[PATCH v4 3/3] m68k: generate new RNG seed on reboot

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

Rather than rebooting into a system with no entropy, regenerate the RNG
seed before rebooting, so that the new system has a fresh seed.

Fixes: a1ee38ab1a75 ("m68k: virt: Use RNG seed from bootinfo block")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 arch/m68k/include/asm/bootinfo.h |  2 ++
 arch/m68k/kernel/process.c       | 11 +++++++++++
 arch/m68k/kernel/setup_mm.c      |  2 ++
 3 files changed, 15 insertions(+)

diff --git a/arch/m68k/include/asm/bootinfo.h b/arch/m68k/include/asm/bootinfo.h
index 81c91af8ec6c..71103530839a 100644
--- a/arch/m68k/include/asm/bootinfo.h
+++ b/arch/m68k/include/asm/bootinfo.h
@@ -28,6 +28,8 @@ void process_uboot_commandline(char *commandp, int size);
 static inline void process_uboot_commandline(char *commandp, int size) {}
 #endif
 
+extern const struct bi_record *rng_seed_record;
+
 #endif /* __ASSEMBLY__ */
 
 
diff --git a/arch/m68k/kernel/process.c b/arch/m68k/kernel/process.c
index 2cb4a61bcfac..c2b20d5d51e2 100644
--- a/arch/m68k/kernel/process.c
+++ b/arch/m68k/kernel/process.c
@@ -29,6 +29,7 @@
 #include <linux/reboot.h>
 #include <linux/init_task.h>
 #include <linux/mqueue.h>
+#include <linux/random.h>
 #include <linux/rcupdate.h>
 #include <linux/syscalls.h>
 #include <linux/uaccess.h>
@@ -36,6 +37,7 @@
 #include <asm/traps.h>
 #include <asm/machdep.h>
 #include <asm/setup.h>
+#include <asm/bootinfo.h>
 
 
 asmlinkage void ret_from_fork(void);
@@ -51,8 +53,17 @@ void arch_cpu_idle(void)
 #endif
 }
 
+const struct bi_record *rng_seed_record;
+
 void machine_restart(char * __unused)
 {
+	if (rng_seed_record && rng_seed_record->size > sizeof(*rng_seed_record) + 2) {
+		u16 len = rounddown_pow_of_two(rng_seed_record->size -
+					       sizeof(*rng_seed_record) - 2);
+		get_random_bytes((u8 *)rng_seed_record->data + 2, len);
+		*(u16 *)rng_seed_record->data = cpu_to_be16(len);
+	}
+
 	if (mach_reset)
 		mach_reset();
 	for (;;);
diff --git a/arch/m68k/kernel/setup_mm.c b/arch/m68k/kernel/setup_mm.c
index e45cc9923703..eacf734bea0e 100644
--- a/arch/m68k/kernel/setup_mm.c
+++ b/arch/m68k/kernel/setup_mm.c
@@ -160,6 +160,8 @@ static void __init m68k_parse_bootinfo(const struct bi_record *record)
 			 * length to prevent kexec from using it.
 			 */
 			memzero_explicit((void *)data, len + 2);
+			/* Store a reference to be filled in on reboot. */
+			rng_seed_record = record;
 			break;
 		}
 
-- 
2.37.3