ksmbd: check the validation of pdu_size in ksmbd_conn_handler_loop

The length field of netbios header must be greater than the SMB header
sizes(smb1 or smb2 header), otherwise the packet is an invalid SMB packet.

If `pdu_size` is 0, ksmbd allocates a 4 bytes chunk to `conn->request_buf`.
In the function `get_smb2_cmd_val` ksmbd will read cmd from
`rcv_hdr->Command`, which is `conn->request_buf + 12`, causing the KASAN
detector to print the following error message:

[    7.205018] BUG: KASAN: slab-out-of-bounds in get_smb2_cmd_val+0x45/0x60
[    7.205423] Read of size 2 at addr ffff8880062d8b50 by task ksmbd:42632/248
...
[    7.207125]  <TASK>
[    7.209191]  get_smb2_cmd_val+0x45/0x60
[    7.209426]  ksmbd_conn_enqueue_request+0x3a/0x100
[    7.209712]  ksmbd_server_process_request+0x72/0x160
[    7.210295]  ksmbd_conn_handler_loop+0x30c/0x550
[    7.212280]  kthread+0x160/0x190
[    7.212762]  ret_from_fork+0x1f/0x30
[    7.212981]  </TASK>

Cc: stable@vger.kernel.org
Reported-by: Chih-Yen Chang <cc85nod@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: namjaejeon

fs/smb/server/connection.c

@@ -294,6 +294,9 @@ bool ksmbd_conn_alive(struct ksmbd_conn *conn)
 	return true;
 }
 
+#define SMB1_MIN_SUPPORTED_HEADER_SIZE (sizeof(struct smb_hdr))
+#define SMB2_MIN_SUPPORTED_HEADER_SIZE (sizeof(struct smb2_hdr) + 4)
+
 /**
  * ksmbd_conn_handler_loop() - session thread to listen on new smb requests
  * @p:		connection instance
@@ -350,6 +353,9 @@ int ksmbd_conn_handler_loop(void *p)
 		if (pdu_size > MAX_STREAM_PROT_LEN)
 			break;
 
+		if (pdu_size < SMB1_MIN_SUPPORTED_HEADER_SIZE)
+			break;
+
 		/* 4 for rfc1002 length field */
 		/* 1 for implied bcc[0] */
 		size = pdu_size + 4 + 1;
@@ -377,6 +383,12 @@ int ksmbd_conn_handler_loop(void *p)
 			continue;
 		}
 
+		if (((struct smb2_hdr *)smb2_get_msg(conn->request_buf))->ProtocolId ==
+		    SMB2_PROTO_NUMBER) {
+			if (pdu_size < SMB2_MIN_SUPPORTED_HEADER_SIZE)
+				break;
+		}
+
 		if (!default_conn_ops.process_fn) {
 			pr_err("No connection request callback\n");
 			break;