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

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