FAILED: patch "[PATCH] media: rainshadow-cec: fix TOCTOU race condition in" failed to apply to 5.4-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 5.4-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.4.y
git checkout FETCH_HEAD
git cherry-pick -x 7af160aea26c7dc9e6734d19306128cce156ec40
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2025082119-december-ranking-b3a3@gregkh' --subject-prefix 'PATCH 5.4.y' HEAD^..

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 7af160aea26c7dc9e6734d19306128cce156ec40 Mon Sep 17 00:00:00 2001
From: Gui-Dong Han <hanguidong02@gmail.com>
Date: Fri, 6 Jun 2025 03:04:59 +0000
Subject: [PATCH] media: rainshadow-cec: fix TOCTOU race condition in
 rain_interrupt()

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>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>

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;