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Subject: Re: [PATCH RFC v4 1/1] random: WARN on large getrandom() waits and
 introduce getrandom2()
To:     Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Cc:     "Ahmed S. Darwish" <darwish.07@gmail.com>,
        Lennart Poettering <mzxreary@0pointer.de>,
        "Theodore Y. Ts'o" <tytso@mit.edu>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Willy Tarreau <w@1wt.eu>,
        Matthew Garrett <mjg59@srcf.ucam.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        linux-man <linux-man@vger.kernel.org>
References: <20190912034421.GA2085@darwi-home-pc>
 <20190912082530.GA27365@mit.edu>
 <CAHk-=wjyH910+JRBdZf_Y9G54c1M=LBF8NKXB6vJcm9XjLnRfg@mail.gmail.com>
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 <008f17bc-102b-e762-a17c-e2766d48f515@gmail.com>
 <20190915052242.GG19710@mit.edu>
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From:   "Alexander E. Patrakov" <patrakov@gmail.com>
Message-ID: <78a4b774-ef6b-62cb-57db-8e1ff8d29f72@gmail.com>
Date:   Fri, 20 Sep 2019 23:15:01 +0500
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20.09.2019 22:52, Andy Lutomirski =D0=BF=D0=B8=D1=88=D0=B5=D1=82:
> I think that, given existing software, we should make two or three
> changes to fix the basic problems here:
>=20
> 1. Add GRND_INSECURE: at least let new applications do the right thing
> going forward.
>=20
> 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.
>=20
> 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().
>=20
> 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=20
example why it is important. I doubt that it is solvable without=20
trusting jitter entropy, or without provoking a dummy read on a random=20
block device, just for timings, or maybe some other interaction with the =

external world - but Willy already said "it seems fishy". However, _if_=20
it is solved, then we don't need GRND_INSECURE, because solving #2 is=20
equivalent to magically making secure random numbers always available.

--=20
Alexander E. Patrakov


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