[Patch v4 08/16] sched/cache: Fix potential NULL mm pointer access

[Tim Chen <tim.c.chen@linux.intel.com>]

From: Chen Yu <yu.c.chen@intel.com>

A concurrent task exit might cause a NULL pointer dereference
in account_mm_sched(). Use the locally cached mm pointer instead,
since the active_mm reference guarantees the structure remains
allocated. Meanwhile, skip the kernel thread because it has
nothing to do with cache aware scheduling.

This bug was reported by sashiko and Vern.

Fixes: df0d98475954 ("sched/cache: Introduce infrastructure for cache-aware load balancing")
Reported-by: Vern Hao <haoxing990@gmail.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Co-developed-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Link: https://lore.kernel.org/all/09cf7ee3-6e27-4505-9692-4b4a4707c8b2@gmail.com/
---
 kernel/sched/fair.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index be96d80c9310..913b09254732 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1649,7 +1649,7 @@ void account_mm_sched(struct rq *rq, struct task_struct *p, s64 delta_exec)
 	if (!mm || !mm->sc_stat.pcpu_sched)
 		return;
 
-	pcpu_sched = per_cpu_ptr(p->mm->sc_stat.pcpu_sched, cpu_of(rq));
+	pcpu_sched = per_cpu_ptr(mm->sc_stat.pcpu_sched, cpu_of(rq));
 
 	scoped_guard (raw_spinlock, &rq->cpu_epoch_lock) {
 		__update_mm_sched(rq, pcpu_sched);
@@ -1689,7 +1689,8 @@ static void task_tick_cache(struct rq *rq, struct task_struct *p)
 	if (!sched_cache_enabled())
 		return;
 
-	if (!mm || !mm->sc_stat.pcpu_sched)
+	if (!mm || p->flags & PF_KTHREAD ||
+	    !mm->sc_stat.pcpu_sched)
 		return;
 
 	epoch = rq->cpu_epoch;
-- 
2.32.0