[PATCH v2] [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, BassCheck[1]. 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.

[1] https://sites.google.com/view/basscheck/

Fixes: 4c07e32884ab6 ("[media] xc4000: Fix get_frequency()")
Cc: stable@vger.kernel.org
Reported-by: BassCheck <bass@buaa.edu.cn>
Signed-off-by: Gui-Dong Han <2045gemini@gmail.com>
---
v2:
* In this patch v2, we've added some information of the static analysis
tool used, as per the researcher guidelines. Also, we've added a cc in the
signed-off-by area, according to the stable-kernel-rules.
  Thank Greg KH for helpful advice.
---
 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