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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Mon, 3 Jan 2022 17:42:54 +0100
Message-ID: <CAHmME9rX5ZnhnO=tBWqZrXwm7xNEdQDptt4FEwzJK_-GVhyerg@mail.gmail.com>
Subject: Re: [PATCH] random: Don't reset crng_init_cnt on urandom_read()
To:     "Theodore Ts'o" <tytso@mit.edu>
Cc:     Jann Horn <jannh@google.com>, LKML <linux-kernel@vger.kernel.org>
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On Mon, Jan 3, 2022 at 5:39 PM Theodore Ts'o <tytso@mit.edu> wrote:
>
> On Mon, Jan 03, 2022 at 05:03:57PM +0100, Jason A. Donenfeld wrote:
> > On Mon, Jan 3, 2022 at 4:59 PM Jann Horn <jannh@google.com> wrote:
> > > This code was inconsistent, and it probably made things worse - just get
> > > rid of it.
> >
> > Rather than adding crng_init_cnt=0 if crng_init<1 to extract_crng_user
> > and get_random_bytes, getting rid of it like this seems probably okay
> > and makes the model simpler. I'll apply this. Thank you.
>
> Ack.  It does mean that we're making a choice that an attacker who is
> carrying out a incremental state tracking attack on the CRNG will make
> /dev/urandom (and getrandom) to make the crng emit "less secure" in
> the period when crng_init is > 0 and < 2.  On the other hand, this
> allows us to get to the state of crng_init=2 faster, where as before,
> the attacker could delay getting us to the state crng_init=1 forever,
> where reads from /dev/urandom would be hence be insecure forever (and
> getrandom() would block forever).

Right. I had a few early drafts of this commit where I was trying to
protect the 0->1 transition from being bruteforced with a trickle of
entropy, and Jann's offline comment was something along the lines of,
"why do we actually care about crng_init==1? it's not secure anyway,"
which seems compelling. Plus, as you point out, letting anything reset
crng_init_cnt (like /dev/urandom reads) means unprivileged userspace
can delay crng_init==2, which seems like a bigger deal.

Jason