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Antw: Re: [kernel-hardening] Re: [PATCH v4 06/13] iscsi: ensure RNG is seeded before use

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  Antw: Re: [kernel-hardening] Re: [PATCH v4 06/13] iscsi:  ensure RNG is seeded before use "Ulrich Windl" <Ulrich.Windl@rz.uni-regensburg.de> - 2017-07-05 09:20 +0200
    Re: Antw: Re: [kernel-hardening] Re: [PATCH v4 06/13] iscsi: ensure  RNG is seeded before use Theodore Ts'o <tytso@mit.edu> - 2017-07-05 14:40 +0200

#1681326 — Antw: Re: [kernel-hardening] Re: [PATCH v4 06/13] iscsi: ensure RNG is seeded before use

>>> Stephan Müller <smueller@chronox.de> schrieb am 26.06.2017 um 19:38 in
Nachricht <1678474.GnYBdSlWgs@tauon.chronox.de>:
> 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


I thought reads from urandom never block by definition. An older manual page
(man urandom) also says: "A  read  from  the  /dev/urandom device will not
block waiting for more entropy."

Regards,
Ulrich

> 
> 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.
> 
> Ciao
> Stephan
> 
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#1681514 — Re: Antw: Re: [kernel-hardening] Re: [PATCH v4 06/13] iscsi: ensure RNG is seeded before use

On Wed, Jul 05, 2017 at 09:03:43AM +0200, Ulrich Windl wrote:
> > 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
> 
> I thought reads from urandom never block by definition. An older manual page
> (man urandom) also says: "A  read  from  the  /dev/urandom device will not
> block waiting for more entropy."

As I said in my original message, I *tried* this as an experiment.
Because lots of security-obsessed people were disputing my
intelligence, my judgement, and in some cases, my paternity becuase I
wouldn't change /dev/urandom not to block.

So I did the experiment so I could give them hard data about why we
couldn't go down that path.

> > 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.

And indeed, making this change brick'ed at least one version of Ubuntu
and one version of CeroWRT, as reported by the kernel's 0-day testing
system.  As a result, this patch (which was always a proof of concept,
not anything I thought had any chance of going upstream), was dropped.

Since in the kernel, We Do Not Break Backwards Compatibility, this is
why we have a new interface --- getrandom(2) --- instead of changing
an existing interface.  (Well, there were multiple good reasons for
getrandom, but this was definitely one of them.)

	    	       	   	    - Ted

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