ksmbd: fix out-of-bound read in smb2_write

ksmbd_smb2_check_message doesn't validate hdr->NextCommand. If
->NextCommand is bigger than Offset + Length of smb2 write, It will
allow oversized smb2 write length. It will cause OOB read in smb2_write.

Cc: stable@vger.kernel.org
Reported-by: zdi-disclosures@trendmicro.com # ZDI-CAN-21164
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/server/smb2misc.c

@@ -351,10 +351,16 @@ int ksmbd_smb2_check_message(struct ksmbd_work *work)
 	int command;
 	__u32 clc_len;  /* calculated length */
 	__u32 len = get_rfc1002_len(work->request_buf);
-	__u32 req_struct_size;
+	__u32 req_struct_size, next_cmd = le32_to_cpu(hdr->NextCommand);
 
-	if (le32_to_cpu(hdr->NextCommand) > 0)
-		len = le32_to_cpu(hdr->NextCommand);
+	if ((u64)work->next_smb2_rcv_hdr_off + next_cmd > len) {
+		pr_err("next command(%u) offset exceeds smb msg size\n",
+				next_cmd);
+		return 1;
+	}
+
+	if (next_cmd > 0)
+		len = next_cmd;
 	else if (work->next_smb2_rcv_hdr_off)
 		len -= work->next_smb2_rcv_hdr_off;