[PATCH v3] coresight: ETR: Fix ETR buffer use-after-free issue

[Xiaoqi Zhuang <xiaoqi.zhuang@oss.qualcomm.com>]

When ETR is enabled as CS_MODE_SYSFS, if the buffer size is changed
and enabled again, currently sysfs_buf will point to the newly
allocated memory(buf_new) and free the old memory(buf_old). But the
etr_buf that is being used by the ETR remains pointed to buf_old, not
updated to buf_new. In this case, it will result in a memory
use-after-free issue.

Fix this by checking ETR's mode before updating and releasing buf_old,
if the mode is CS_MODE_SYSFS, then skip updating and releasing it.

Fixes: bd2767ec3df2 ("coresight: Fix run time warnings while reusing ETR buffer")
Signed-off-by: Xiaoqi Zhuang <xiaoqi.zhuang@oss.qualcomm.com>
---
Changes in v3:
- Add a fix tag for the fix patch.
- Link to v2: https://lore.kernel.org/r/20251021-fix_etr_issue-v2-1-80c40c9cac8c@oss.qualcomm.com

Changes in v2:
- Exit earlier to avoid allocating memory unnecessarily.
- Link to v1: https://lore.kernel.org/r/20251020-fix_etr_issue-v1-1-902ab51770b4@oss.qualcomm.com
---
 drivers/hwtracing/coresight/coresight-tmc-etr.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/hwtracing/coresight/coresight-tmc-etr.c b/drivers/hwtracing/coresight/coresight-tmc-etr.c
index b07fcdb3fe1a..800be06598c1 100644
--- a/drivers/hwtracing/coresight/coresight-tmc-etr.c
+++ b/drivers/hwtracing/coresight/coresight-tmc-etr.c
@@ -1250,6 +1250,13 @@ static struct etr_buf *tmc_etr_get_sysfs_buffer(struct coresight_device *csdev)
 	 * with the lock released.
 	 */
 	raw_spin_lock_irqsave(&drvdata->spinlock, flags);
+
+	/*
+	 * If the ETR is already enabled, continue with the existing buffer.
+	 */
+	if (coresight_get_mode(csdev) == CS_MODE_SYSFS)
+		goto out;
+
 	sysfs_buf = READ_ONCE(drvdata->sysfs_buf);
 	if (!sysfs_buf || (sysfs_buf->size != drvdata->size)) {
 		raw_spin_unlock_irqrestore(&drvdata->spinlock, flags);

---
base-commit: 98ac9cc4b4452ed7e714eddc8c90ac4ae5da1a09
change-id: 20251020-fix_etr_issue-02c706dbc899

Best regards,
-- 
Xiaoqi Zhuang <xiaoqi.zhuang@oss.qualcomm.com>