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Message-ID: <20250430084021.348347-1-arighi@nvidia.com>
Date: Wed, 30 Apr 2025 10:40:21 +0200
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>,
David Vernet <void@...ifault.com>,
Changwoo Min <changwoo@...lia.com>
Cc: linux-kernel@...r.kernel.org
Subject: [PATCH sched_ext/for-6.16] sched_ext: Avoid NULL scx_root deref in __scx_exit()
A sched_ext scheduler may trigger __scx_exit() from a BPF timer
callback, where scx_root may not be safely dereferenced.
This can lead to a NULL pointer dereference as shown below (triggered by
scx_tickless):
BUG: kernel NULL pointer dereference, address: 0000000000000330
...
CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.14.0-virtme #1 PREEMPT(full)
RIP: 0010:__scx_exit+0x2b/0x190
...
Call Trace:
<IRQ>
scx_bpf_get_idle_smtmask+0x59/0x80
bpf_prog_8320d4217989178c_dispatch_all_cpus+0x35/0x1b6
...
bpf_prog_97f847d871513f95_sched_timerfn+0x4c/0x264
bpf_timer_cb+0x7a/0x140
__hrtimer_run_queues+0x1f9/0x3a0
hrtimer_run_softirq+0x8c/0xd0
handle_softirqs+0xd3/0x3d0
__irq_exit_rcu+0x9a/0xc0
irq_exit_rcu+0xe/0x20
Fix this by checking for a valid scx_root and adding proper RCU
protection.
Fixes: 48e1267773866 ("sched_ext: Introduce scx_sched")
Signed-off-by: Andrea Righi <arighi@...dia.com>
---
kernel/sched/ext.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)
diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 66d21c7cdff7d..1ee5a0088b1a1 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -5212,12 +5212,19 @@ static void scx_error_irq_workfn(struct irq_work *irq_work)
static __printf(3, 4) void __scx_exit(enum scx_exit_kind kind, s64 exit_code,
const char *fmt, ...)
{
- struct scx_exit_info *ei = scx_root->exit_info;
+ struct scx_sched *sch;
+ struct scx_exit_info *ei;
int none = SCX_EXIT_NONE;
va_list args;
- if (!atomic_try_cmpxchg(&scx_root->exit_kind, &none, kind))
- return;
+ rcu_read_lock();
+ sch = rcu_dereference(scx_root);
+ if (!sch)
+ goto out_unlock;
+ ei = sch->exit_info;
+
+ if (!atomic_try_cmpxchg(&sch->exit_kind, &none, kind))
+ goto out_unlock;
ei->exit_code = exit_code;
#ifdef CONFIG_STACKTRACE
@@ -5235,7 +5242,9 @@ static __printf(3, 4) void __scx_exit(enum scx_exit_kind kind, s64 exit_code,
ei->kind = kind;
ei->reason = scx_exit_reason(ei->kind);
- irq_work_queue(&scx_root->error_irq_work);
+ irq_work_queue(&sch->error_irq_work);
+out_unlock:
+ rcu_read_unlock();
}
static struct scx_sched *scx_alloc_and_add_sched(struct sched_ext_ops *ops)
--
2.49.0
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