linux-kernel - Re: [PATCH] random: Don't reset crng_init_cnt on urandom

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Message-ID: <YdMmnnAkLpk81mYN@mit.edu>
Date:   Mon, 3 Jan 2022 11:38:54 -0500
From:   "Theodore Ts'o" <tytso@....edu>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Jann Horn <jannh@...gle.com>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] random: Don't reset crng_init_cnt on urandom_read()

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@...gle.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).

						- Ted