is this 193 magic number?
> --
> Michal Hocko
> SUSE Labs