[PATCH v4] 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().
Check offset is valid before doing memcmp.

Signed-off-by: Chris Lu <chris.lu@mediatek.com>
Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
---
v2: fix typo
v3: fix bot checking error
v4: reduce variable 'offset' declaration in v3
---
 drivers/bluetooth/btmtk.c | 12 ++++++------
 drivers/bluetooth/btmtk.h |  1 +
 2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/drivers/bluetooth/btmtk.c b/drivers/bluetooth/btmtk.c
index 786f775196ae..9cc789272ab7 100644
--- a/drivers/bluetooth/btmtk.c
+++ b/drivers/bluetooth/btmtk.c
@@ -395,12 +395,12 @@ int btmtk_process_coredump(struct hci_dev *hdev, struct sk_buff *skb)
 
 		/* 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);
-		}
+		    skb->len > MTK_COREDUMP_END_LEN)
+			if (!memcmp((char *)&skb->data[skb->len - MTK_COREDUMP_END_LEN],
+			    MTK_COREDUMP_END, MTK_COREDUMP_END_LEN - 1)) {
+				bt_dev_info(hdev, "Mediatek coredump end");
+				hci_devcd_complete(hdev);
+			}
 
 		break;
 	}
diff --git a/drivers/bluetooth/btmtk.h b/drivers/bluetooth/btmtk.h
index 68309dfe076a..56f5502baadf 100644
--- a/drivers/bluetooth/btmtk.h
+++ b/drivers/bluetooth/btmtk.h
@@ -24,6 +24,7 @@
 
 #define MTK_COREDUMP_SIZE		(1024 * 1000)
 #define MTK_COREDUMP_END		"coredump end"
+#define MTK_COREDUMP_END_LEN		(sizeof(MTK_COREDUMP_END))
 #define MTK_COREDUMP_NUM		255
 
 enum {
-- 
2.18.0