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

From: Kees Cook <keescook@chromium.org>
To: Vito Caputo <vcaputo@pengaru.com>
Cc: "Jann Horn" <jannh@google.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Josh Poimboeuf" <jpoimboe@redhat.com>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"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>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Kenta.Tada@sony.com" <Kenta.Tada@sony.com>,
	"Daniel Bristot de Oliveira" <bristot@redhat.com>,
	"Michael Weiß" <michael.weiss@aisec.fraunhofer.de>,
	"Anand K Mistry" <amistry@google.com>,
	"Alexey Gladkov" <legion@kernel.org>,
	"Michal Hocko" <mhocko@suse.com>, "Helge Deller" <deller@gmx.de>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"Andrea Righi" <andrea.righi@canonical.com>,
	"Ohhoon Kwon" <ohoono.kwon@samsung.com>,
	"Kalesh Singh" <kaleshsingh@google.com>,
	"YiFei Zhu" <yifeifz2@illinois.edu>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	linux-kernel@vger.kernel.org, x86@kernel.org,
	linux-fsdevel@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH] proc: Disable /proc/$pid/wchan
Date: Thu, 23 Sep 2021 18:42:04 -0700	[thread overview]
Message-ID: <202109231839.33EF45C785@keescook> (raw)
In-Reply-To: <20210924013408.mqw42x4lhqeq5ios@shells.gnugeneration.com>

On Thu, Sep 23, 2021 at 06:34:08PM -0700, Vito Caputo wrote:
> On Thu, Sep 23, 2021 at 06:16:16PM -0700, Kees Cook wrote:
> > On Thu, Sep 23, 2021 at 05:22:30PM -0700, Vito Caputo wrote:
> > > Instead of unwinding stacks maybe the kernel should be sticking an
> > > entrypoint address in the current task struct for get_wchan() to
> > > access, whenever userspace enters the kernel?
> > 
> > wchan is supposed to show where the kernel is at the instant the
> > get_wchan() happens. (i.e. recording it at syscall entry would just
> > always show syscall entry.)
> > 
> 
> And you have the syscall # onhand when performing the syscall entry,
> no?
> 
> The point is, if the alternative is to always get 0 from
> /proc/PID/wchan when a process is sitting in ioctl(), I'd be perfectly
> happy to get back sys_ioctl.  I'm under the impression there's quite a
> bit of vendor-specific flexibility here in terms of how precise WCHAN
> is.

Oh, yeah, if you're happy with syscall-level granularity, that'd be
totally fine by me too.

> If it's possible to preserve the old WCHAN precision I'm all for it.
> But if we've become so paranoid about leaking anything about the
> kernel to userspace that this is untenable, even if it just spits out
> the syscall being performed that has value.

I'd like to find a middle ground -- wchan has always seemed like a info
leak, even with only symbols. And it doesn't help that walking the stack
from outside the current task is difficult. :)

-Kees

-- 
Kees Cook