[PATCH v6 2/6] sched/debug: Protect lockless rq->rd access in print_dl_rq()

[Aaron Tomlin <atomlin@atomlin.com>]

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.

However, rq_attach_root() previously updated rq->rd using a plain C store
without an RCU publication barrier (i.e., rcu_assign_pointer()). Without a
release memory barrier on the writer side, CPU or compiler reordering could
allow the new rq->rd pointer store to become visible to other CPUs before
the initialization writes to rd->dl_bw are committed.

Furthermore, because print_dl_rq() did not hold an RCU read lock while
dereferencing cpu_rq(cpu)->rd, an RCU grace period could elapse
concurrently while debugfs is reading the file, allowing
free_rootdomain() to execute kfree(old_rd) and causing a use-after-free
race condition when print_dl_rq() reads dl_bw->bw.

Resolve this by using rcu_assign_pointer(rq->rd, rd) in rq_attach_root() to
guarantee a release memory barrier when publishing a root_domain.
Finally, fetch rq->rd using guard(rcu)() and rcu_dereference() in print_dl_rq().

Fixes: 02968ccf7b80 ("sched: add /proc/sched_debug file")
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
---
 kernel/sched/debug.c    | 3 ++-
 kernel/sched/topology.c | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 72236db67983..b8be86bf8849 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -1172,7 +1172,8 @@ 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;
+	guard(rcu)();
+	dl_bw = &rcu_dereference(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);
 
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 21e816ad23ee..40c3bcf82a68 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -496,7 +496,7 @@ void rq_attach_root(struct rq *rq, struct root_domain *rd)
 	}
 
 	atomic_inc(&rd->refcount);
-	rq->rd = rd;
+	rcu_assign_pointer(rq->rd, rd);
 
 	cpumask_set_cpu(rq->cpu, rd->span);
 	if (cpumask_test_cpu(rq->cpu, cpu_active_mask))
-- 
2.55.0