linux-kernel - Re: IRQF_SAMPLE

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Date:	Mon, 6 Apr 2009 11:44:32 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Jeff Garzik <jeff@...zik.org>
Cc:	Robin Getz <rgetz@...ckfin.uclinux.org>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Chris Peterson <cpeterso@...terso.com>,
	Matt Mackall <mpm@...enic.com>,
	David Miller <davem@...emloft.net>
Subject: Re: IRQF_SAMPLE_RANDOM question...

On Mon, 06 Apr 2009 14:40:05 -0400
Jeff Garzik <jeff@...zik.org> wrote:

> Robin Getz wrote:
> > Although there was some discussion  
> > http://thread.gmane.org/gmane.linux.kernel/680723
> > 
> > about removing IRQF_SAMPLE_RANDOM from the remaining network drivers in May of 
> > 2008, but they still appears to be there in 2.6.29.
> > 
> > drivers/net/ibmlana.c
> > drivers/net/macb.c
> > drivers/net/3c523.c
> > drivers/net/3c527.c
> > drivers/net/netxen/netxen_nic_main.c
> > drivers/net/cris/eth_v10.c
> > drivers/net/xen-netfront.c
> > drivers/net/atlx/atl1.c
> > drivers/net/qla3xxx.c
> > drivers/net/tg3.c
> > drivers/net/niu.c
> > 
> > So what is the plan? If I send a patch to add IRQF_SAMPLE_RANDOM to others 
> > (like the Blackfin) networking drivers - will it get rejected?
> > 
> > We have lots of embedded headless systems (no keyboard/mouse, no soundcard, no 
> > video) systems with *no* sources of entropy - and people using SSL.
> > 
> > I didn't really find any docs which describe what should have 
> > IRQF_SAMPLE_RANDOM on it or not. I did find Matt Mackall describing it as:
> >> We currently assume that IRQF_SAMPLE_RANDOM means 'this is a completely
> >> trusted unobservable entropy source' which is obviously wrong for
> >> network devices but is right for some other classes of device.
> > 
> > Currently - I see most things I see using IRQF_SAMPLE_RANDOM would also fail 
> > the "completely unobservable" test. Other than the TRNG that are inside the 
> > CPU - what does pass?
> 
> IMO it's not observation but rather that a remote host is essentially 
> your source of entropy -- which means your source of entropy is 
> potentially controllable or influenced by an attacker.
> 
> Furthermore, with hardware interrupt mitigation, non-trivial traffic 
> levels can imply that interrupts are delivered with timer-based 
> regularity.  This, too, may clearly be influenced by a remote attacker.
> 
> Thus I think IRQF_SAMPLE_RANDOM should be banned from network drivers... 
>   but that is not a universal opinion.
> 
> 	Jeff

The real problem one is xen-netfront. Because 1) it is least random,
the attacker might be another VM 2) the VM is most in need of random
samples because it doesn't have real hardware.
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