linux-ext4 - Re: [PATCH RFC v4 1/1] random: WARN on large getrandom() waits and introduce getrandom2()

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Date:   Fri, 20 Sep 2019 23:15:01 +0500
From:   "Alexander E. Patrakov" <patrakov@...il.com>
To:     Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Ahmed S. Darwish" <darwish.07@...il.com>,
        Lennart Poettering <mzxreary@...inter.de>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Willy Tarreau <w@....eu>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-man <linux-man@...r.kernel.org>
Subject: Re: [PATCH RFC v4 1/1] random: WARN on large getrandom() waits and
 introduce getrandom2()

20.09.2019 22:52, Andy Lutomirski пишет:
> I think that, given existing software, we should make two or three
> changes to fix the basic problems here:
> 
> 1. Add GRND_INSECURE: at least let new applications do the right thing
> going forward.
> 
> 2. Fix what is arguably a straight up kernel bug, not even an ABI
> issue: when a user program is blocking in getrandom(..., 0), the
> kernel happily sits there doing absolutely nothing and deadlocks the
> system as a result.  This IMO isn't an ABI issue -- it's an
> implementation problem.  How about we make getrandom() (probably
> actually wait_for_random_bytes()) do something useful to try to seed
> the RNG if the system is otherwise not doing IO.
> 
> 3. Optionally, entirely in user code: Get glibc to add new *library*
> functions: getentropy_secure_blocking() and getentropy_insecure() or
> whatever they want to call them.  Deprecate getentropy().
> 
> I think #2 is critical.  Right now, suppose someone has a system that
> neets to do a secure network request (a la Red Hat's Clevis).  I have
> no idea what Clevis actually does, but it wouldn't be particularly
> crazy to do a DH exchange or sign with an EC key to ask some network
> server to help unlock a dm-crypt volume.  If the system does this at
> boot, it needs to use getrandom(..., 0), GRND_EXPLICIT, or whatever,
> because it NEEDS a secure random number.  No about of ABI fiddling
> will change this.  The kernel should *work* in this case rather than
> deadlocking.

Let me express a little bit of disagreement with the logic here.

I do agree that #2 is critical, and the Clevis use case is a perfect 
example why it is important. I doubt that it is solvable without 
trusting jitter entropy, or without provoking a dummy read on a random 
block device, just for timings, or maybe some other interaction with the 
external world - but Willy already said "it seems fishy". However, _if_ 
it is solved, then we don't need GRND_INSECURE, because solving #2 is 
equivalent to magically making secure random numbers always available.

-- 
Alexander E. Patrakov


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