| Message ID | 20200910022435.2773735-1-guro@fb.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | mm: memcg/slab: fix racy access to page->mem_cgroup in mem_cgroup_from_obj() | expand |
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 --- 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>
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
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> --- mm/memcontrol.c | 11 +++++++++++ 1 file changed, 11 insertions(+)