[PATCH] media: rainshadow-cec: fix TOCTOU race condition in rain_interrupt()

[Gui-Dong Han <hanguidong02@gmail.com>]

In the interrupt handler rain_interrupt(), the buffer full check on
rain->buf_len is performed before acquiring rain->buf_lock. This
creates a Time-of-Check to Time-of-Use (TOCTOU) race condition, as
rain->buf_len is concurrently accessed and modified in the work
handler rain_irq_work_handler() under the same lock.

Multiple interrupt invocations can race, with each reading buf_len
before it becomes full and then proceeding. This can lead to both
interrupts attempting to write to the buffer, incrementing buf_len
beyond its capacity (DATA_SIZE) and causing a buffer overflow.

Fix this bug by moving the spin_lock() to before the buffer full
check. This ensures that the check and the subsequent buffer modification
are performed atomically, preventing the race condition. An corresponding
spin_unlock() is added to the overflow path to correctly release the
lock.

This possible bug was found by an experimental static analysis tool
developed by our team.

Fixes: 0f314f6c2e77 ("[media] rainshadow-cec: new RainShadow Tech HDMI CEC driver")
Cc: stable@vger.kernel.org
Signed-off-by: Gui-Dong Han <hanguidong02@gmail.com>
---
 drivers/media/cec/usb/rainshadow/rainshadow-cec.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/media/cec/usb/rainshadow/rainshadow-cec.c b/drivers/media/cec/usb/rainshadow/rainshadow-cec.c
index ee870ea1a886..6f8d6797c614 100644
--- a/drivers/media/cec/usb/rainshadow/rainshadow-cec.c
+++ b/drivers/media/cec/usb/rainshadow/rainshadow-cec.c
@@ -171,11 +171,12 @@ static irqreturn_t rain_interrupt(struct serio *serio, unsigned char data,
 {
 	struct rain *rain = serio_get_drvdata(serio);
 
+	spin_lock(&rain->buf_lock);
 	if (rain->buf_len == DATA_SIZE) {
+		spin_unlock(&rain->buf_lock);
 		dev_warn_once(rain->dev, "buffer overflow\n");
 		return IRQ_HANDLED;
 	}
-	spin_lock(&rain->buf_lock);
 	rain->buf_len++;
 	rain->buf[rain->buf_wr_idx] = data;
 	rain->buf_wr_idx = (rain->buf_wr_idx + 1) & 0xff;
-- 
2.25.1