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qrexec: add info about new DispVM options in Qubes 4.0

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Marek Marczykowski-Górecki 2016-08-17 01:01:42 +02:00
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services/qrexec3.md
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@ -127,10 +127,12 @@ means "new VM created for this particular request," so it is never a
source of request). Currently there is no way to specify source VM by
type. Whenever a rpc request for action X is received, the first line in
`/etc/qubes-rpc/policy/X` that match srcvm/destvm is consulted to determine
whether to allow rpc, what user account the program should run in target
VM under, and what VM to redirect the execution to. If the policy file does
not exits, user is prompted to create one; if still there is no policy file
after prompting, the action is denied.
whether to allow rpc, what user account the program should run in target VM
under, and what VM to redirect the execution to. Note that if the request is
redirected (`target=` parameter), policy action remains the same - even if
there is another rule which would otherwise deny such request. If the policy
file does not exits, user is prompted to create one; if still there is no
policy file after prompting, the action is denied.
In the target VM, the `/etc/qubes-rpc/RPC_ACTION_NAME` must exist, containing
the file name of the program that will be invoked, or being that program itself
@ -156,6 +158,31 @@ be fatal to Qubes security. On the other hand, this mechanism allows to
delegate processing of untrusted input to less privileged (or throwaway)
AppVMs, thus wise usage of it increases security.
### Extra keywords available in Qubes 4.0 and later
**This section is about not yet released version, some details may change**
In Qubes 4.0, target VM can be specified also as `$dispvm:DISP_VM`, which is
very similar to `$dispvm` but force using particular VM (`DISP_VM`) as a base
VM to be started as Disposable VM. For example:
anon-whonix $dispvm:anon-whonix-dvm allow
Adding such policy itself will not force usage of this particular `DISP_VM` -
it will only allow it when specified by the caller. But `$dispvm:DISP_VM` can
also be used as target in request redirection, so _it is possible_ to force
particular `DISP_VM` usage, when caller didn't specified it:
anon-whonix $dispvm allow,target=$dispvm:anon-whonix-dvm
Note that without redirection, this rule would allow using default Disposable
VM (`default_dispvm` VM property, which itself defaults to global
`default_dispvm` property).
Also note that the request will be allowed (`allow` action) even if there is no
second rule allowing calls to `$dispvm:anon-whonix-dvm`, or even if
there is a rule explicitly denying it. This is because the redirection happen
_after_ considering the action.
### Service argument in policy
Sometimes just service name isn't enough to make reasonable qrexec policy. One