linux-kernel - Re: Hashed pointer issues

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Message-ID: <20180430124135.0cce92e3@gandalf.local.home>
Date:   Mon, 30 Apr 2018 12:41:35 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        tcharding <me@...in.cc>
Subject: Re: Hashed pointer issues

On Mon, 30 Apr 2018 16:31:52 +0000
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Mon, Apr 30, 2018 at 9:11 AM Kees Cook <keescook@...omium.org> wrote:
> 
> > I (or other folks?) had proposed this before, but, AIUI, Linus remains
> > opposed.  
> 
> Yeah, I hate this, because it will make people paper over their problems by
> just booting with that option.
> 
> I think it should just be fixed.
> 
> Is there really any reason why trace buffers have to be dumped so early
> that the entropy hasn't even taken yet?

In this case, an RCU stall was triggering and a "dump on oops" would
dump the trace buffers, (before entropy was established).

> 
> And if we really want a command line option, can we make that still hash
> the pointer, just force the entropy early. That way kernel developers that
> test that command line option are still testing the *hashing*, they just
> are missing the good entropy.
> 

That may work too. Even with bad entropy, pointers at start up are hard
to come by, and I can't see a hacker being able to use them as it only
happens once during boot.

-- Steve