linux-kernel - Re: [kernel-hardening] [PATCH v3] printk: hash addresses printed with %p

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Message-ID: <CAHmME9oGm3uHx40Qd=VfhhqPkh2mPy+cOqAodynnEONnKOe-3g@mail.gmail.com>
Date:   Wed, 18 Oct 2017 02:59:17 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     "Tobin C. Harding" <me@...in.cc>
Cc:     kernel-hardening@...ts.openwall.com,
        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 <will.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: [kernel-hardening] [PATCH v3] printk: hash addresses printed with %p

Hi Tobin,

You submitted v3 without replying to my v2 comments. I'll give a
condensed version of those here for convenience.

> diff --git a/include/linux/siphash.h b/include/linux/siphash.h
> +unsigned long siphash_1ulong(const unsigned long a, const siphash_key_t *key);

Don't add this function here. It's not the right signature anyway.
> +unsigned long siphash_1ulong(const unsigned long first, const siphash_key_t *key)
> +{

Likewise, remove this.

> +/* Maps a pointer to a 32 bit unique identifier. */
> +static char *ptr_to_id(char *buf, char *end, void *ptr, struct printf_spec spec)
> +{
> +       static siphash_key_t ptr_secret __read_mostly;
> +       static atomic_t have_key = ATOMIC_INIT(0);
> +       unsigned long hashval;
> +
> +        if (atomic_xchg(&have_key, 1) == 0)
> +               get_random_bytes(&ptr_secret, sizeof(ptr_secret));

This isn't safe. Initialize ptr_secret in the callback function
provided to add_random_ready_callback. Before ptr_secret is
initialized, you should simply return a stub literal string from that
function of something like "(pointer value)".

> +       hashval = siphash_1ulong((unsigned long)ptr, &ptr_secret);

Replace this with:

#ifdef CONFIG_64BIT
hashval = (unsigned long)siphash_1u64((u64)ptr, key);
#else
hashval = (unsigned long)siphash_1u32((u32)ptr, key);
#endif

However, in another thread, Linus mentioned that he'd prefer all the
obfuscated values actually be 32-bit. So, this then looks like:

unsigned int hashval;
...
#ifdef CONFIG_64BIT
hashval = (unsigned int)siphash_1u64((u64)ptr, key);
#else
hashval = (unsigned int)siphash_1u32((u32)ptr, key);
#endif