[PATCH] [media] xc4000: Fix atomicity violation in xc4000_get_frequency

[Gui-Dong Han <2045gemini@gmail.com>]

In xc4000_get_frequency():
	*freq = priv->freq_hz + priv->freq_offset;
The code accesses priv->freq_hz and priv->freq_offset without holding any
lock.

In xc4000_set_params():
	// Code that updates priv->freq_hz and priv->freq_offset
	...

xc4000_get_frequency() and xc4000_set_params() may execute concurrently,
risking inconsistent reads of priv->freq_hz and priv->freq_offset. Since
these related data may update during reading, it can result in incorrect
frequency calculation, leading to atomicity violations.

This possible bug is found by an experimental static analysis tool
developed by our team. This tool analyzes the locking APIs to extract
function pairs that can be concurrently executed, and then analyzes the
instructions in the paired functions to identify possible concurrency bugs
including data races and atomicity violations. The above possible bug is
reported when our tool analyzes the source code of Linux 6.2.

To address this issue, it is proposed to add a mutex lock pair in
xc4000_get_frequency() to ensure atomicity. With this patch applied, our
tool no longer reports the possible bug, with the kernel configuration
allyesconfig for x86_64. Due to the lack of associated hardware, we cannot
test the patch in runtime testing, and just verify it according to the
code logic.

Fixes: 4c07e32884ab6 ("[media] xc4000: Fix get_frequency()")
Reported-by: BassCheck <bass@buaa.edu.cn>
Signed-off-by: Gui-Dong Han <2045gemini@gmail.com>
---
 drivers/media/tuners/xc4000.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/media/tuners/xc4000.c b/drivers/media/tuners/xc4000.c
index 57ded9ff3f04..29bc63021c5a 100644
--- a/drivers/media/tuners/xc4000.c
+++ b/drivers/media/tuners/xc4000.c
@@ -1515,10 +1515,10 @@ static int xc4000_get_frequency(struct dvb_frontend *fe, u32 *freq)
 {
 	struct xc4000_priv *priv = fe->tuner_priv;
 
+	mutex_lock(&priv->lock);
 	*freq = priv->freq_hz + priv->freq_offset;
 
 	if (debug) {
-		mutex_lock(&priv->lock);
 		if ((priv->cur_fw.type
 		     & (BASE | FM | DTV6 | DTV7 | DTV78 | DTV8)) == BASE) {
 			u16	snr = 0;
@@ -1529,8 +1529,8 @@ static int xc4000_get_frequency(struct dvb_frontend *fe, u32 *freq)
 				return 0;
 			}
 		}
-		mutex_unlock(&priv->lock);
 	}
+	mutex_unlock(&priv->lock);
 
 	dprintk(1, "%s()\n", __func__);
 
-- 
2.34.1