[PATCH v2] sched/fair: Fix invalid pointer dereference in child_cfs_rq_on_list()

[Aboorva Devarajan <aboorvad@linux.ibm.com>]

In child_cfs_rq_on_list(), leaf_cfs_rq_list.prev is expected to point to
a valid cfs_rq->leaf_cfs_rq_list in the hierarchy. However, in some
cases leaf_cfs_rq_list.prev and rq->tmp_alone_branch can point to the
list head (rq->leaf_cfs_rq_list) instead of another cfs_rq->leaf_cfs_rq_list.

The function does not handle this case, leading to incorrect pointer
calculations and unintended memory accesses, which can result in a kernel
crash.

A recent attempt to reorder fields in struct rq exposed this issue by
modifying memory offsets and affecting how pointer computations are
resolved. While the problem existed before, it was previously masked by
specific field arrangement. The reordering caused erroneous pointer
accesses, leading to a NULL dereference and a crash, as seen in the
following trace:

[    2.152852] Call Trace:
[    2.152855] __update_blocked_fair+0x45c/0x6a0 (unreliable)
[    2.152862] sched_balance_update_blocked_averages+0x11c/0x24c
[    2.152869] sched_balance_softirq+0x60/0x9c
[    2.152876] handle_softirqs+0x148/0x3b4
[    2.152884] do_softirq_own_stack+0x40/0x54
[    2.152891] __irq_exit_rcu+0x18c/0x1b4
[    2.152897] irq_exit+0x20/0x38
[    2.152903] timer_interrupt+0x174/0x30c
[    2.152910] decrementer_common_virt+0x28c/0x290
[    2.059873] systemd[1]: Hostname set to ...
[    2.152682] BUG: Unable to handle kernel data access on read at 0x100000125
[    2.152717] Faulting instruction address: 0xc0000000001c0270
[    2.152724] Oops: Kernel access of bad area, sig: 7 [#1]
..

To fix this, introduce a check to detect when prev points to the list head
(&rq->leaf_cfs_rq_list). If this condition is met, return early to prevent
the use of an invalid prev_cfs_rq.

Fixes: fdaba61ef8a2 ("sched/fair: Ensure that the CFS parent is added after unthrottling")
Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
---
 kernel/sched/fair.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1c0ef435a7aa..52ac22465a24 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4045,15 +4045,24 @@ static inline bool child_cfs_rq_on_list(struct cfs_rq *cfs_rq)
 {
 	struct cfs_rq *prev_cfs_rq;
 	struct list_head *prev;
+	struct rq *rq;
+
+	rq = rq_of(cfs_rq);
 
 	if (cfs_rq->on_list) {
 		prev = cfs_rq->leaf_cfs_rq_list.prev;
 	} else {
-		struct rq *rq = rq_of(cfs_rq);
-
 		prev = rq->tmp_alone_branch;
 	}
 
+	/*
+	 * Return early if prev points to rq->leaf_cfs_rq_list, the
+	 * list head and not a valid list node, to avoid computing
+	 * an invalid prev_cfs_rq.
+	 */
+	if (prev == &rq->leaf_cfs_rq_list)
+		return false;
+
 	prev_cfs_rq = container_of(prev, struct cfs_rq, leaf_cfs_rq_list);
 
 	return (prev_cfs_rq->tg->parent == cfs_rq->tg);
-- 
2.43.5