linux-kernel - Re: [PATCH v2] x86, random: Fix get_random_bytes() warning in x86 start

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Date:   Fri, 1 Feb 2019 22:02:40 -0500
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Prarit Bhargava <prarit@...hat.com>
CC:     <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, <x86@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rik van Riel <riel@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Kees Cook <keescook@...omium.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Kate Stewart <kstewart@...uxfoundation.org>
Subject: Re: [PATCH v2] x86, random: Fix get_random_bytes() warning in x86
 start_kernel

On Fri, Feb 01, 2019 at 01:08:31PM -0500, Prarit Bhargava wrote:
> After 43838a23a05f ("random: fix crng_ready() test") early boot calls to
> get_random_bytes() will warn on x86 because the crng is not initialized.
> For example,
> 
> random: get_random_bytes called from start_kernel+0x8e/0x587 with crng_init=0
> 
> x86 only uses get_random_bytes() for better randomization of the stack
> canary value so the warning is of no consequence.
> 
> Test if the crng is initialized before calling get_random_bytes().  If it
> is not available then attempt to read from the hardware random generator,
> before finally using the TSC.

If you want to trust the CPU's hardware number generator, there is a
way to do this already.  Simply enable CONFIG_RANDOM_TRUST_CPU, or set
the boot command line option "random.trust_cpu=on".

Also, relying on the TSC for entropy is not something we should be
recommending.

So, NAK.

						- Ted