linux-kernel - Re: [PATCH v2] mm/mempolicy: fix mpol_new leak in shared_policy

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Message-ID: <YjmD6BR0GiYF/j+X@dhcp22.suse.cz>
Date:   Tue, 22 Mar 2022 09:08:08 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, kosaki.motohiro@...fujitsu.com,
        mgorman@...e.de, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/mempolicy: fix mpol_new leak in
 shared_policy_replace

On Tue 22-03-22 09:50:35, Miaohe Lin wrote:
> On 2022/3/21 20:12, Michal Hocko wrote:
> > On Tue 22-03-22 16:34:56, 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.
> > 
> > I would just add:
> > 
> > This would happen if mempolicy was updated on the shared shmem file
> > while the sp->lock has been dropped during the memory allocation.
> > 
> 
> Do you mean the below commit log?
> 
> """
> 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. This would happen if mempolicy was updated on the
> shared shmem file while the sp->lock has been dropped during the memory
> allocation.
> 
> This issue could be triggered easily with the below code snippet if
> there're many processes doing the below work at the same time:
> 
>   shmid = shmget((key_t)5566, 1024 * PAGE_SIZE, 0666|IPC_CREAT);
>   shm = shmat(shmid, 0, 0);
>   loop many times {
>     mbind(shm, 1024 * PAGE_SIZE, MPOL_LOCAL, mask, maxnode, 0);
>     mbind(shm + 128 * PAGE_SIZE, 128 * PAGE_SIZE, MPOL_DEFAULT, mask,
>           maxnode, 0);
>   }
> """

Yes, LGTM.
Thanks!
-- 
Michal Hocko
SUSE Labs