smb: client: fix integer underflow in receive_encrypted_read()

In receive_encrypted_read(), the length of data to read from the socket
is computed as:

  len = le32_to_cpu(tr_hdr->OriginalMessageSize) -
        server->vals->read_rsp_size;

OriginalMessageSize comes from the server's transform header and is
untrusted. If a malicious server sends a value smaller than
read_rsp_size, the unsigned subtraction wraps to a very large value
(~4GB). This value is then passed to netfs_alloc_folioq_buffer() and
cifs_read_iter_from_socket(), causing either a massive allocation
attempt that fails with -ENOMEM (DoS), or under extreme memory
pressure, potential heap corruption.

Fix by adding a check that OriginalMessageSize is at least
read_rsp_size before the subtraction. On failure, jump to
discard_data to drain the remaining PDU from the socket, preventing
desync of subsequent reads on the connection.

Signed-off-by: Dudu Lu <phx0fer@gmail.com>
Reviewed-by: Enzo Matsumiya <ematsumiya@suse.de>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: phx0fer

fs/smb/client/smb2ops.c

@@ -4943,6 +4943,14 @@ receive_encrypted_read(struct TCP_Server_Info *server, struct mid_q_entry **mid,
 		goto free_dw;
 	server->total_read += rc;
 
+	if (le32_to_cpu(tr_hdr->OriginalMessageSize) <
+	    server->vals->read_rsp_size) {
+		cifs_server_dbg(VFS, "OriginalMessageSize %u too small for read response (%zu)\n",
+			le32_to_cpu(tr_hdr->OriginalMessageSize),
+			server->vals->read_rsp_size);
+		rc = -EINVAL;
+		goto discard_data;
+	}
 	len = le32_to_cpu(tr_hdr->OriginalMessageSize) -
 		server->vals->read_rsp_size;
 	dw->len = len;