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.
> 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);
> }
>
> Fixes: 42288fe366c4 ("mm: mempolicy: Convert shared_policy mutex to spinlock")
> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # 3.8
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Thanks a lot!
> ---
> v1->v2:
> Add reproducer snippet and Cc stable.
> Thanks Michal Hocko for review and comment!
> ---
> mm/mempolicy.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index a2516d31db6c..4cdd425b2752 100644
> --- 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;
> }
>
> --
> 2.23.0
--
Michal Hocko
SUSE Labs
