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66 lines
4.0 KiB
Markdown
66 lines
4.0 KiB
Markdown
---
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layout: wiki
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title: Qrexec2Implementation
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permalink: /wiki/Qrexec2Implementation/
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---
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Implementation of qrexec in Qubes R2
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====================================
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This page describes implementation of the [qrexec framework](/wiki/Qrexec) in Qubes OS R2. Note that the implementation has changed significantly in Qubes R3 (see [Qrexec3Implementation](/wiki/Qrexec3Implementation)), although the user API reminded backwards compatible (i.e. qrexec apps written for Qubes R2 should run without modifications on Qubes R3).
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Dom0 tools implementation
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-------------------------
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Players:
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- `/usr/lib/qubes/qrexec-daemon` \<- started by mgmt stack (qubes.py) when a VM is started.
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- `/usr/lib/qubes/qrexec-policy` \<- internal program used to evaluate the policy file and making the 2nd half of the connection.
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- `/usr/lib/qubes/qrexec-client` \<- raw command line tool that talks to the daemon via unix socket (`/var/run/qubes/qrexec.XID`)
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Note: neither of the above tools are designed to be used by users.
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Linux VMs implementation
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------------------------
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Players:
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- `/usr/lib/qubes/qrexec-agent` \<- started by VM bootup scripts, a daemon.
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- `/usr/lib/qubes/qubes-rpc-multiplexer` \<- executes the actual service program, as specified in VM's `/etc/qubes-rpc/qubes.XYZ`
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- `/usr/lib/qubes/qrexec-client-vm` \<- raw command line tool that talks to the agent
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Note: neither of the above tools are designed to be used by users. `qrexec-client-vm` is designed to be wrapped up by Qubes apps.
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Windows VMs implemention
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------------------------
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TODO
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All the pieces together at work
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-------------------------------
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Note: this section is not needed to use qrexec for writing Qubes apps. Also note the qrexec framework implemention in Qubes R3 significantly differs from what is described in this section.
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The VM-VM channels in Qubes R2 are made via "gluing" two VM-Dom0 and Dom0-VM vchan connections:
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[![No image "qrexec2-internals.png" attached to Qrexec2Implementation](/chrome/common/attachment.png "No image "qrexec2-internals.png" attached to Qrexec2Implementation")](/attachment/wiki/Qrexec2Implementation/qrexec2-internals.png)
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Note: Dom0 never examines the actual data flowing in neither of the two vchan connections.
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When a user in a source VM executes `qrexec-client-vm` utility, the following steps are taken:
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- `qrexec-client-vm` connects to `qrexec-agent`'s `/var/run/qubes/qrexec-agent-fdpass` unix socket 3 times. Reads 4 bytes from each of them, which is the fd number of the accepted socket in agent. These 3 integers, in text, concatenated, form "connection identifier" (CID)
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- `qrexec-client-vm` writes to `/var/run/qubes/qrexec-agent` fifo a blob, consisting of target vmname, rpc action, and CID
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- `qrexec-client-vm` executes the rpc client, passing the above mentioned unix sockets as process stdin/stdout, and optionally stderr (if the PASS\_LOCAL\_STDERR env variable is set)
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- `qrexec-agent` passes the blob to `qrexec-daemon`, via MSG\_AGENT\_TO\_SERVER\_TRIGGER\_CONNECT\_EXISTING message over vchan
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- `qrexec-daemon` executes `qrexec-policy`, passing source vmname, target vmname, rpc action, and CID as cmdline arguments
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- `qrexec-policy` evaluates the policy file. If successful, creates a pair of `qrexec-client` processes, whose stdin/stdout are cross-connencted.
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- The first `qrexec-client` connects to the src VM, using the `-c ClientID` parameter, which results in not creating a new process, but connecting to the existing process file descriptors (these are the fds of unix socket created in step 1).
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- The second `qrexec-client` connects to the target VM, and executes `qubes-rpc-multiplexer` command there with the rpc action as the cmdline argument. Finally, `qubes-rpc-multiplexer` executes the correct rpc server on the target.
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- In the above step, if the target VM is `$dispvm`, the dispvm is created via the `qfile-daemon-dvm` program. The latter waits for the `qrexec-client` process to exit, and then destroys the dispvm.
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Protocol description ("wire-level" spec)
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----------------------------------------
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TODO
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