Currently, we use PermitRead/PermitWrite/PermitCert permission flags to determine which operations are allowed for a LocalAPI client.
These checks are performed when localapi.Handler handles a request. Additionally, certain operations (e.g., changing the serve config)
requires the connected user to be a local admin. This approach is inherently racey and is subject to TOCTOU issues.
We consider it to be more critical on Windows environments, which are inherently multi-user, and therefore we prevent more than one
OS user from connecting and utilizing the LocalBackend at the same time. However, the same type of issues is also applicable to other
platforms when switching between profiles that have different OperatorUser values in ipn.Prefs.
We'd like to allow more than one Windows user to connect, but limit what they can see and do based on their access rights on the device
(e.g., an local admin or not) and to the currently active LoginProfile (e.g., owner/operator or not), while preventing TOCTOU issues on Windows
and other platforms. Therefore, we'd like to pass an actor from the LocalAPI to the LocalBackend to represent the user performing the operation.
The LocalBackend, or the profileManager down the line, will then check the actor's access rights to perform a given operation on the device
and against the current (and/or the target) profile.
This PR does not change the current permission model in any way, but it introduces the concept of an actor and includes some preparatory
work to pass it around. Temporarily, the ipnauth.Actor interface has methods like IsLocalSystem and IsLocalAdmin, which are only relevant
to the current permission model. It also lacks methods that will actually be used in the new model. We'll be adding these gradually in the next
PRs and removing the deprecated methods and the Permit* flags at the end of the transition.
Updates tailscale/corp#18342
Signed-off-by: Nick Khyl <nickk@tailscale.com>
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