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Date: Mon, 26 Jun 2017 19:38:06 +0200
From: Stephan Müller <>
To: "Nicholas A. Bellinger" <>
Cc: Lee Duncan <>, "Jason A. Donenfeld" <>, Theodore Ts'o <>, Linux Crypto Mailing List <>, LKML <>,, Greg Kroah-Hartman <>, David Miller <>, Eric Biggers <>, Chris Leech <>,, target-devel <>
Subject: Re: Re: [PATCH v4 06/13] iscsi: ensure RNG is seeded before use

Am Montag, 26. Juni 2017, 03:23:09 CEST schrieb Nicholas A. Bellinger:

Hi Nicholas,

> Hi Stephan, Lee & Jason,
> (Adding target-devel CC')
> Apologies for coming late to the discussion.  Comments below.
> On Sun, 2017-06-18 at 10:04 +0200, Stephan Müller wrote:
> > Am Samstag, 17. Juni 2017, 05:45:57 CEST schrieb Lee Duncan:
> > 
> > Hi Lee,
> > 
> > > In your testing, how long might a process have to wait? Are we talking
> > > seconds? Longer? What about timeouts?
> > 
> > In current kernels (starting with 4.8) this timeout should clear within a
> > few seconds after boot.
> > 
> > In older kernels (pre 4.8), my KVM takes up to 90 seconds to reach that
> > seeding point. I have heard that on IBM System Z this trigger point
> > requires minutes to be reached.
> I share the same concern as Lee wrt to introducing latency into the
> existing iscsi-target login sequence.
> Namely in the use-cases where a single node is supporting ~1K unique
> iscsi-target IQNs, and fail-over + re-balancing of IQNs where 100s of
> login attempts are expected to occur in parallel.
> If environments exist that require non trivial amounts of time for RNG
> seeding to be ready for iscsi-target usage, then enforcing this
> requirement at iscsi login time can open up problems, especially when
> iscsi host environments may be sensitive to login timeouts, I/O
> timeouts, et al.
> That said, I'd prefer to simply wait for RNG to be seeded at modprobe
> iscsi_target_mod time, instead of trying to enforce randomness during
> login.
> This way, those of use who have distributed storage platforms can know
> to avoid putting a node into a state to accept iscsi-target IQN export
> migration, before modprobe iscsi_target_mod has successfully loaded and
> RNG seeding has been confirmed.
> WDYT..?

We may have a chicken and egg problem when the wait is at the modprobe time. 
Assume the modprobe happens during initramfs time to get access to the root 
file system. In this case, you entire boot process will lock up for an 
indefinite amount of time. The reason is that in order to obtain events 
detected by the RNG, devices need to be initialized and working. Such devices 
commonly start working after the the root partition is mounted as it contains 
all drivers, all configuration information etc.

Note, during the development of my /dev/random implementation, I added the 
getrandom-like blocking behavior to /dev/urandom (which is the equivalent to 
Jason's patch except that it applies to user space). The boot process locked 
up since systemd wanted data from /dev/urandom while it processed the 
initramfs. As it did not get any, the boot process did not commence that could 
deliver new events to be picked up by the RNG.

As I do not have such an iscsi system, I cannot test Jason's patch. But maybe 
the mentioned chicken-and-egg problem I mentioned above is already visible 
with the current patch as it will lead to a blocking of the mounting of the 
root partition in case the root partition is on an iscsi target.


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