linux-kernel - Re: [PATCH,RFC] random: collect cpu randomness

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Message-ID: <20140203163729.GA13634@thunk.org>
Date:	Mon, 3 Feb 2014 11:37:29 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Jörn Engel <joern@...fs.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	macro@...ux-mips.org, ralf@...ux-mips.org, dave.taht@...il.com,
	blogic@...nwrt.org, andrewmcgr@...il.com, smueller@...onox.de,
	geert@...ux-m68k.org, tg@...bsd.de
Subject: Re: [PATCH,RFC] random: collect cpu randomness

On Mon, Feb 03, 2014 at 10:50:42AM -0500, Jörn Engel wrote:
> If the measurement event is an interrupt and the CPU has a
> cycle-counter, you are set.  On interesting systems lacking a
> cycle-counter, we still have a high-resolution counter or sorts that
> is the CPU itself.
> 
> Instruction pointer and stack pointer for both kernel and userland are
> one way to read out the "counter".  Main problem here are tight loops
> where your "counter" is not high-resolution at all.  But something
> within the CPU is constantly changing.  And that something tends to be
> contained in the registers.
> 
> How about taking the saved registers from the interrupted CPU, xor'ing
> them all and calling the result random_get_entropy() on systems
> lacking a good cycles-counter?

So we could take the struct pt_regs which we get from get_irq_regs(),
XOR them together and use them to feed into input[2] amd input[3] in
add_interrupt_randomness().  Or some other way of distributing the
values of all of the irq registers into the __u32 input[4] array.

That would probably be a good and useful thing to do.  Was that
basically what you were suggesting?

						- Ted
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