Re: [PATCH 09/41] mm: rcu safe VMA freeing

[Suren Baghdasaryan]

On Tue, Jan 17, 2023 at 6:25 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Mon 09-01-23 12:53:04, Suren Baghdasaryan wrote:
> [...]
> >  void vm_area_free(struct vm_area_struct *vma)
> >  {
> >       free_anon_vma_name(vma);
> > +#ifdef CONFIG_PER_VMA_LOCK
> > +     call_rcu(&vma->vm_rcu, __vm_area_free);
> > +#else
> >       kmem_cache_free(vm_area_cachep, vma);
> > +#endif
>
> Is it safe to have vma with already freed vma_name? I suspect this is
> safe because of mmap_lock but is there any reason to split the freeing
> process and have this potential UAF lurking?

It should be safe because VMA is either locked or has been isolated
while locked, so no page fault handlers should have access to it. But
you are right, moving free_anon_vma_name() into __vm_area_free() does
seem safer. Will make the change in the next rev.

>
> >  }
> >
> >  static void account_kernel_stack(struct task_struct *tsk, int account)
> > --
> > 2.39.0
>
> --
> Michal Hocko
> SUSE Labs