Re: [PATCH] proc: Disable /proc/$pid/wchan

[Kees Cook <keescook@chromium.org>]

On Thu, Sep 23, 2021 at 07:13:06PM -0700, Andrew Morton wrote:
> On Thu, 23 Sep 2021 16:31:05 -0700 Kees Cook <keescook@chromium.org> wrote:
> 
> > The /proc/$pid/wchan file has been broken by default on x86_64 for 4
> > years now[1].
> 
> [1] is hard to decrypt.  I think it would be better if this changelog
> were to describe the problem directly, completely and succinctly?
> 
> > As this remains a potential leak of either kernel
> > addresses (when symbolization fails) or limited observation of kernel
> > function progress, just remove the contents for good.
> > 
> > Unconditionally set the contents to "0" and also mark the wchan
> > field in /proc/$pid/stat with 0.
> > 
> > This leaves kernel/sched/fair.c as the only user of get_wchan(). But
> > again, since this was broken for 4 years, was this profiling logic
> > actually doing anything useful?
> 
> Agree that returning a hard-wired "0\n" is the way to go.

I must NAK my own patch. ;) It looks like this would be a breaking
userspace-visible change[1].

We need to fix the two bugs though:

1) wchan broken under ORC (patch exists in the thread at [1])

2) wchan leaking raw addresses (152c432b128c needs reverting from v5.12 and later)

-Kees

[1] https://lore.kernel.org/lkml/20210924054647.v6x6risoa4jhuu6s@shells.gnugeneration.com

-- 
Kees Cook