x/crypto/ssh: allow to change allowed authentication methods

[drakkan]

Proposal Details

Now that CL 516355 has been merged a server can dynamically change allowed authentication methods for a given user by returning a PartialSuccessError.

When a PartialSuccessError is returned, the "partial success" boolean field in SSH_MSG_USERAUTH_FAILURE is set to true.

I propose adding a new error that allows to change authentication methods without returning a partial success error

// ChangeAuthMethodsError can be returned by any of the [ServerConfig]
// authentication callbacks to change the allowed authentication methods.
type ChangeAuthMethodsError struct {
	Next ServerAuthCallbacks
}

func (e *ChangeAuthMethodsError) Error() string {
	// We return a generic error string.
	return "ssh: authentication failed"
}

This is a cleaner approach to allowing modification of authentication methods and will reuse the logic already implemented for PartialSuccessError.

[gopherbot]

Change https://go.dev/cl/578735 mentions this issue: ssh: allow to dynamically change authentication methods

[ianlancetaylor]

CC @golang/security

[awly]

The idea behind the proposal is sound: having a way to switch auth methods without returning PartialSuccess.

I suggest creating an interface (internal or exported) that both PartialSuccessError and ChangeAuthMethodsError implement to use in the type assertion here.

// ChangeAuthMethods can be implemented by any error returned from auth callbacks to 
// change the set of auth callbacks used for further attempts on this connection.
type ChangeAuthMethods interface {
    NextAuthMethods() ServerAuthCallbacks
}

func (e *PartialSuccessError) NextAuthMethods() ServerAuthCallbacks { return e.Next }
func (e *ChangeAuthMethodsError) NextAuthMethods() ServerAuthCallbacks { return e.Next }

An exported interface has the benefit of using error values to affect other behaviors, like sending banner messages.

[FiloSottile]

I don't think a ChangeAuthMethods interface composes well with BannerError for example, because we'd have to add NextAuthMethods to BannerError ourselves.

Instead, we already have a mechanism to nest error types, which is Unwrap.

We can document that we check for both BannerError, PartialSuccessError, and ChangeAuthMethodsError with errors.As, and then implementations can just compose them by nesting a ChangeAuthMethodsError in a BannerError (or more complex custom nesting by using their own type which Unwraps to all of those).

[awly]

Good point, Unwrap is a better way to combine errors. Should we implement Unwrap for all 3 proposed error types, so they can be combined in an arbitrary order?

[FiloSottile]

I think Unwrap makes sense on errors that are likely to be wrappers, and it feels like BannerError is much more likely to be a wrapper ("something else went wrong and I want to send you a banner to explain it") than PartialSuccessError and ChangeAuthMethodsError (which often will just be "everything is fine, I just learned more about you and want you to continue with these auth methods").

Also, is a PartialSuccessError that wraps a non-nil error... not a partial success? It'd be a bit confusing.

The nice thing about errors.As is that if you do need to make PartialSuccessError wrap something, you can implement your own partialSuccessErrorWrapper type with a As(interface{}) bool method (or your own error type that returns a PartialSuccessError and another error from an Unwrap() []error method).

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[rsc]

@drakkan and @FiloSottile what is the current status of this discussion. Is there a consensus for something? If so, what specifically?

[drakkan]

@rsc I think we should implement the original proposal and not add Unwrap to ChangeAuthMethodsError and PartialSuccessError.

I think it makes sense to check for both ChangeAuthMethodsError and PartialSuccessError using errors.As. The use case is if you want to return a partial success or change the allowed authentication methods and at the same time you want to send a banner with some explanations to the user: in this case the authentication callback can return a BannerError wrapping ChangeAuthMethodsError or PartialSuccessError. However this is an implementation detail we can discuss during the CL review