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

[Michal Hocko <mhocko@suse.com>]

On Wed 16-03-22 14:39:37, Miaohe Lin wrote:
> On 2022/3/15 23:27, Michal Hocko wrote:
> > 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.
> > 
> 
> This is found via code investigation. I think this could be triggered if there
> are many concurrent mpol_set_shared_policy in place. But the user-visible effect
> might be obscure as only sizeof(struct mempolicy) bytes leaks possiblely every time.
> 
> > These details would be really interesting for the changelog so that we
> > can judge how important this would be.
> 
> This might not be that important as this issue should have been well-concealed for
> almost ten years (since commit 42288fe366c4 ("mm: mempolicy: Convert shared_policy mutex to spinlock")).

I think it is really worth to drill down to the bottom of the issue.
While theoretically possible can be a good enough to justify the change
it is usually preferred to describe the underlying problem for future
maintainability. 
-- 
Michal Hocko
SUSE Labs