Re: [PATCH] mm: memcg/slab: fix racy access to page->mem_cgroup in mem_cgroup_from_obj()

[Shakeel Butt]

On Thu, Sep 10, 2020 at 3:43 PM Roman Gushchin <guro@xxxxxx> wrote:
>
> Forgot to cc stable@, an updated version is below.
>
> Thanks!
>
> --
>
> From fe61af45ae570b143ca783ba4d013a0a2b923a15 Mon Sep 17 00:00:00 2001
> From: Roman Gushchin <guro@xxxxxx>
> Date: Wed, 9 Sep 2020 12:19:37 -0700
> Subject: [PATCH] mm: memcg/slab: fix racy access to page->mem_cgroup in
>  mem_cgroup_from_obj()
>
> mem_cgroup_from_obj() checks the lowest bit of the page->mem_cgroup
> pointer to determine if the page has an attached obj_cgroup vector
> instead of a regular memcg pointer. If it's not set, it simple returns
> the page->mem_cgroup value as a struct mem_cgroup pointer.
>
> The commit 10befea91b61 ("mm: memcg/slab: use a single set of
> kmem_caches for all allocations") changed the moment when this bit
> is set: if previously it was set on the allocation of the slab page,
> now it can be set well after, when the first accounted object is
> allocated on this page.
>
> It opened a race: if page->mem_cgroup is set concurrently after the
> first page_has_obj_cgroups(page) check, a pointer to the obj_cgroups
> array can be returned as a memory cgroup pointer.
>
> A simple check for page->mem_cgroup pointer for NULL before the
> page_has_obj_cgroups() check fixes the race. Indeed, if the pointer
> is not NULL, it's either a simple mem_cgroup pointer or a pointer
> to obj_cgroup vector. The pointer can be asynchronously changed
> from NULL to (obj_cgroup_vec | 0x1UL), but can't be changed
> from a valid memcg pointer to objcg vector or back.
>
> If the object passed to mem_cgroup_from_obj() is a slab object
> and page->mem_cgroup is NULL, it means that the object is not
> accounted, so the function must return NULL.
>
> I've discovered the race looking at the code, so far I haven't seen it
> in the wild.
>
> Fixes: 10befea91b61 ("mm: memcg/slab: use a single set of kmem_caches for all allocations")
> Signed-off-by: Roman Gushchin <guro@xxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

I think this patch is good to have as it will make
mem_cgroup_from_obj() more future proof.

Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>