linux-ext4 - Re: Linux 5.3-rc8

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Date:   Sun, 15 Sep 2019 20:56:39 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Lennart Poettering <mzxreary@...inter.de>,
        "Alexander E. Patrakov" <patrakov@...il.com>,
        "Ahmed S. Darwish" <darwish.07@...il.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jan Kara <jack@...e.cz>, Ray Strode <rstrode@...hat.com>,
        William Jon McCann <mccann@....edu>,
        zhangjs <zachary@...shancloud.com>, linux-ext4@...r.kernel.org,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: Linux 5.3-rc8

On Sun, Sep 15, 2019 at 8:40 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> If you want secure keys, you can't rely on a blocking model, because
> it ends up not working. Blocking leads to problems.

Side note: I'd argue that (despite my earlier mis-understanding) the
only really valid use of "block until there is entropy" is the
systemd-random-seed model that blocks not because it wants a secure
key, but blocks because it wants to save the (now properly) random
seed for later.

So apologies to Lennart - he was very much right, and I mis-understood
Ahmed's bug report. Systemd was blameless, and blocked correctly.

While blocking for actual random keys was the usual bug, just for that
silly and pointless MIT cookie that doesn't even need the secure
randomness.

But because the getrandom() interface was mis-designed (and only
_looks_ like a more convenient interface for /dev/urandom, without
being one), the MIT cookie code got the blocking whether it wanted to
or not.

Just say no to blocking for key data.

            Linus