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Date: Fri, 27 Jul 2018 11:41:05 -0700
From: Mark Salyzyn <>
To: Steven Rostedt <>
Cc: Nick Desaulniers <>, Jann Horn <>,,, 
	Kees Cook <>, Mark Salyzyn <>, 
	LKML <>,, 
	Android Kernel Team <>,,
Subject: Re: [PATCH] tracing: do not leak kernel addresses

Any system can chose to change the permissions of a sysfs node, default,
DAC (and MAC) is just layers of multi-level security (or lack thereof). As
well intentioned as a default DAC is in the kernel, leaking kernel
addresses is still an attack surface that we want to close tightly.

For instance on Android:

     chmod 0755 /sys/kernel/debug/tracing

is in the common init.rc file ...

If DAC has been adjusted at runtime to permit access to the node, I would
think that if the caller does not have all the credentials/capabilities, we
do want the addresses to be abstracted by the kernel.

-- Mark

On Fri, Jul 27, 2018 at 11:31 AM, Steven Rostedt <>

> On Fri, 27 Jul 2018 11:13:51 -0700
> Nick Desaulniers <> wrote:
> > I found the internal bug report (reported Jan '17, you'll have to
> > forgive me if my memory of the issue is hazy, or if the fix used at
> > the time wasn't perfect), which was reported against the Nexus 6.
> > >From the report, it was possible to `cat
> > /sys/kernel/debug/tracing/printk_formats` without being root, which I
> > can't do on my workstations much more modern kernel (Nexus 6 was
> > 3.10).  So I guess the question is what governs access to files below
> > /sys/kernel/debug, and why was it missing from those kernels?  I
> > assume some check was added, but either not backported to 3.10 stable
> > (or more likely not pulled in to Nexus 6's kernel through stable;
> > Android is now in a much better place for that kind of issue).
> As of commit 82aceae4f0d4 ("debugfs: more tightly restrict default
> mount mode") /sys/kernel/debug has been default mounted as 0700 (root
> only). But that was introduced in 3.7. Not sure why your 3.10 kernel
> didn't have that. Perhaps there's another commit that fixed
> permissions not being inherited?
> -- Steve
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