[RFC PATCH for tip] sched: Fix concurrency ID handling of usermodehelper kthreads

[Mathieu Desnoyers]

sched_mm_cid_after_execve does not expect NULL t->mm, but it may happen
if a usermodehelper kthread fails when attempting to execute a binary.

sched_mm_cid_fork can be issued from a usermodehelper kthread, which has
t->flags PF_KTHREAD set.

Fixes: af7f588d8f73 ("sched: Introduce per-memory-map concurrency ID")
Reported-by: kernel test robot <yujie.liu@xxxxxxxxx>
Link: https://lore.kernel.org/oe-lkp/202212301353.5c959d72-yujie.liu@xxxxxxxxx
Reported-by: Borislav Petkov <bp@xxxxxxxxx>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
---
 kernel/sched/core.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 048ec2417990..f99ee69867e3 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -11343,8 +11343,8 @@ void sched_mm_cid_after_execve(struct task_struct *t)
 	struct mm_struct *mm = t->mm;
 	unsigned long flags;
 
-	WARN_ON_ONCE((t->flags & PF_KTHREAD) || !t->mm);
-
+	if (!mm)
+		return;
 	local_irq_save(flags);
 	t->mm_cid = mm_cid_get(mm);
 	t->mm_cid_active = 1;
@@ -11354,7 +11354,7 @@ void sched_mm_cid_after_execve(struct task_struct *t)
 
 void sched_mm_cid_fork(struct task_struct *t)
 {
-	WARN_ON_ONCE((t->flags & PF_KTHREAD) || !t->mm || t->mm_cid != -1);
+	WARN_ON_ONCE(!t->mm || t->mm_cid != -1);
 	t->mm_cid_active = 1;
 }
 #endif
-- 
2.25.1