Can KEYCTL_SESSION_TO_PARENT be dropped entirely? -- was Re: [PATCH v2 1/2] KEYS: use synchronous task work for changing parent credentials

[Jann Horn]

On Thu, Aug 15, 2024 at 9:46 PM David Howells <dhowells at redhat.com> wrote:
> Jann Horn <jannh at google.com> wrote:
>
> > Rewrite keyctl_session_to_parent() to run task work on the parent
> > synchronously, so that any errors that happen in the task work can be
> > plumbed back into the syscall return value in the child.
>
> The main thing I worry about is if there's a way to deadlock the child and the
> parent against each other.  vfork() for example.

Yes - I think it would work fine for scenarios like using
KEYCTL_SESSION_TO_PARENT from a helper binary against the shell that
launched the helper (which I think is the intended usecase?), but
there could theoretically be constellations where it would cause an
(interruptible) hang if the parent is stuck in
uninterruptible/killable sleep.

I think vfork() is rather special in that it does a killable wait for
the child to exit or execute; and based on my understanding of the
intended usecase of KEYCTL_SESSION_TO_PARENT, I think normally
KEYCTL_SESSION_TO_PARENT would only be used by a child that has gone
through execve?


> > +     if (task_work_cancel(parent, &ctx.work)) {
> > +             /*
> > +              * We got interrupted and the task work was canceled before it
> > +              * could execute.
> > +              * Use -ERESTARTNOINTR instead of -ERESTARTSYS for
> > +              * compatibility - the manpage does not list -EINTR as a
> > +              * possible error for keyctl().
> > +              */
>
> I think returning EINTR is fine, provided that if we return EINTR, the change
> didn't happen.  KEYCTL_SESSION_TO_PARENT is only used by the aklog, dlog and
> klog* OpenAFS programs AFAIK, and only if "-setpag" is set as a command line
> option.  It also won't be effective if you strace the program.

Ah, I didn't even know about those.

The users I knew of are the command-line tools "keyctl new_session"
and "e4crypt new_session" (see
https://codesearch.debian.net/search?q=KEYCTL_SESSION_TO_PARENT&literal=1,
which indexes code that's part of Debian).

> Maybe the AFS people can say whether it's even worth keeping the functionality
> rather than just dropping KEYCTL_SESSION_TO_PARENT?

I think this would break the tools "keyctl new_session" and "e4crypt
new_session" - though I don't know if anyone actually uses those
invocations.