[Devel] [PATCH vz10] memcg: guard mm_overdraft() against NULL memcg (cgroup_disable=memory)

[Konstantin Khorenko]

get_mem_cgroup_from_mm() returns NULL when mem_cgroup_disabled() is true
(e.g. the kernel was booted with "cgroup_disable=memory"). mm_overdraft()
dereferenced the result unconditionally:

	memcg = get_mem_cgroup_from_mm(mm);
	overdraft = memcg->overdraft;     /* NULL deref */
	css_put(&memcg->css);

It is called from oom_badness() (*overdraft = mm_overdraft(p->mm)) on
every OOM scoring pass, so any OOM kill on a memory-cgroup-disabled
kernel NULL-derefs. Return 0 (no overdraft) when memcg is NULL.

Fixes: 36eac5599341 ("memcg: add oom_guarantee")
https://virtuozzo.atlassian.net/browse/VSTOR-132310
Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>
---
 include/linux/memcontrol.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 23ab42e5978e..bf2440e36941 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -907,6 +907,8 @@ static inline unsigned long mm_overdraft(struct mm_struct *mm)
 	unsigned long overdraft;
 
 	memcg = get_mem_cgroup_from_mm(mm);
+	if (!memcg)
+		return 0;
 	overdraft = memcg->overdraft;
 	css_put(&memcg->css);
 
-- 
2.47.1