Re: [PATCH] mm/mempolicy: fix potential mpol_new leak in shared_policy_replace

[Michal Hocko <mhocko@suse.com>]

On Tue 15-03-22 21:42:29, Miaohe Lin wrote:
> On 2022/3/15 0:44, Michal Hocko wrote:
> > On Fri 11-03-22 17:36:24, Miaohe Lin wrote:
> >> If mpol_new is allocated but not used in restart loop, mpol_new will be
> >> freed via mpol_put before returning to the caller. But refcnt is not
> >> initialized yet, so mpol_put could not do the right things and might
> >> leak the unused mpol_new.
> > 
> > The code is really hideous but is there really any bug there? AFAICS the
> > new policy is only allocated in if (n->end > end) branch and that one
> > will set the reference count on the retry. Or am I missing something?
> > 
> 
> Many thanks for your comment.
> IIUC, new policy is allocated via the below code:
> 
> shared_policy_replace:
> 	alloc_new:
> 		write_unlock(&sp->lock);
> 		ret = -ENOMEM;
> 		n_new = kmem_cache_alloc(sn_cache, GFP_KERNEL);
> 		if (!n_new)
> 			goto err_out;
> 		mpol_new = kmem_cache_alloc(policy_cache, GFP_KERNEL);
> 		if (!mpol_new)
> 			goto err_out;
> 		goto restart;
> 
> And mpol_new' reference count will be set before used in n->end > end case. But
> if that is "not" the case, i.e. mpol_new is not inserted into the rb_tree, mpol_new
> will be freed via mpol_put before return:

One thing I have missed previously is that the lock is dropped during
the allocation so I guess the memory policy could have been changed
during that time. Is this possible? Have you explored this possibility?
Is this a theoretical problem or it can be triggered intentionally.

These details would be really interesting for the changelog so that we
can judge how important this would be.
-- 
Michal Hocko
SUSE Labs