um: seed rng using host OS rng

UML generally does not provide access to special CPU instructions like
RDRAND, and execution tends to be rather deterministic, with no real
hardware interrupts, making good randomness really very hard, if not
all together impossible. Not only is this a security eyebrow raiser, but
it's also quite annoying when trying to do various pieces of UML-based
automation that takes a long time to boot, if ever.

Fix this by trivially calling getrandom() in the host and using that
seed as "bootloader randomness", which initializes the rng immediately
at UML boot.

The old behavior can be restored the same way as on any other arch, by
way of CONFIG_TRUST_BOOTLOADER_RANDOMNESS=n or
random.trust_bootloader=0. So seen from that perspective, this just
makes UML act like other archs, which is positive in its own right.

Additionally, wire up arch_get_random_{int,long}() in the same way, so
that reseeds can also make use of the host RNG, controllable by
CONFIG_TRUST_CPU_RANDOMNESS and random.trust_cpu, per usual.

Cc: stable@vger.kernel.org
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

1 files changed, 30 insertions, 0 deletions

diff --git a/arch/um/include/asm/archrandom.h b/arch/um/include/asm/archrandom.h
new file mode 100644
index 000000000000..2f24cb96391d
--- /dev/null
+++ b/arch/um/include/asm/archrandom.h
@@ -0,0 +1,30 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_UM_ARCHRANDOM_H__
+#define __ASM_UM_ARCHRANDOM_H__
+
+#include <linux/types.h>
+
+/* This is from <os.h>, but better not to #include that in a global header here. */
+ssize_t os_getrandom(void *buf, size_t len, unsigned int flags);
+
+static inline bool __must_check arch_get_random_long(unsigned long *v)
+{
+	return os_getrandom(v, sizeof(*v), 0) == sizeof(*v);
+}
+
+static inline bool __must_check arch_get_random_int(unsigned int *v)
+{
+	return os_getrandom(v, sizeof(*v), 0) == sizeof(*v);
+}
+
+static inline bool __must_check arch_get_random_seed_long(unsigned long *v)
+{
+	return false;
+}
+
+static inline bool __must_check arch_get_random_seed_int(unsigned int *v)
+{
+	return false;
+}
+
+#endif