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

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Message-ID: <20190204155535.GC9802@mit.edu>
Date:   Mon, 4 Feb 2019 10:55:35 -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 Sun, Feb 03, 2019 at 08:09:37AM -0500, Prarit Bhargava wrote:
> Ted, the bug I'm trying to fix is the warning:
> 
> random: get_random_bytes called from start_kernel+0x8e/0x587 with crng_init=0
> 
> during early boot.  Even with the kernel parameter the warning appears.

Sometimes the warnings are real, and shouldn't be suppressed.  A
Debian maintainer once tried to suppress a compile-time warning, and
it was disastrous for security.  :-)

What line number is that corresponding to?  It sounds like something
is trying to use get_random_bytes() before the random driver was
initialized, and so the first question is does it really need to call
get_random_bytes() then or can it be moved?

> > Also, relying on the TSC for entropy is not something we should be
> > recommending.
> 
> The current code uses the TSC.  It is not something new I'm introducing.

But we don't *rely* on it.  That's a big difference.

       	     	       	    	     - Ted