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Begin revising RPC examples section in qrexec.md

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- Put both RPC examples under one heading
- Add a table for the needed example policies
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pierwill 2019-08-26 08:48:31 -05:00
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developer/services/qrexec.md
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@ -213,7 +213,11 @@ See below for an example service using an argument.
<!-- TODO document "Yes to All" authorization if it is reintroduced -->
### Qubes RPC example
## Qubes RPC examples
To demostrate some of the possibilities afforded by the qrexec framework, here are two examples of custom RPC services.
### Simple RPC service (addition)
As a demonstration, we can create an RPC service that adds two integers in a target domain (the server, call it "anotherVM") and returns back the result to the invoker (the client, "someVM").
In someVM, create a file with the following contents and save it with the path `/usr/bin/our_test_add_client`:
@ -253,46 +257,41 @@ We should get "3" as answer.
**Note:** For a real world example of writing a qrexec service, see this [blog post](https://blog.invisiblethings.org/2013/02/21/converting-untrusted-pdfs-into-trusted.html).
### Qubes RPC example - with argument usage
### RPC service with argument (file reader)
We will show the necessary files to create an RPC call that reads a specific file from a predefined directory on the target.
Besides really naive storage, it may be a very simple password manager.
This example uses an [argument](#service-argument-in-policy) to the policy to create an RPC call that reads a specific file from a predefined directory on the target.
<!-- Besides really naive storage, it may be a very simple password manager. -->
Additionally, in this example a simplified workflow will be used - server code placed directly in the service definition file (in `/etc/qubes-rpc` directory).
And no separate client script will be used.
* RPC server code (*/etc/qubes-rpc/test.File*)
RPC server code (*/etc/qubes-rpc/test.File*)
#!/bin/sh
argument="$1" # service argument, also available as $QREXEC_SERVICE_ARGUMENT
if [ -z "$argument" ]; then
echo "ERROR: No argument given!"
exit 1
fi
# service argument is already sanitized by qrexec framework and it is
# guaranteed to not contain any space or /, so no need for additional path
# sanitization
cat "/home/user/rpc-file-storage/$argument"
#!/bin/sh
argument="$1" # service argument, also available as $QREXEC_SERVICE_ARGUMENT
if [ -z "$argument" ]; then
echo "ERROR: No argument given!"
exit 1
fi
cat "/home/user/rpc-file-storage/$argument"
* specific policy file in dom0 (*/etc/qubes-rpc/policy/test.File+testfile1* )
(The service argument is already sanitized by qrexec framework. It is guaranteed to not contain any spaces or slashes, so there sould be no need for additional path sanitization.)
source_vm1 target_vm allow
We'll create three policy files in dom0:
* another specific policy file in dom0 (*/etc/qubes-rpc/policy/test.File+testfile2* )
| Path to file in `dom0` | Policy contents |
|-------------------------------------------+----------------------------|
| /etc/qubes-rpc/policy/test.File | @anyvm @anyvm deny |
| /etc/qubes-rpc/policy/test.File+testfile1 | source_vm1 target_vm allow |
| /etc/qubes-rpc/policy/test.File+testfile2 | source_vm2 target_vm allow |
source_vm2 target_vm allow
invoke RPC from `source_vm1` via
* default policy file in dom0 (*/etc/qubes-rpc/policy/test.File* )
/usr/lib/qubes/qrexec-client-vm target_vm test.File+testfile1
@anyvm @anyvm deny
and we should get content of `/home/user/rpc-file-storage/testfile1` as answer.
* invoke RPC from `source_vm1` via
also possible to invoke RPC from `source_vm2` via
/usr/lib/qubes/qrexec-client-vm target_vm test.File+testfile1
/usr/lib/qubes/qrexec-client-vm target_vm test.File+testfile2
and we should get content of `/home/user/rpc-file-storage/testfile1` as answer.
* also possible to invoke RPC from `source_vm2` via
/usr/lib/qubes/qrexec-client-vm target_vm test.File+testfile2
But when invoked with other argument or from different VM, it should be denied.
But when invoked with other argument or from different VM, it should be denied.