random: add arch_get_random_*long_early()

Some architectures (e.g. arm64) can have heterogeneous CPUs, and the
boot CPU may be able to provide entropy while secondary CPUs cannot. On
such systems, arch_get_random_long() and arch_get_random_seed_long()
will fail unless support for RNG instructions has been detected on all
CPUs. This prevents the boot CPU from being able to provide
(potentially) trusted entropy when seeding the primary CRNG.

To make it possible to seed the primary CRNG from the boot CPU without
adversely affecting the runtime versions of arch_get_random_long() and
arch_get_random_seed_long(), this patch adds new early versions of the
functions used when initializing the primary CRNG.

Default implementations are provided atop of the existing
arch_get_random_long() and arch_get_random_seed_long() so that only
architectures with such constraints need to provide the new helpers.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <[e-mail]>
Cc: Mark Brown <[e-mail]>
Cc: Theodore Ts'o <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Theodore Ts'o <[e-mail]>

drivers/char/random.c

···
 	return arch_init;
 }
 
+static bool __init crng_init_try_arch_early(struct crng_state *crng)
+{
+	int		i;
+	bool		arch_init = true;
+	unsigned long	rv;
+
+	for (i = 4; i < 16; i++) {
+		if (!arch_get_random_seed_long_early(&rv) &&
+		    !arch_get_random_long_early(&rv)) {
+			rv = random_get_entropy();
+			arch_init = false;
+		}
+		crng->state[i] ^= rv;
+	}
+
+	return arch_init;
+}
+
 static void crng_initialize_secondary(struct crng_state *crng)
 {
 	memcpy(&crng->state[0], "expand 32-byte k", 16);
···
 {
 	memcpy(&crng->state[0], "expand 32-byte k", 16);
 	_extract_entropy(&input_pool, &crng->state[4], sizeof(__u32) * 12, 0);
-	if (crng_init_try_arch(crng) && trust_cpu) {
+	if (crng_init_try_arch_early(crng) && trust_cpu) {
 		invalidate_batched_entropy();
 		numa_crng_init();
 		crng_init = 2;

include/linux/random.h

···
 #ifndef _LINUX_RANDOM_H
 #define _LINUX_RANDOM_H
 
+#include <linux/bug.h>
+#include <linux/kernel.h>
 #include <linux/list.h>
 #include <linux/once.h>
 
···
 static inline bool __must_check arch_get_random_seed_int(unsigned int *v)
 {
 	return false;
+}
+#endif
+
+/*
+ * Called from the boot CPU during startup; not valid to call once
+ * secondary CPUs are up and preemption is possible.
+ */
+#ifndef arch_get_random_seed_long_early
+static inline bool __init arch_get_random_seed_long_early(unsigned long *v)
+{
+	WARN_ON(system_state != SYSTEM_BOOTING);
+	return arch_get_random_seed_long(v);
+}
+#endif
+
+#ifndef arch_get_random_long_early
+static inline bool __init arch_get_random_long_early(unsigned long *v)
+{
+	WARN_ON(system_state != SYSTEM_BOOTING);
+	return arch_get_random_long(v);
 }
 #endif