Re: [PATCH v2 2/3] seccomp: Introduce addfd ioctl to seccomp user notifier

[Christian Brauner]

On Sat, May 30, 2020 at 05:17:24AM +0200, Jann Horn wrote:
> On Sat, May 30, 2020 at 4:43 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> > I mean, yes, that's certainly better, but it just seems a shame that
> > everyone has to do the get_unused/put_unused dance just because of how
> > SCM_RIGHTS does this weird put_user() in the middle.
> >
> > Can anyone clarify the expected failure mode from SCM_RIGHTS? Can we
> > move the put_user() after instead?
> 
> Honestly, I think trying to remove file descriptors and such after
> -EFAULT is a waste of time. If userspace runs into -EFAULT, userspace

Agreed, we've never bothered with trying to recover from EFAULT. Just
look at kernel/fork.c:_do_fork():
	if (clone_flags & CLONE_PARENT_SETTID)
		put_user(nr, args->parent_tid);

we don't even bother even though we technically could.

> is beyond saving and can't really do much other than exit immediately.
> There are a bunch of places that will change state and then throw
> -EFAULT at the end if userspace supplied an invalid address, because
> trying to hold locks across userspace accesses just in case userspace
> supplied a bogus address is kinda silly (and often borderline
> impossible).
> 
> You can actually see that even scm_detach_fds() currently just
> silently swallows errors if writing some header fields fails at the
> end.

There's really no point in trying to save a broken scm message imho.