linux-kernel - Re: [PATCH RFC v0] random: block in /dev/urandom

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Date:   Mon, 14 Feb 2022 15:53:13 +0100
From:   Lennart Poettering <mzxreary@...inter.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv@...ts.infradead.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-m68k@...ts.linux-m68k.org,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-mips@...r.kernel.org,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Eric Biggers <ebiggers@...gle.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH RFC v0] random: block in /dev/urandom

On Mo, 14.02.22 15:13, Jason A. Donenfeld (Jason@...c4.com) wrote:

> Hi Lennart,
>
> On Mon, Feb 14, 2022 at 9:53 AM Lennart Poettering <mzxreary@...inter.de> wrote:
> > So, systemd uses (potentially half-initialized) /dev/urandom for
> > seeding its hash tables. For that its kinda OK if the random values
> > have low entropy initially, as we'll automatically reseed when too
> > many hash collisions happen, and then use a newer (and thus hopefully
> > better) seed, again acquired through /dev/urandom. i.e. if the seeds
> > are initially not good enough to thwart hash collision attacks, once
> > the hash table are actually attacked we'll replace the seeds with
> > someting better. For that all we need is that the random pool
> > eventually gets better, that's all.
> >
> > So for that usecase /dev/urandom behaving the way it so far does is
> > kinda nice.
>
> Oh that's an interesting point. But that sounds to me like the problem
> with this patch is not that it makes /dev/urandom block (its primary
> purpose) but that it also removes GRND_INSECURE (a distraction). So
> perhaps an improved patch would be something like the below, which
> changes /dev/urandom for new kernels but doesn't remove GRND_INSECURE.
> Then your hash tables could continue to use GRND_INSECURE and all would
> be well.  (And for kernels without getrandom(), they'd just fall back to
> /dev/urandom like normal which would have old semantics, so works.)

In fact, systemd already uses getrandom(GRND_INSECURE) for this, if it
is supported, and falls back to /dev/urandom only if it is not. So as
long as GRND_INSECURE remains available we are good.

Lennart

--
Lennart Poettering, Berlin