linux-kernel - Re: [PATCH v4 3/3] vsprintf: Use hw RNG for ptr

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Date:   Wed, 16 May 2018 07:09:55 +1000
From:   "Tobin C. Harding" <me@...in.cc>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Theodore Ts'o <tytso@....edu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/3] vsprintf: Use hw RNG for ptr_key

On Tue, May 15, 2018 at 09:47:44AM -0400, Steven Rostedt wrote:
> On Tue, 15 May 2018 13:06:26 +1000
> "Tobin C. Harding" <me@...in.cc> wrote:
> 
> > Currently we must wait for enough entropy to become available before
> > hashed pointers can be printed.  We can remove this wait by using the
> > hw RNG if available.
> > 
> > Use hw RNG to get keying material.
> > 
> > Suggested-by: Kees Cook <keescook@...omium.org>
> > Signed-off-by: Tobin C. Harding <me@...in.cc>
> > ---
> >  lib/vsprintf.c | 19 ++++++++++++++++---
> >  1 file changed, 16 insertions(+), 3 deletions(-)
> > 
> > diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> > index b82f0c6c2aec..3697a19c2b25 100644
> > --- a/lib/vsprintf.c
> > +++ b/lib/vsprintf.c
> > @@ -1657,9 +1657,8 @@ char *device_node_string(char *buf, char *end, struct device_node *dn,
> >  static bool have_filled_random_ptr_key __read_mostly;
> >  static siphash_key_t ptr_key __read_mostly;
> >  
> > -static void fill_random_ptr_key(struct random_ready_callback *unused)
> > +static void ptr_key_ready(void)
> >  {
> > -	get_random_bytes(&ptr_key, sizeof(ptr_key));
> >  	/*
> >  	 * have_filled_random_ptr_key==true is dependent on get_random_bytes().
> >  	 * ptr_to_id() needs to see have_filled_random_ptr_key==true
> 
> Nothing to do with this patch, but I believe there's a missing memory
> barrier in the code.
> 
> Right after this we have:
> 
> 	smp_mb();
> 	WRITE_ONCE(have_filled_random_ptr_key, true);
> 
> Where the comment says that have_filled_random_ptr_key must be set
> after ptr_key has been updated. But there's no memory barrier on the
> read side. In fact, I think this could be a smp_wmb() instead of a
> smp_mb(). The read side has:
> 
> 	if (unlikely(!have_filled_random_ptr_key))
> 		return string(buf, end, "(ptrval)", spec);
> 
> /* Missing memory barrier smp_rmb() here. */
> 
> 	hashval = (unsigned long)siphash_1u64((u64)ptr, &ptr_key);
> 
> Thus we can have something like:
> 
> 	CPU0				CPU1
> 	----				----
> 				   load ptr_key = 0
>    store ptr_key = random
>    smp_mb()
>    store have_filled_random_ptr_key
> 
> 				   load have_filled_random_ptr_key = true
> 
> 				    BAD BAD BAD!
> 
> I'll send a patch.

Awesome reviewing.  Thanks for catching this.


	Tobin