Re: [PATCH v3] mm/mempolicy: fix mpol_new leak in shared_policy_replace

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 26 Mar 2022 14:46:28 +0800 Miaohe Lin <linmiaohe@huawei.com> wrote:

> On 2022/3/26 8:29, Andrew Morton wrote:
> > On Tue, 22 Mar 2022 18:43:45 +0800 Miaohe Lin <linmiaohe@huawei.com> 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. 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
> >> are 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);
> >>   }
> >>
> >> ...
> >>
> >> --- a/mm/mempolicy.c
> >> +++ b/mm/mempolicy.c
> >> @@ -2733,6 +2733,7 @@ static int shared_policy_replace(struct shared_policy *sp, unsigned long start,
> >>  	mpol_new = kmem_cache_alloc(policy_cache, GFP_KERNEL);
> >>  	if (!mpol_new)
> >>  		goto err_out;
> >> +	refcount_set(&mpol_new->refcnt, 1);
> >>  	goto restart;
> >>  }
> > 
> > Two other sites in this file do
> > 
> > 	atomic_set(&policy->refcnt, 1);
> > 
> > 
> > Could we please instead have a little helper function which does the
> > kmem_cache_alloc()+refcount_set()?> .
> 
> There are usecases like below:
> 
> 	struct mempolicy *new = kmem_cache_alloc(policy_cache, GFP_KERNEL);
> 	*new = *old;
> 	^^^^^^^^^^^^
> 	refcount_set(&new->refcnt, 1);
> 
> If we use helper function to do kmem_cache_alloc()+refcount_set() above, separate
> refcount_set(&new->refcnt, 1) is still needed as old is copied to new and overwrites
> the refcnt field. So that little helper function might not work. Or am I miss something?
> 

Hm, spose so.  I guess the helper doesn't add much in that case.

Can we please redo this on mainline?  I'm not happy with the bloat
which refcnt_t adds and I think I'll drop
mm-mempolicy-convert-from-atomic_t-to-refcount_t-on-mempolicy-refcnt.patch.