qubes-doc/Qrexec3Implementation.md

layout	title	permalink
wiki	Qrexec3Implementation	/wiki/Qrexec3Implementation/

Implementation of qrexec in Qubes R2

This page describes implementation of the qrexec framework in Qubes OS R2.

Qrexec framework consists of a number of processes communicating with each other using common IPC protocol (described in detail below). Components residing in the same domain use pipes as the underlying transport medium, while components in separate domains use vchan link.

Dom0 tools implementation

• /usr/lib/qubes/qrexec-daemon <- one instance is required for every active domain. Responsible for:
  • Handling execution and service requests from dom0 (source: qrexec-client).
  • Handling service requests from the associated domain (source: qrexec-client-vm, then qrexec-agent).

Command line: qrexec-daemon domain-id domain-name [default user]

domain-id: numeric Qubes ID assigned to the associated domain.

domain-name: associated domain name.

default user: optional. If passed, qrexec-daemon uses this user as default for all execution requests that don't specify one.

• /usr/lib/qubes/qrexec-policy <- internal program used to evaluate the RPC policy and deciding whether a RPC call should be allowed.
• /usr/lib/qubes/qrexec-client <- used to pass execution and service requests to qrexec-daemon. Command line parameters:

-d target-domain-name Specifies the target for the execution/service request.

-l local-program Optional. If present, local-program is executed and its stdout/stdin are used when sending/receiving data to/from the remote peer.

-e Optional. If present, stdout/stdin are not connected to the remote peer. Only process creation status code is received.

-c <request-id,src-domain-name,src-domain-id> Used for connecting a VM-VM service request by qrexec-policy. Details described below in the service example.

cmdline Command line to pass to qrexec-daemon as the execution/service request. Service request format is described below in the service example.

Note: none of the above tools are designed to be used by users directly.

VM tools implementation

• qrexec-agent <- one instance runs in each active domain. Responsible for:
  • Handling service requests from qrexec-client-vm and passing them to connected qrexec-daemon in dom0.
  • Executing associated qrexec-daemon execution/service requests.

Command line parameters: none.

• qrexec-client-vm <- runs in an active domain. Used to pass service requests to qrexec-agent.

Command line: qrexec-client-vm target-domain-name service-name local-program [local program arguments]

target-domain-name Target domain for the service request. Source is the current domain.

service-name Requested service name.

local-program local-program is executed locally and its stdin/stdout are connected to the remote service endpoint.

Qrexec protocol details

Qrexec protocol is message-based. All messages share a common header followed by an optional data packet.

/* uniform for all peers, data type depends on message type */
struct msg_header {
   uint32_t type;           /* message type */
   uint32_t len;            /* data length */
};

When two peers establish connection, the server sends MSG_HELLO followed by peer_info struct:

struct peer_info {
   uint32_t version; /* qrexec protocol version */
};

The client then should reply with its own MSG_HELLO and peer_info. If protocol versions don't match, the connection is closed. TODO: fallback for backwards compatibility.

Details of all possible use cases and the messages involved are described below.

dom0: request execution of some_command in domX and pass stdin/stdout

• qrexec-client is invoked in dom0 as follows:

qrexec-client -d domX [-l local_program] user:some_command

user may be substituted with the literal DEFAULT. In that case, default Qubes user will be used to execute some_command.

• qrexec-client sets QREXEC_REMOTE_DOMAIN environment variable to domX.

• If local_program is set, qrexec-client executes it and uses that child's stdin/stdout in place of its own when exchanging data with qrexec-agent later.

• qrexec-client connects to domX's qrexec-daemon.

• qrexec-daemon sends MSG_HELLO followed by peer_info to qrexec-client.

• qrexec-client replies with MSG_HELLO followed by peer_info to qrexec-daemon.

• qrexec-client sends MSG_EXEC_CMDLINE followed by exec_params to qrexec-daemon

   /* variable size */
   struct exec_params {
      uint32_t connect_domain; /* target domain id */
      uint32_t connect_port;   /* target vchan port for i/o exchange */
      char cmdline[0];         /* command line to execute, size = msg_header.len - sizeof(struct exec_params) */
   };
  

  In this case, connect_domain and connect_port are set to 0.

• qrexec-daemon replies to qrexec-client with MSG_EXEC_CMDLINE followed by exec_params, but with empty cmdline field. connect_domain is set to Qubes ID of domX and connect_port is set to a vchan port allocated by qrexec-daemon.

• qrexec-daemon sends MSG_EXEC_CMDLINE followed by exec_params to the associated domX qrexec-agent over vchan. connect_domain is set to 0 (dom0), connect_port is the same as sent to qrexec-client. cmdline is unchanged except that the literal DEFAULT is replaced with actual user name, if present.

• qrexec-client disconnects from qrexec-daemon.

• qrexec-client starts a vchan server using the details received from qrexec-daemon and waits for connection from domX's qrexec-agent.

• domX's qrexec-agent receives MSG_EXEC_CMDLINE followed by exec_params from qrexec-daemon over vchan.

• domX's qrexec-agent connects to qrexec-client over vchan using the details from exec_params.

• domX's qrexec-agent executes some_command in domX and connects the child's stdin/stdout to the data vchan. If the process creation fails, qrexec-agent sends MSG_DATA_EXIT_CODE to qrexec-client followed by the status code (int) and disconnects from the data vchan.

• Data read from some_command's stdout is sent to the data vchan using MSG_DATA_STDOUT by qrexec-agent. qrexec-client passes data received as MSG_DATA_STDOUT to its own stdout (or to local_program's stdin if used).

• qrexec-client sends data read from local stdin (or local_program's stdout if used) to qrexec-agent over the data vchan using MSG_DATA_STDIN. qrexec-agent passes data received as MSG_DATA_STDIN to some_command's stdin.

• MSG_DATA_STDOUT or MSG_DATA_STDIN with data len field set to 0 in msg_header is an EOF marker. Peer receiving such message should close the associated input/output pipe.

• When some_command terminates, domX's qrexec-agent sends MSG_DATA_EXIT_CODE to qrexec-client followed by the exit code (int). qrexec-agent then disconnects from the data vchan.