[PATCH] kernel/sched: Fix sched_fork() access an invalid sched_task_group

[Zhang Qiao]

There is a small race between copy_process() and sched_fork()
where child->sched_task_group point to an already freed pointer.

parent doing fork()      | someone moving the parent
				to another cgroup
-------------------------------+-------------------------------
copy_process()
    + dup_task_struct()<1>
				parent move to another cgroup,
				and free the old cgroup. <2>
    + sched_fork()
      + __set_task_cpu()<3>
      + task_fork_fair()
        + sched_slice()<4>

In the worst case, this bug can lead to "use-after-free" and
cause panic as shown above,
(1)parent copy its sched_task_group to child at <1>;
(2)someone move the parent to another cgroup and free the old
   cgroup at <2>;
(3)the sched_task_group and cfs_rq that belong to the old cgroup
   will be accessed at <3> and <4>, which cause a panic:

[89249.732198] BUG: unable to handle kernel NULL pointer
dereference at 0000000000000000
[89249.732701] PGD 8000001fa0a86067 P4D 8000001fa0a86067 PUD
2029955067 PMD 0
[89249.733005] Oops: 0000 [#1] SMP PTI
[89249.733288] CPU: 7 PID: 648398 Comm: ebizzy Kdump: loaded
Tainted: G           OE    --------- -  - 4.18.0.x86_64+ #1
[89249.734318] RIP: 0010:sched_slice+0x84/0xc0
 ....
[89249.737910] Call Trace:
[89249.738181]  task_fork_fair+0x81/0x120
[89249.738457]  sched_fork+0x132/0x240
[89249.738732]  copy_process.part.5+0x675/0x20e0
[89249.739010]  ? __handle_mm_fault+0x63f/0x690
[89249.739286]  _do_fork+0xcd/0x3b0
[89249.739558]  do_syscall_64+0x5d/0x1d0
[89249.739830]  entry_SYSCALL_64_after_hwframe+0x65/0xca
[89249.740107] RIP: 0033:0x7f04418cd7e1

When a new process is forked, cgroup_post_fork() associates it
with the cgroup of its parent. Therefore this commit move the
__set_task_cpu() and task_fork() that access some cgroup-related
fields(sched_task_group and cfs_rq) to sched_post_fork() and
call sched_post_fork() after cgroup_post_fork().

Fixes: 8323f26ce342 ("sched: Fix race in task_group")
Signed-off-by: Zhang Qiao <zhangqiao22@xxxxxxxxxx>
---
 kernel/fork.c       |  2 +-
 kernel/sched/core.c | 33 +++++++++++++--------------------
 2 files changed, 14 insertions(+), 21 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index bc94b2cc5995..a4acd87f00f9 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2330,8 +2330,8 @@ static __latent_entropy struct task_struct *copy_process(
 	write_unlock_irq(&tasklist_lock);
 
 	proc_fork_connector(p);
-	sched_post_fork(p);
 	cgroup_post_fork(p, args);
+	sched_post_fork(p);
 	perf_event_fork(p);
 
 	trace_task_newtask(p, clone_flags);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 20ffcc044134..041f21e8b266 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4069,8 +4069,6 @@ int sysctl_schedstats(struct ctl_table *table, int write, void *buffer,
  */
 int sched_fork(unsigned long clone_flags, struct task_struct *p)
 {
-	unsigned long flags;
-
 	__sched_fork(clone_flags, p);
 	/*
 	 * We mark the process as NEW here. This guarantees that
@@ -4116,24 +4114,6 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
 
 	init_entity_runnable_average(&p->se);
 
-	/*
-	 * The child is not yet in the pid-hash so no cgroup attach races,
-	 * and the cgroup is pinned to this child due to cgroup_fork()
-	 * is ran before sched_fork().
-	 *
-	 * Silence PROVE_RCU.
-	 */
-	raw_spin_lock_irqsave(&p->pi_lock, flags);
-	rseq_migrate(p);
-	/*
-	 * We're setting the CPU for the first time, we don't migrate,
-	 * so use __set_task_cpu().
-	 */
-	__set_task_cpu(p, smp_processor_id());
-	if (p->sched_class->task_fork)
-		p->sched_class->task_fork(p);
-	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
-
 #ifdef CONFIG_SCHED_INFO
 	if (likely(sched_info_on()))
 		memset(&p->sched_info, 0, sizeof(p->sched_info));
@@ -4151,6 +4131,19 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
 
 void sched_post_fork(struct task_struct *p)
 {
+	unsigned long flags;
+
+	raw_spin_lock_irqsave(&p->pi_lock, flags);
+	rseq_migrate(p);
+	/*
+	 * We're setting the CPU for the first time, we don't migrate,
+	 * so use __set_task_cpu().
+	 */
+	__set_task_cpu(p, smp_processor_id());
+	if (p->sched_class->task_fork)
+		p->sched_class->task_fork(p);
+	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+
 	uclamp_post_fork(p);
 }
 
-- 
2.17.1