[PATCH v2] m68k: virt: use RNG seed from bootinfo block

["Jason A. Donenfeld" <Jason@zx2c4.com>]

Other virt VMs can pass RNG seeds via the "rng-seed" device tree
property or via UEFI, but m68k doesn't have either. Instead it has its
own bootinfo protocol. So this commit adds support for receiving a RNG
seed from it, which will be used at the earliest possible time in boot,
just like device tree.

Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 arch/m68k/include/uapi/asm/bootinfo-virt.h | 7 +++++++
 arch/m68k/virt/config.c                    | 9 +++++++++
 2 files changed, 16 insertions(+)

diff --git a/arch/m68k/include/uapi/asm/bootinfo-virt.h b/arch/m68k/include/uapi/asm/bootinfo-virt.h
index e4db7e2213ab..0cb2c2a41610 100644
--- a/arch/m68k/include/uapi/asm/bootinfo-virt.h
+++ b/arch/m68k/include/uapi/asm/bootinfo-virt.h
@@ -13,6 +13,13 @@
 #define BI_VIRT_VIRTIO_BASE	0x8004
 #define BI_VIRT_CTRL_BASE	0x8005
 
+/* A random seed used to initialize the RNG. Record format:
+ *
+ *   - length       [ 2 bytes, 16-bit big endian ]
+ *   - seed data    [ `length` bytes ]
+ */
+#define BI_VIRT_RNG_SEED	0x8006
+
 #define VIRT_BOOTI_VERSION	MK_BI_VERSION(2, 0)
 
 #endif /* _UAPI_ASM_M68K_BOOTINFO_MAC_H */
diff --git a/arch/m68k/virt/config.c b/arch/m68k/virt/config.c
index 632ba200ad42..645acc6918b2 100644
--- a/arch/m68k/virt/config.c
+++ b/arch/m68k/virt/config.c
@@ -2,6 +2,7 @@
 
 #include <linux/reboot.h>
 #include <linux/serial_core.h>
+#include <linux/random.h>
 #include <clocksource/timer-goldfish.h>
 
 #include <asm/bootinfo.h>
@@ -92,6 +93,14 @@ int __init virt_parse_bootinfo(const struct bi_record *record)
 		data += 4;
 		virt_bi_data.virtio.irq = be32_to_cpup(data);
 		break;
+	case BI_VIRT_RNG_SEED: {
+		u16 len = be16_to_cpup(data);
+		add_bootloader_randomness(data + 2, len);
+		/* Zero the data to preserve forward secrecy, and zero the
+		 * length to prevent kexec from using it. */
+		memzero_explicit((void *)data, len + 2);
+		break;
+	}
 	default:
 		unknown = 1;
 		break;
-- 
2.35.1