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Date: Tue, 17 Oct 2017 17:13:10 -0700
From: Kees Cook <keescook@...omium.org>
To: "Tobin C. Harding" <me@...in.cc>
Cc: Steven Rostedt <rostedt@...dmis.org>, 
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Paolo Bonzini <pbonzini@...hat.com>, 
	Tycho Andersen <tycho@...ker.com>, "Roberts, William C" <william.c.roberts@...el.com>, 
	Tejun Heo <tj@...nel.org>, Jordan Glover <Golden_Miller83@...tonmail.ch>, 
	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>, Catalin Marinas <catalin.marinas@....com>, 
	Will Deacon <will.deacon@....com>, Chris Fries <cfries@...gle.com>, 
	Dave Weinstein <olorin@...gle.com>, Daniel Micay <danielmicay@...il.com>, 
	Djalal Harouni <tixxdz@...il.com>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] printk: hash addresses printed with %p

On Tue, Oct 17, 2017 at 4:15 PM, Tobin C. Harding <me@...in.cc> wrote:
> On Tue, Oct 17, 2017 at 09:31:19AM -0400, Steven Rostedt wrote:
>> On Tue, 17 Oct 2017 15:52:51 +1100
>> "Tobin C. Harding" <me@...in.cc> wrote:
>>
>> > Currently there are many places in the kernel where addresses are being
>> > printed using an unadorned %p. Kernel pointers should be printed using
>> > %pK allowing some control via the kptr_restrict sysctl. Exposing addresses
>> > gives attackers sensitive information about the kernel layout in memory.
>> >
>> > We can reduce the attack surface by hashing all addresses printed with
>> > %p. This will of course break some users, forcing code printing needed
>> > addresses to be updated.
>> >
>> > For what it's worth, usage of unadorned %p can be broken down as follows
>> >
>> >     git grep '%p[^KFfSsBRrbMmIiEUVKNhdDgCGO]' | wc -l
>>
>> Does %p[FfSs] leak addresses? Well, I guess it does if they are not
>> found in kallsyms, but otherwise you have:
>>
>>   function+0x<offset>
>
> You are correct %pF and %pS print an offset. Does this provide an attack vector,
> I didn't think so but I'm no security expert. If they do then we need to amend
> those calls also.

They haven't traditionally been a big deal. If they turn out to be
problematic, we can deal with it then, IMO.

-Kees

-- 
Kees Cook
Pixel Security

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