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SubjectRe: [PATCH] memcg: do not expose uninitialized mem_cgroup_per_node to world
On Wed 01-06-11 12:44:04, Igor Mammedov wrote:
> Freshly allocated 'mem_cgroup_per_node' list entries must be
> initialized before the rest of the kernel can see them. Otherwise
> zero initialized list fields can lead to race condition at
> mem_cgroup_force_empty_list:
> pc = list_entry(list->prev, struct page_cgroup, lru);
> where 'pc' will be something like 0xfffffffc if list->prev is 0
> and cause page fault later when 'pc' is dereferenced.

Have you ever seen such a race? I do not see how this could happen.
mem_cgroup_force_empty_list is called only from
mem_cgroup_force_empty_write (aka echo whatever > group/force_empty)
or mem_cgroup_pre_destroy when the group is destroyed.

The initialization code is, however, called before a group is
given for use AFICS.

I am not saying tha the change is bad, I like it, but I do not think it
is a fix for potential race condition.

>
> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> ---
> mm/memcontrol.c | 2 +-
> 1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index bd9052a..ee7cb4c 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4707,7 +4707,6 @@ static int alloc_mem_cgroup_per_zone_info(struct mem_cgroup *mem, int node)
> if (!pn)
> return 1;
>
> - mem->info.nodeinfo[node] = pn;
> for (zone = 0; zone < MAX_NR_ZONES; zone++) {
> mz = &pn->zoneinfo[zone];
> for_each_lru(l)
> @@ -4716,6 +4715,7 @@ static int alloc_mem_cgroup_per_zone_info(struct mem_cgroup *mem, int node)
> mz->on_tree = false;
> mz->mem = mem;
> }
> + mem->info.nodeinfo[node] = pn;
> return 0;
> }
>
> --
> 1.7.1
>
> --
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--
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9
Czech Republic


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