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

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Date:   Thu, 19 Oct 2017 07:49:06 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     "Tobin C. Harding" <me@...in.cc>
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

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);