Problems using RemoteForward for gpg-agent with multiple sessions

[Brandon Cheng]

On Mon, 13 Jun 2022, Damien Miller wrote:

> On Fri, 10 Jun 2022, Brandon Cheng wrote:
> > To complete this option, how might the server determine the unique
> > path? I'm leaning towards SetEnv and updating it to understand
> > %-expansions. (If it doesn't already.)
> >
> >   Host example
> >     RemoteForward /tmp/%R.sock /home/local/.gnupg/S.gpg-agent.extra
> >     SetEnv SSH_R_EXPANSION=%R
>
> I think that could be made to work - we'd need ssh's main() to
> generate a single random token per connection, though it might be
> possible for a user to shoot themselves in the foot if they reused
> the token for multiple things and exposed it to an on-host attacker.
> We generally avoid leaving footguns around.

I think you're right. Although the random token derived socket paths
don't need to be secret for this use case, I could see users relying on
the random token for other use cases requiring secrecy.

Thanks for keeping security in mind. Although this could work, I'd
be nervous about creating a new common pattern of security
vulnerabilities.

On Mon, 13 Jun 2022, Damien Miller wrote:

> Another option: remote port-forwarding has a notion of
> server-allocated ports, that are communicated back to the client. We
> could do something similar for Unix domain sockets too, but with a
> server-generated /tmp path instead of a port.

> The downside is that this is only really useful when used in
> conjunction with ControlPath, and they take a fair bit more connection
> setup.

Does the server-generated path need to be communicated back to the
client? Taking a spin on the "Possibility 1: New RemoteForward syntax"
suggestion from the first email, could the server-generated tmp path be
placed in an $SSH_REMOTE_FORWARD_SOCKS_DIR environment variable?

  Host example
    RemoteForward 0:example-foo /home/local/.foo.sock
    RemoteForward 0:example-bar /home/local/.bar.sock

  $ ssh example
  $ echo $SSH_REMOTE_FORWARD_SOCKS_DIR
  /tmp/ssh-UIzrILLqkZ
  $ ls -l $SSH_REMOTE_FORWARD_SOCKS_DIR
  example-foo.sock
  example-bar.sock

Some immediate caveats I can think of:

  - This deviates from the TCP port behavior, which can be surprising.
  - All forwarded sockets share the same server-generated path. Is this
    a security concern? I think any unique path -> envvar approach would
    leak RemoteForward socket paths to on-host attackers anyway, even if
    each RemoteForward had a unique environment variable. By design this
    is intentionally available to on-host processes.