Patch "media: xc4000: Fix atomicity violation in xc4000_get_frequency" has been added to the 6.8-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    media: xc4000: Fix atomicity violation in xc4000_get_frequency

to the 6.8-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     media-xc4000-fix-atomicity-violation-in-xc4000_get_f.patch
and it can be found in the queue-6.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 27c6a4da02ffc5ddc8cadb08f026eeaff170ea57
Author: Gui-Dong Han <2045gemini@xxxxxxxxx>
Date:   Fri Dec 22 13:50:30 2023 +0800

    media: xc4000: Fix atomicity violation in xc4000_get_frequency
    
    [ Upstream commit 36d503ad547d1c75758a6fcdbec2806f1b6aeb41 ]
    
    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: 4c07e32884ab ("[media] xc4000: Fix get_frequency()")
    Cc: stable@xxxxxxxxxxxxxxx
    Reported-by: BassCheck <bass@xxxxxxxxxxx>
    Signed-off-by: Gui-Dong Han <2045gemini@xxxxxxxxx>
    Signed-off-by: Hans Verkuil <hverkuil-cisco@xxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/drivers/media/tuners/xc4000.c b/drivers/media/tuners/xc4000.c
index 57ded9ff3f043..29bc63021c5aa 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__);