[PATCH v5 3/6] media: meson: vdec: Set cur_sess before hardware vdec_poweron()

[Anand Moon <linux.amoon@gmail.com>]

vdec_poweron() initializes hardware and unmasks device interrupts. If an
interrupt fires before core->cur_sess is set, vdec_isr() dereferences a
NULL pointer when updating sess->last_irq_jiffies, leading to a kernel
panic.

Fix this by assigning core->cur_sess and updating sess->status under
core->lock before calling vdec_poweron(). This ensures the interrupt
handler always sees a valid session pointer. On the error path, clear
core->cur_sess and reset sess->status to STATUS_STOPPED to avoid stale
references.

Following change also strengthens the hardware exclusivity check by
holding core->lock during session assignment, preventing concurrent
sessions from racing through cur_sess == NULL and corrupting hardware
state.

Cc: Nicolas Dufresne <nicolas@ndufresne.ca>
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260521090944.F35401F00A3D@smtp.kernel.org/
Fixes: 3e7f51bd9607 ("media: meson: add v4l2 m2m video decoder driver")
Signed-off-by: Anand Moon <linux.amoon@gmail.com>
---
v5: [Critical] Race condition between hardware power-on and `core->cur_sess`
 initialization leads to a NULL pointer dereference in the IRQ handler.
---
 drivers/staging/media/meson/vdec/vdec.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/staging/media/meson/vdec/vdec.c b/drivers/staging/media/meson/vdec/vdec.c
index e72f54af026e..52ace4de967c 100644
--- a/drivers/staging/media/meson/vdec/vdec.c
+++ b/drivers/staging/media/meson/vdec/vdec.c
@@ -334,6 +334,11 @@ static int vdec_start_streaming(struct vb2_queue *q, unsigned int count)
 	atomic_set(&sess->esparser_queued_bufs, 0);
 	v4l2_ctrl_s_ctrl(sess->ctrl_min_buf_capture, 1);
 
+	mutex_lock(&core->lock);
+	core->cur_sess = sess;
+	sess->status = STATUS_INIT;
+	mutex_unlock(&core->lock);
+
 	ret = vdec_poweron(sess);
 	if (ret)
 		goto vififo_free;
@@ -344,12 +349,14 @@ static int vdec_start_streaming(struct vb2_queue *q, unsigned int count)
 		sess->recycle_thread = kthread_run(vdec_recycle_thread, sess,
 						   "vdec_recycle");
 
-	sess->status = STATUS_INIT;
-	core->cur_sess = sess;
 	schedule_work(&sess->esparser_queue_work);
 	return 0;
 
 vififo_free:
+	mutex_lock(&core->lock);
+	core->cur_sess = NULL;
+	sess->status = STATUS_STOPPED;
+	mutex_unlock(&core->lock);
 	dma_free_coherent(sess->core->dev, sess->vififo_size,
 			  sess->vififo_vaddr, sess->vififo_paddr);
 bufs_done:
-- 
2.50.1