Bluetooth: btmtk: Fix null pointer when processing coredump

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.

---
v2: fix typo

---
 drivers/bluetooth/btmtk.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

index 786f775196ae..0f290430ae0e 100644
@@ -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.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

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Dear submitter,

Thank you for submitting the patches to the linux bluetooth mailing list. This is a CI test results with your patch series:
PW Link:https://patchwork.kernel.org/project/bluetooth/list/?series=764635

---Test result---

Test Summary:

CheckPatch                    FAIL      0.98 seconds
GitLint                       PASS      0.34 seconds
SubjectPrefix                 PASS      0.13 seconds
BuildKernel                   PASS      33.41 seconds
CheckAllWarning               PASS      37.07 seconds
CheckSparse                   PASS      41.83 seconds
CheckSmatch                   PASS      112.94 seconds
BuildKernel32                 PASS      32.24 seconds
TestRunnerSetup               PASS      494.86 seconds
TestRunner_l2cap-tester       PASS      23.46 seconds
TestRunner_iso-tester         PASS      42.33 seconds
TestRunner_bnep-tester        PASS      10.63 seconds
TestRunner_mgmt-tester        PASS      220.73 seconds
TestRunner_rfcomm-tester      PASS      16.10 seconds
TestRunner_sco-tester         PASS      17.10 seconds
TestRunner_ioctl-tester       PASS      18.45 seconds
TestRunner_mesh-tester        PASS      13.72 seconds
TestRunner_smp-tester         PASS      14.62 seconds
TestRunner_userchan-tester    PASS      11.37 seconds
IncrementalBuild              PASS      30.90 seconds

Details
##############################
Test: CheckPatch - FAIL
Desc: Run checkpatch.pl script
Output:
[v2] Bluetooth: btmtk: Fix null pointer when processing coredump
WARNING: Co-developed-by: must be immediately followed by Signed-off-by:
#82:

---

total: 0 errors, 1 warnings, 30 lines checked

NOTE: For some of the reported defects, checkpatch may be able to
mechanically convert to the typical style using --fix or --fix-inplace.

/github/workspace/src/src/13309551.patch has style problems, please review.

NOTE: Ignored message types: UNKNOWN_COMMIT_ID

NOTE: If any of the errors are false positives, please report
them to the maintainer, see CHECKPATCH in MAINTAINERS.

---
Regards,
Linux Bluetooth