[PATCH 2/2] memcg: Close the race between migration and installing bprm->mm as mm

[ebiederm@xmission.com (Eric W. Biederman)]

Oleg pointed out that there is a race at exec time between when
bprm->mm is initialized and the exec'ing task being migrated to a
different memory control group.

Ractor the code in memcontrol so exec_mmap can use the same code as as
fork to ensure that task->memcg == task->mm->memcg.

Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 fs/exec.c                  | 2 ++
 include/linux/memcontrol.h | 5 +++++
 mm/memcontrol.c            | 4 ++--
 3 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index a8be9318d1a8..32461a1543fc 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1044,6 +1044,8 @@ static int exec_mmap(struct mm_struct *mm)
 		return 0;
 	}
 	mmdrop(active_mm);
+	/* The tsk may have migrated before the new mm was attached */
+	mm_sync_memcg_from_task(tsk);
 	return 0;
 }
 
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 147e04bfcaee..9b68d9f2740e 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -402,6 +402,7 @@ static inline bool mem_cgroup_is_descendant(struct mem_cgroup *memcg,
 }
 
 void mm_update_memcg(struct mm_struct *mm, struct mem_cgroup *new);
+void mm_sync_memcg_from_task(struct task_struct *tsk);
 
 static inline bool mm_match_cgroup(struct mm_struct *mm,
 				   struct mem_cgroup *memcg)
@@ -786,6 +787,10 @@ static inline void mm_update_memcg(struct mm_struct *mm, struct mem_cgroup *new)
 {
 }
 
+static inline void mm_sync_memcg_from_task(struct task_struct *tsk)
+{
+}
+
 static inline bool mm_match_cgroup(struct mm_struct *mm,
 		struct mem_cgroup *memcg)
 {
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index d74aeba7dfed..552657613c0b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5063,7 +5063,7 @@ static void mem_cgroup_attach(struct cgroup_taskset *tset)
 	}
 }
 
-static void mem_cgroup_fork(struct task_struct *tsk)
+void mm_sync_memcg_from_task(struct task_struct *tsk)
 {
 	struct cgroup_subsys_state *css;
 
@@ -5377,7 +5377,7 @@ struct cgroup_subsys memory_cgrp_subsys = {
 	.attach = mem_cgroup_attach,
 	.cancel_attach = mem_cgroup_cancel_attach,
 	.post_attach = mem_cgroup_move_task,
-	.fork = mem_cgroup_fork,
+	.fork = mm_sync_memcg_from_task,
 	.bind = mem_cgroup_bind,
 	.dfl_cftypes = memory_files,
 	.legacy_cftypes = mem_cgroup_legacy_files,
-- 
2.14.1