Re: [PATCH] mm/z3fold: Fix z3fold_buddy_slots use after free

[Henry Burns]

On Tue, Jul 2, 2019 at 12:45 AM Vitaly Wool <vitalywool@xxxxxxxxx> wrote:
>
> Hi Henry,
>
> On Mon, Jul 1, 2019 at 8:31 PM Henry Burns <henryburns@xxxxxxxxxx> wrote:
> >
> > Running z3fold stress testing with address sanitization
> > showed zhdr->slots was being used after it was freed.
> >
> > z3fold_free(z3fold_pool, handle)
> >   free_handle(handle)
> >     kmem_cache_free(pool->c_handle, zhdr->slots)
> >   release_z3fold_page_locked_list(kref)
> >     __release_z3fold_page(zhdr, true)
> >       zhdr_to_pool(zhdr)
> >         slots_to_pool(zhdr->slots)  *BOOM*
>
> Thanks for looking into this. I'm not entirely sure I'm all for
> splitting free_handle() but let me think about it.
>
> > Instead we split free_handle into two functions, release_handle()
> > and free_slots(). We use release_handle() in place of free_handle(),
> > and use free_slots() to call kmem_cache_free() after
> > __release_z3fold_page() is done.
>
> A little less intrusive solution would be to move backlink to pool
> from slots back to z3fold_header. Looks like it was a bad idea from
> the start.
>
> Best regards,
>    Vitaly

We still want z3fold pages to be movable though. Wouldn't moving
the backink to the pool from slots to z3fold_header prevent us from
enabling migration?