Re: [PATCH] mm: memcg/slab: fix racy access to page->mem_cgroup in mem_cgroup_from_obj()
Forgot to cc stable@, an updated version is below.
Thanks!
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From fe61af45ae570b143ca783ba4d013a0a2b923a15 Mon Sep 17 00:00:00 2001
From: Roman Gushchin guro@fb.com 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@fb.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Vlastimil Babka vbabka@suse.cz Cc: Shakeel Butt shakeelb@google.com Cc: stable@vger.kernel.org --- mm/memcontrol.c | 11 +++++++++++ 1 file changed, 11 insertions(+)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 75cd1a1e66c8..093526fec4bf 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -2923,6 +2923,17 @@ struct mem_cgroup *mem_cgroup_from_obj(void *p)
page = virt_to_head_page(p);
+ /* + * If page->mem_cgroup is set, it's either a simple mem_cgroup pointer + * or a pointer to obj_cgroup vector. In the latter case the lowest + * bit of the pointer is set. + * The page->mem_cgroup 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 (!page->mem_cgroup) + return NULL; + /* * Slab objects are accounted individually, not per-page. * Memcg membership data for each individual object is saved in
On Thu, Sep 10, 2020 at 3:43 PM Roman Gushchin guro@fb.com wrote:
Forgot to cc stable@, an updated version is below.
Thanks!
--
From fe61af45ae570b143ca783ba4d013a0a2b923a15 Mon Sep 17 00:00:00 2001 From: Roman Gushchin guro@fb.com 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@fb.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Vlastimil Babka vbabka@suse.cz Cc: Shakeel Butt shakeelb@google.com Cc: stable@vger.kernel.org
Is the caller list_lru_from_kmem() the concern or is this more about making mem_cgroup_from_obj() more future proof?
Also have you taken a look at [1]? I am still trying to figure out how that is possible.
[1] https://lore.kernel.org/lkml/20200901075321.GL4299@shao2-debian/
On Fri, Sep 11, 2020 at 09:21:49AM -0700, Shakeel Butt wrote:
On Thu, Sep 10, 2020 at 3:43 PM Roman Gushchin guro@fb.com wrote:
Forgot to cc stable@, an updated version is below.
Thanks!
--
From fe61af45ae570b143ca783ba4d013a0a2b923a15 Mon Sep 17 00:00:00 2001 From: Roman Gushchin guro@fb.com 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@fb.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Vlastimil Babka vbabka@suse.cz Cc: Shakeel Butt shakeelb@google.com Cc: stable@vger.kernel.org
Is the caller list_lru_from_kmem() the concern or is this more about making mem_cgroup_from_obj() more future proof?
I was doing some refactorings around (see https://lore.kernel.org/linux-mm/20200910202659.1378404-1-guro@fb.com/T/#t ), and just noticed it from looking at the code. I'm not aware of any real life consequences at the moment.
Also have you taken a look at [1]? I am still trying to figure out how that is possible.
[1] https://lore.kernel.org/lkml/20200901075321.GL4299@shao2-debian/
Hm, yeah, it's complicated. At the very first glance it looks like that the obj_cgroups vector is placed onto the very same page it describes, or at least it shares the kmem_cache with it, with some bad consequences. Could be something SLAB-specific, newer saw anything like that with SLUB. Or maybe it's completely unrelated and has been attributed to this commit by mistake.
I've spent several hours running the provided test in a loop, but wasn't lucky enough to trigger it. Did you try?
Thanks!
Roman
On Fri, Sep 11, 2020 at 2:34 PM Roman Gushchin guro@fb.com wrote:
[snip]
Also have you taken a look at [1]? I am still trying to figure out how that is possible.
[1] https://lore.kernel.org/lkml/20200901075321.GL4299@shao2-debian/
Hm, yeah, it's complicated. At the very first glance it looks like that the obj_cgroups vector is placed onto the very same page it describes, or at least it shares the kmem_cache with it, with some bad consequences. Could be something SLAB-specific, newer saw anything like that with SLUB. Or maybe it's completely unrelated and has been attributed to this commit by mistake.
I've spent several hours running the provided test in a loop, but wasn't lucky enough to trigger it. Did you try?
Yeah same, no success in reproducing it.
On Thu, Sep 10, 2020 at 3:43 PM Roman Gushchin guro@fb.com wrote:
Forgot to cc stable@, an updated version is below.
Thanks!
--
From fe61af45ae570b143ca783ba4d013a0a2b923a15 Mon Sep 17 00:00:00 2001 From: Roman Gushchin guro@fb.com 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@fb.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Vlastimil Babka vbabka@suse.cz Cc: Shakeel Butt shakeelb@google.com Cc: stable@vger.kernel.org
I think this patch is good to have as it will make mem_cgroup_from_obj() more future proof.
Reviewed-by: Shakeel Butt shakeelb@google.com
linux-stable-mirror@lists.linaro.org
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Roman Gushchin -
Shakeel Butt