ksmbd: fix slab-out-of-bounds read in smb2_handle_negotiate

Check request_buf length first to avoid out-of-bounds read by
req->DialectCount.

[ 3350.990282] BUG: KASAN: slab-out-of-bounds in smb2_handle_negotiate+0x35d7/0x3e60
[ 3350.990282] Read of size 2 at addr ffff88810ad61346 by task kworker/5:0/276
[ 3351.000406] Workqueue: ksmbd-io handle_ksmbd_work
[ 3351.003499] Call Trace:
[ 3351.006473]  <TASK>
[ 3351.006473]  dump_stack_lvl+0x8d/0xe0
[ 3351.006473]  print_report+0xcc/0x620
[ 3351.006473]  kasan_report+0x92/0xc0
[ 3351.006473]  smb2_handle_negotiate+0x35d7/0x3e60
[ 3351.014760]  ksmbd_smb_negotiate_common+0x7a7/0xf00
[ 3351.014760]  handle_ksmbd_work+0x3f7/0x12d0
[ 3351.014760]  process_one_work+0xa85/0x1780

Cc: stable@vger.kernel.org
Signed-off-by: Kuan-Ting Chen <h3xrabbit@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/server/smb2pdu.c

@@ -1053,16 +1053,16 @@ int smb2_handle_negotiate(struct ksmbd_work *work)
 		return rc;
 	}
 
-	if (req->DialectCount == 0) {
-		pr_err("malformed packet\n");
+	smb2_buf_len = get_rfc1002_len(work->request_buf);
+	smb2_neg_size = offsetof(struct smb2_negotiate_req, Dialects);
+	if (smb2_neg_size > smb2_buf_len) {
 		rsp->hdr.Status = STATUS_INVALID_PARAMETER;
 		rc = -EINVAL;
 		goto err_out;
 	}
 
-	smb2_buf_len = get_rfc1002_len(work->request_buf);
-	smb2_neg_size = offsetof(struct smb2_negotiate_req, Dialects);
-	if (smb2_neg_size > smb2_buf_len) {
+	if (req->DialectCount == 0) {
+		pr_err("malformed packet\n");
 		rsp->hdr.Status = STATUS_INVALID_PARAMETER;
 		rc = -EINVAL;
 		goto err_out;