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

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Message-Id: <7D3731D2-348F-4561-A52D-AA7DAAEE258B@amacapital.net>
Date:   Fri, 20 Sep 2019 11:29:52 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     "Alexander E. Patrakov" <patrakov@...il.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "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()



> On Sep 20, 2019, at 11:15 AM, Alexander E. Patrakov <patrakov@...il.com> wrote:
> 
> 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.
> 
> 

I beg to differ. There is a big difference between “do your best *right now*” and “give me a real secure result in a vaguely timely manner”.

For example, the former is useful for ASLR or hash table randomization. The latter is not.