[PATCH] Bluetooth: btmtk: Fix null pointer when processing coredump

[Chris Lu <chris.lu@mediatek.com>]

There may be a potential null pointer risk if offset value is
less than 0 when doing memcmp in btmtk_process_coredump().
Checking offset is valid before doing memcmp.

Signed-off-by: Chris Lu <chris.lu@mediatek.com>
Co-developed-by: Sean Wang <sean.wang@mediatek.com>
---
 drivers/bluetooth/btmtk.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/drivers/bluetooth/btmtk.c b/drivers/bluetooth/btmtk.c
index 786f775196ae..9ae46b4c98b9 100644
--- a/drivers/bluetooth/btmtk.c
+++ b/drivers/bluetooth/btmtk.c
@@ -370,7 +370,7 @@ EXPORT_SYMBOL_GPL(btmtk_register_coredump);
 int btmtk_process_coredump(struct hci_dev *hdev, struct sk_buff *skb)
 {
 	struct btmediatek_data *data = hci_get_priv(hdev);
-	int err;
+	int err, offset;
 
 	if (!IS_ENABLED(CONFIG_DEV_COREDUMP))
 		return 0;
@@ -392,15 +392,15 @@ int btmtk_process_coredump(struct hci_dev *hdev, struct sk_buff *skb)
 		if (err < 0)
 			break;
 		data->cd_info.cnt++;
+		offset = skb->len - sizeof(MTK_COREDUMP_END);
 
 		/* Mediatek coredump data would be more than MTK_COREDUMP_NUM */
-		if (data->cd_info.cnt > MTK_COREDUMP_NUM &&
-		    skb->len > sizeof(MTK_COREDUMP_END) &&
-		    !memcmp((char *)&skb->data[skb->len - sizeof(MTK_COREDUMP_END)],
-			    MTK_COREDUMP_END, sizeof(MTK_COREDUMP_END) - 1)) {
-			bt_dev_info(hdev, "Mediatek coredump end");
-			hci_devcd_complete(hdev);
-		}
+		if (data->cd_info.dump_cnt > MTK_COREDUMP_NUM && offset > 0)
+			if (!memcmp((char *)&skb->data[offset], MTK_COREDUMP_END,
+				    sizeof(MTK_COREDUMP_END) - 1)) {
+				bt_dev_info(hdev, "Mediatek coredump end");
+				hci_devcd_complete(hdev);
+			}
 
 		break;
 	}
-- 
2.18.0