[PATCH v4 3/5] sched/debug: Protect lockless p->mm access in sched_show_numa()

[Aaron Tomlin]

In sched_show_numa(), p->mm is checked locklessly and then passed to the
P(mm->numa_scan_seq) macro. This results in a double-evaluation of p->mm.

If the task exits concurrently via exit_mm(p) between the check and the
macro expansion, p->mm can be set to NULL on another CPU. The second
evaluation then dereferences a NULL pointer.

Fix this TOCTOU race by reading p->mm once into a local variable using
READ_ONCE(p->mm) before checking and dereferencing its numa_scan_seq
field.

Fixes: b32e86b4301e ("sched/numa: Add debugging")
Reported-by: sashiko-bot <sashiko-bot@xxxxxxxxxx>
Signed-off-by: Aaron Tomlin <atomlin@xxxxxxxxxxx>
---
 kernel/sched/debug.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index c3409166a288..f5494e02f600 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -1317,8 +1317,10 @@ void print_numa_stats(struct seq_file *m, int node, unsigned long tsf,
 static void sched_show_numa(struct task_struct *p, struct seq_file *m)
 {
 #ifdef CONFIG_NUMA_BALANCING
-	if (p->mm)
-		P(mm->numa_scan_seq);
+	struct mm_struct *mm = READ_ONCE(p->mm);
+
+	if (mm)
+		__PS("mm->numa_scan_seq", mm->numa_scan_seq);
 
 	P(numa_pages_migrated);
 	P(numa_preferred_nid);
-- 
2.55.0