linux-kernel - Re: [PATCH] random: Move rand

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Date:   Fri, 12 Oct 2018 10:42:39 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Kees Cook <keescook@...omium.org>
Cc:     "Theodore Ts'o" <tytso@....edu>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Laura Abbott <labbott@...hat.com>,
        Daniel Micay <danielmicay@...il.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        tcharding <me@...in.cc>, "Jason A. Donenfeld" <Jason@...c4.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        linux-rtc@...r.kernel.org
Subject: Re: [PATCH] random: Move rand_initialize() earlier

On Fri, Oct 12, 2018 at 12:54 AM Kees Cook <keescook@...omium.org> wrote:
>
> Right now rand_initialize() is run as an early_initcall(), but it only
> depends on timekeeping_init() (for mixing ktime_get_real() into the
> pools). However, the call to boot_init_stack_canary() for stack canary
> initialization runs earlier, which triggers a warning at boot:
>
> random: get_random_bytes called from start_kernel+0x357/0x548 with crng_init=0
>
> Instead, this moves rand_initialize() to after timekeeping_init(), and moves
> canary initialization here as well.
>
> Note that this warning may still remain for machines that do not have
> UEFI RNG support (which initializes the RNG pools during setup_arch()),
> or for x86 machines without RDRAND (or booting without "random.trust=on"
> or CONFIG_RANDOM_TRUST_CPU=y).
>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> Alternatively, ktime_get_real() could get mixed into the pools after
> timekeeping_init(), and rand_initialize() could be run MUCH early,
> like after setup_arch()...

I wonder if mixing in ktime_get_real() is flawed to start with:
This depends on read_persistent_clock64() actually returning
a meaningful time, but in many cases it does not; x86 being
a notable exception.

We have three ways of setting the initial time:

* At early boot using read_persistent_clock64(). This may read
  the time from the hypervisor (if any), and on some older
  architectures that have a custom RTC abstraction, it
  reads from the RTC directly. We generally move away from
  the RTC method in favor of the proper rtc class interfaces
  (see below)

* At late_initcall time, in rtc_hctosys(). If an RTC driver has
  been loaded successfully at this time, it will be read from
  there. We also move away from this.

* From user space, which reads the RTC time or NTP,
  and then sets the system time from that.

The latter two end up in do_settimeofday64(), but there
is no mixing in of the time into the random seed that I can
see, and it's not clear whether there should be (it
can be called with arbitrary times from user space,
provided CAP_SYS_TIME permissions).

       Arnd