[PATCH v3 2/4] sched/debug: Protect lockless rq->rd access in print_dl_rq()

In print_dl_rq(), cpu_rq(cpu)->rd is dereferenced locklessly to display
deadline bandwidth statistics.

During CPU hot-unplug or cgroup cpuset repartitioning events,
partition_sched_domains() calls cpu_attach_domain(), which executes
rq_attach_root() to detach the CPU from its root_domain. When the
reference count of the detached root_domain drops to zero,
rq_attach_root() calls call_rcu(&old_rd->rcu, free_rootdomain) to
schedule memory teardown after an RCU grace period.

Because print_dl_rq() does not hold an RCU read lock while dereferencing
cpu_rq(cpu)->rd, an RCU grace period can elapse concurrently while
debugfs is reading the file. This allows free_rootdomain() to execute
kfree(old_rd), introducing a use-after-free race condition when
print_dl_rq() reads dl_bw->bw.

Fix this by fetching rq->rd using READ_ONCE() inside an RCU read-side
critical section. Holding the RCU read lock guarantees that the struct
root_domain memory remains valid while being accessed.

Fixes: 02968ccf7b80 ("sched: add /proc/sched_debug file")
Reported-by: sashiko-bot <[email protected]>
Signed-off-by: Aaron Tomlin <[email protected]>
---
 kernel/sched/debug.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 78fc02d71710..e9d40a660346 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -1081,6 +1081,7 @@ void print_rt_rq(struct seq_file *m, int cpu, struct rt_rq *rt_rq)
 void print_dl_rq(struct seq_file *m, int cpu, struct dl_rq *dl_rq)
 {
 	struct dl_bw *dl_bw;
+	struct root_domain *rd;
 
 	SEQ_printf(m, "\n");
 	SEQ_printf(m, "dl_rq[%d]:\n", cpu);
@@ -1089,9 +1090,14 @@ void print_dl_rq(struct seq_file *m, int cpu, struct dl_rq *dl_rq)
 	SEQ_printf(m, "  .%-30s: %lu\n", #x, (unsigned long)(dl_rq->x))
 
 	PU(dl_nr_running);
-	dl_bw = &cpu_rq(cpu)->rd->dl_bw;
-	SEQ_printf(m, "  .%-30s: %lld\n", "dl_bw->bw", dl_bw->bw);
-	SEQ_printf(m, "  .%-30s: %lld\n", "dl_bw->total_bw", dl_bw->total_bw);
+	rcu_read_lock();
+	rd = READ_ONCE(cpu_rq(cpu)->rd);
+	if (rd) {
+		dl_bw = &rd->dl_bw;
+		SEQ_printf(m, "  .%-30s: %lld\n", "dl_bw->bw", dl_bw->bw);
+		SEQ_printf(m, "  .%-30s: %lld\n", "dl_bw->total_bw", dl_bw->total_bw);
+	}
+	rcu_read_unlock();
 
 #undef PU
 }
-- 
2.55.0