Re: [PATCH v25 01/12] Linux Random Number Generator

Nicolai Stange
2019-11-16T18:13:29Z

Hi Stephan,

Stephan Müller <smueller@chronox.de> writes:

> +/* Initialize the default DRNG during boot */

I think that this can get called a bit too early through the
get_random_bytes() invoked from e.g. boot_init_stack_canary(): in
start_kernel(), there is

boot_init_stack_canary();

time_init();

On ARM (at least with arm_arch_timer.c), get_cycles() would return 0
until

time_init() => timer_probe() => arch_timer_of_init() =>
arch_timer_common_init() => arch_timer_arch_init() =>
arch_timer_delay_timer_register() => register_current_timer_delay()

has executed and thus, ...

> +void lrng_drngs_init_cc20(void)
> +{
> + unsigned long flags = 0;
> +
> + if (lrng_get_available())
> + return;
> +
> + lrng_sdrng_lock(&lrng_sdrng_init, &flags);
> + if (lrng_get_available()) {
> + lrng_sdrng_unlock(&lrng_sdrng_init, &flags);
> + return;
> + }
> +
> + if (random_get_entropy() || random_get_entropy()) {
> + /*
> + * As the highres timer is identified here, previous interrupts
> + * obtained during boot time are treated like a lowres-timer
> + * would have been present.
> + */
> + lrng_pool_configure(true, LRNG_IRQ_ENTROPY_BITS);
> + } else {
> + lrng_health_disable();
> + lrng_pool_configure(false, LRNG_IRQ_ENTROPY_BITS *
> + LRNG_IRQ_OVERSAMPLING_FACTOR);
> + pr_warn("operating without high-resolution timer and applying "
> + "IRQ oversampling factor %u\n",
> + LRNG_IRQ_OVERSAMPLING_FACTOR);

... LRNG thinks that no high-res timer is available even though there
is:

[ 0.000000] lrng_sdrng: operating without high-resolution timer and applying IRQ oversampling factor 10
[ 0.000000] lrng_chacha20: ChaCha20 core initialized
[ 0.000000] lrng_chacha20: ChaCha20 core initialized
[ 0.000014] sched_clock: 32 bits at 1000kHz, resolution 1000ns, wraps every 2147483647500ns
[ 0.000036] clocksource: timer: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 1911260446275 ns
[ 0.000114] bcm2835: system timer (irq = 27)
[ 0.000594] arch_timer: cp15 timer(s) running at 19.20MHz (phys).
[ 0.000613] clocksource: arch_sys_counter: mask: 0xffffffffffffff max_cycles: 0x46d987e47, max_idle_ns: 440795202767 ns
[ 0.000631] sched_clock: 56 bits at 19MHz, resolution 52ns, wraps every 4398046511078ns
[ 0.000645] Switching to timer-based delay loop, resolution 52ns

Note that this last line comes from aforementioned
register_current_timer_delay().

Similarly, get_random_bytes() can get called quite early through
WARN() => warn_slowpath_fmt() => __warn() => print_oops_end_marker() =>
init_oops_id().

Perhaps it would make sense not to do the (pool + health test)
initalization "on-demand", but rather make sure it happens at some
well-defined point after time_init()? Or at least that the pool +
the health tests get reconfigured eventually?

Thanks,

Nicolai

P.S: include/linux/lrng.h needs an #include <linux/errno.h> for
CONFIG_LRNG_DRNG_SWITCH=n

> + }
> +
> + lrng_sdrng_reset(&lrng_sdrng_init);
> + lrng_cc20_init_state(&secondary_chacha20);
> + lrng_state_init_seed_work();
> + lrng_sdrng_unlock(&lrng_sdrng_init, &flags);
> +
> + lrng_sdrng_lock(&lrng_sdrng_atomic, &flags);
> + lrng_sdrng_reset(&lrng_sdrng_atomic);
> + /*
> + * We do not initialize the state of the atomic DRNG as it is identical
> + * to the secondary DRNG at this point.
> + */
> + lrng_sdrng_unlock(&lrng_sdrng_atomic, &flags);
> +
> + lrng_trng_init();
> +
> + lrng_set_available();
> +}
> +
> +/* Reset LRNG such that all existing entropy is gone */

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