On 05/11/2014 04:01 PM, Stephan Mueller wrote:
> Hi Peter,
>
> some time back when the RDRAND instruction was debated, a patch was offered
> for driver/char/random.c that in essence turned /dev/random into a frontend
> for RDRAND in case that instruction was available. The patch kind of
> monopolized the noise sources such that if a user space "random number hog"
> pulled data from /dev/random endlessly, the (almost) only noise source was
> RDRAND. As that patch treated RDRAND to provide entropy, the blocking
> behavior
> went away for /dev/random.
>
This is false in a number of ways.
First of all... we NEVER pulled either /dev/random or /dev/urandom
directly from RDRAND. We used RDRAND directly for kernel internal
randomness uses. Users did object to this.
> That patch did not sit well with some developers and it got finally changed
> such that the output of RDRAND is now just XORed with the output of the
> "classic" /dev/random behavior -- /dev/random is still blocking.
Mixing in RDRAND into /dev/random and /dev/urandom is actually
> With the current development cycle for 3.15, the function arch_random_refill
> is added as presented in [1]. It now uses RDSEED instead of RDRAND. Yet, the
> way this function is called in random_read seems (as I have no system with an
> RDSEED, I cannot test) to show the very same behavior as the aforementioned
> RDRAND patch: the blocking behavior of /dev/random will be gone and RDSEED
> will monopolize the noise sources in case of a user space hog.
There is a huge difference between this and what people objected to
earlier: we filter everything through the kernel random number pool
system, which would require a herculean mathematical effort to reverse
even if the output of RDSEED was 100% predictable.
> Note, I do not see an issue with the patch that adds RDSEED as part of
> add_interrupt_randomness outlined in [2]. The reason is that this patch does
> not monopolizes the noise sources.
>
> I do not want to imply that Intel (or any other chip manufacturer that will
> hook into arch_random_refill) intentionally provides bad entropy (and this
> email shall not start a discussion about entropy again), but I would like to
> be able to only use noise sources that I can fully audit. As it is with
> hardware, I am not able to see what it is doing.
I have to point out the irony in this given your previous proposals,
however...
> Thus, may I ask that arch_random_refill is revised such that it will not
> monopolize the noise sources? If somebody wants that, he can easily use rngd.
Feel free to build the kernel without CONFIG_ARCH_RANDOM, or use the
"nordrand" option to the kernel. These options are there for a reason.
Now when you mention it, though, the nordrand option should turn off
RDSEED as well as RDRAND. It currently doesn't; that is a bug, plain
and simple.
-hpa
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