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

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
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