linux-kernel - Re: [PATCH v5] printk: hash addresses printed with %p

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Message-ID: <20171022233216.GM29874@eros>
Date:   Mon, 23 Oct 2017 10:32:16 +1100
From:   "Tobin C. Harding" <me@...in.cc>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        kernel-hardening@...ts.openwall.com, Theodore Ts'o <tytso@....edu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.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 <wilal.deacon@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        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 v5] printk: hash addresses printed with %p

On Thu, Oct 19, 2017 at 07:49:06AM +0200, Jason A. Donenfeld wrote:
> A small detail carried over from the other thread:
> 
> >
> > but a bigger problem might the following thing:
> >
> > vscnprintf()
> >  pointer()
> >   ptr_to_id()
> >    initialize_ptr_secret()
> >     get_random_bytes()
> >      _get_random_bytes()
> >       extract_crng()
> >        _extract_crng()
> >         spin_lock_irqsave(&crng->lock, flags);   <<<<<
> >
> >
> > this, once again, can deadlock. can it? just like before:
> 
> So, actually, then, we need to do this as an initcall. Fortunately,
> that simplifies things greatly. Here's a rough sketch of what that
> looks like, which you'll probably need to debug and refine:
> 
> 
> 
> static siphash_key_t ptr_secret __ro_after_init;
> static DEFINE_STATIC_KEY_TRUE(no_ptr_secret);
> 
> static char *ptr_to_id(char *buf, char *end, void *ptr, struct printf_spec spec)
> {
>     if (static_branch_unlikely(&no_ptr_secret))
>         return "(pointer value)";
> 
>     hashval = ....
> 
> }
> 
> static void fill_random_ptr_key(struct random_ready_callback *rdy)
> {
>     get_random_bytes(&ptr_secret, sizeof(ptr_secret));
>     static_branch_disable(&no_ptr_secret);
> }
> 
> static struct random_ready_callback random_ready = {
>     .func = fill_random_ptr_key
> };
> 
> static int __init initialize_ptr_random(void)
> {
>     int ret = add_random_ready_callback(&random_ready);
> 
>     if (!ret)
>         return 0;
>     else if (ret == -EALREADY) {
>         fill_random_ptr_key(&random_ready);
>         return 0;
>     }
> 
>     return ret;
> }
> early_initcall(initialize_ptr_random);

Thanks for this Jason. This is _conceptually_ what I wanted since before v1, I obviously did not ask
the right questions. Not to worry, we got there in the end. The process works, thanks to every
bodies patience :)

Implemented for v6 as suggested (including __read_mostly), you even got it fast for the usual
case. Thanks, I learned a whole bunch from this email.

Tobin.