linux-kernel - Re: BPF RNG

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Message-ID: <CAGXu5jKj=SfzZqiPtRir7ombU7cLd=sD0e2ehoY=4+y7pZjyjA@mail.gmail.com>
Date:   Tue, 16 Apr 2019 17:58:56 -0500
From:   Kees Cook <keescook@...gle.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Hannes Frederic Sowa <hannes@...essinduktion.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: BPF RNG

On Tue, Apr 16, 2019 at 10:13 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 04/16/2019 03:55 PM, Kees Cook wrote:
> > [correcting Alexei's email address and resending...]
> >
> > On Tue, Apr 16, 2019 at 8:54 AM Kees Cook <keescook@...gle.com> wrote:
> >>
> >> Hi,
> >>
> >> In looking at prandom_u32() users, I noticed that BPF uses its own
> >> state variable with bpf_user_rnd_u32(). It appears that this state is
>
> Correct, we split that off from the main one.
>
> >> never reseeded like regular prandom_u32(). (See __prandom_timer().) Is
> >> this intentional, or should reseeding be happening?
>
> The prng is not to be used in any security-critical context. That said,
> we optionally could perform reseeding from time to time, though that
> should probably be done generically, so that all "external"
> prandom_u32_state() users could register themselves upon init whether
> they opt-in for periodic reseeding or not.

Yeah, it seemed like it should be built into the API. I'll add this to
the TODO list. ;)

Thanks for double-checking.

-- 
Kees Cook