linux-kernel - Re: [PATCH 01/11] random: don't feed stack data into pool when interrupt regs NULL

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Date:	Tue, 1 Oct 2013 08:44:24 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 01/11] random: don't feed stack data into pool when
 interrupt regs NULL

On Mon, Sep 30, 2013 at 08:51:43PM +0000, Luck, Tony wrote:
> > In this case fast_mix would use two uninitialized ints from the stack
> > and mix it into the pool.
> 
> Is the concern here is that an attacker might know (or be able to control) what is on
> the stack - and so get knowledge of what is being mixed into the pool?

Yes, this is a bogus complaint.

> > In this case set the input to 0.
> 
> And the fix is to guarantee that everyone knows what is being mixed in? (!)
> 
> Wouldn't it be better to adjust the "nbytes" parameter to
> 
> 	fast_mix(..., ..., sizeof (input));
> 
> to only mix in the part of input[] that we successfully initialized?

The changes queued for the next merge window in the random tree solve
this problem slightly differently:

	...
	input[0] = cycles ^ j_high ^ irq;
	input[1] = now ^ c_high;
	ip = regs ? instruction_pointer(regs) : _RET_IP_;
	input[2] = ip;
	input[3] = ip >> 32;

	fast_mix(fast_pool, input);
	...

(Note the lack of nbytes parameter in fast_mix --- there are some
optimizations so that we mix in the changes by 32-bit words, instead
of bytes, and the number of 32-bit words is fixed to 4, since it's the
only way fast_mix is called).

_RET_IP_ isn't that much better than 0, but it's at least kernel
specific, and I figured it was better to shut up bogus warnings, as
opposed to trying to depend on stack garbage (which would likely be
kernel specific anyway).

Cheers,

						- Ted
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