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Date: Wed, 4 Oct 2017 10:28:13 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Kees Cook <keescook@...omium.org>
Cc: "Tobin C. Harding" <me@...in.cc>, Greg KH <gregkh@...uxfoundation.org>, 
	Petr Mladek <pmladek@...e.com>, Joe Perches <joe@...ches.com>, Ian Campbell <ijc@...lion.org.uk>, 
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>, 
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, LKML <linux-kernel@...r.kernel.org>, 
	Catalin Marinas <catalin.marinas@....com>, Will Deacon <will.deacon@....com>, 
	Steven Rostedt <rostedt@...dmis.org>, William Roberts <william.c.roberts@...el.com>, 
	Chris Fries <cfries@...gle.com>, Dave Weinstein <olorin@...gle.com>
Subject: Re: [RFC V2 4/6] lib: vsprintf: default
 kptr_restrict to the maximum value

On Wed, Oct 4, 2017 at 10:08 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> So I honestly doubt the value of kptr_restrict. Any *sane* policy
> pretty much has to be in the caller, and by thinking about what you
> print out. IOW, things like proc_pid_wchan().

Looking at /proc/kallsyms is actually a prime example of this.

IOW, the old "open /proc/kallsyms as a normal user, then make it stdin
for some suid-root program that can be fooled to output it probably
works on it.

So kptr_restrict ends up being entirely the wrong thing to do there.

The only value in kptr_restrict ends up being as a complete hack,
where you say "I trust nobody" and make %p almost entirely useless.

And as mentioned, that will just make people use %x instead, or
randomly sprinkle the new "I didn't really mean this" modifiers like
the already discussed pr_debug() case.

So even when kptr_restrict "works", it ends up just fighting itself.
And most of the time it just doesn't work.

                Linus

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