Re: [patch 1/2] mm: memcontrol: fix lockless reclaim hierarchyiterator

[Michal Hocko]

On Wed 05-06-13 18:53:45, Johannes Weiner wrote:
> The lockless reclaim hierarchy iterator currently has a misplaced
> barrier that can lead to use-after-free crashes.
> 
> The reclaim hierarchy iterator consist of a sequence count and a
> position pointer that are read and written locklessly, with memory
> barriers enforcing ordering.
> 
> The write side sets the position pointer first, then updates the
> sequence count to "publish" the new position.  Likewise, the read side
> must read the sequence count first, then the position.  If the
> sequence count is up to date, it's guaranteed that the position is up
> to date as well:
> 
>   writer:                         reader:
>   iter->position = position       if iter->sequence == expected:
>   smp_wmb()                           smp_rmb()
>   iter->sequence = sequence           position = iter->position
> 
> However, the read side barrier is currently misplaced, which can lead
> to dereferencing stale position pointers that no longer point to valid
> memory.  Fix this.
> 
> Reported-by: Tejun Heo <tj@xxxxxxxxxx>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: stable@xxxxxxxxxx [3.10+]

Acked-by: Michal Hocko <mhocko@xxxxxxx>

> ---
>  mm/memcontrol.c | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 010d6c1..e2cbb44 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1199,7 +1199,6 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
>  
>  			mz = mem_cgroup_zoneinfo(root, nid, zid);
>  			iter = &mz->reclaim_iter[reclaim->priority];
> -			last_visited = iter->last_visited;
>  			if (prev && reclaim->generation != iter->generation) {
>  				iter->last_visited = NULL;
>  				goto out_unlock;
> @@ -1218,13 +1217,12 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
>  			 * is alive.
>  			 */
>  			dead_count = atomic_read(&root->dead_count);
> -			smp_rmb();
> -			last_visited = iter->last_visited;
> -			if (last_visited) {
> -				if ((dead_count != iter->last_dead_count) ||
> -					!css_tryget(&last_visited->css)) {
> +			if (dead_count == iter->last_dead_count) {
> +				smp_rmb();
> +				last_visited = iter->last_visited;
> +				if (last_visited &&
> +				    !css_tryget(&last_visited->css))
>  					last_visited = NULL;
> -				}
>  			}
>  		}
>  
> -- 
> 1.8.3
> 

-- 
Michal Hocko
SUSE Labs
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