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Date:   Thu, 23 Sep 2021 23:04:11 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
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        "H. Peter Anvin" <hpa@zytor.com>, Jens Axboe <axboe@kernel.dk>,
        Mark Rutland <mark.rutland@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Stefan Metzmacher <metze@samba.org>,
        Andy Lutomirski <luto@kernel.org>,
        Lai Jiangshan <laijs@linux.alibaba.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
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        Michael =?iso-8859-1?Q?Wei=DF?= 
        <michael.weiss@aisec.fraunhofer.de>,
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        Ohhoon Kwon <ohoono.kwon@samsung.com>,
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        linux-kernel@vger.kernel.org, x86@kernel.org,
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Subject: Re: [PATCH] proc: Disable /proc/$pid/wchan
Message-ID: <202109232301.B0B9753D@keescook>
References: <20210923233105.4045080-1-keescook@chromium.org>
 <20210923191306.664d39866761778a4a6ea56c@linux-foundation.org>
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On Thu, Sep 23, 2021 at 07:13:06PM -0700, Andrew Morton wrote:
> On Thu, 23 Sep 2021 16:31:05 -0700 Kees Cook <keescook@chromium.org> wrote:
> 
> > The /proc/$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