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

["Tobin C. Harding" <me@tobin.cc>]

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.