linux-kernel - [PATCH] media: bt8xx: Fix a possible data race in buffer

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Date:   Wed, 28 Jun 2023 15:19:51 +0800
From:   Tuo Li <islituo@...il.com>
To:     mchehab@...nel.org, hverkuil-cisco@...all.nl
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        baijiaju1990@...look.com, Tuo Li <islituo@...il.com>,
        BassCheck <bass@...a.edu.cn>
Subject: [PATCH] media: bt8xx: Fix a possible data race in buffer_queue()

The variable btv->loop_irq is often protected by the lock btv->s_lock when
is accessed. Here is an example in bttv_irq_timeout():

  spin_lock_irqsave(&btv->s_lock,flags); 
  ...
  btv->loop_irq = 0;
  ...
  spin_unlock_irqrestore(&btv->s_lock,flags);

However, it is accessed without holding the lock btv->s_lock in 
buffer_queue():

  btv->loop_irq |= 1;

And thus a data race can occur.
To fix this possible data race, a lock and unlock pair is added when
accessing the variable btv->loop_irq in buffer_queue().

Reported-by: BassCheck <bass@...a.edu.cn>
Signed-off-by: Tuo Li <islituo@...il.com>
---
 drivers/media/pci/bt8xx/bttv-driver.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/media/pci/bt8xx/bttv-driver.c b/drivers/media/pci/bt8xx/bttv-driver.c
index 734f02b91aa3..241c51951627 100644
--- a/drivers/media/pci/bt8xx/bttv-driver.c
+++ b/drivers/media/pci/bt8xx/bttv-driver.c
@@ -1617,13 +1617,16 @@ buffer_queue(struct videobuf_queue *q, struct videobuf_buffer *vb)
 	struct bttv_buffer *buf = container_of(vb,struct bttv_buffer,vb);
 	struct bttv_fh *fh = q->priv_data;
 	struct bttv    *btv = fh->btv;
+	unsigned long flags;
 
+	spin_lock_irqsave(&btv->s_lock,flags);
 	buf->vb.state = VIDEOBUF_QUEUED;
 	list_add_tail(&buf->vb.queue,&btv->capture);
 	if (!btv->curr.frame_irq) {
 		btv->loop_irq |= 1;
 		bttv_set_dma(btv, 0x03);
 	}
+	spin_unlock_irqrestore(&btv->s_lock,flags);
 }
 
 static void buffer_release(struct videobuf_queue *q, struct videobuf_buffer *vb)
-- 
2.34.1