Re: [PATCH v8 3/3] arm64: Use v8.5-RNG entropy for KASLR seed

When seeding KALSR on a system where we have architecture level random
number generation make use of that entropy, mixing it in with the seed
passed by the bootloader. Since this is run very early in init before
feature detection is complete we open code rather than use archrandom.h.

Signed-off-by: Mark Brown <broonie@kernel.org>
---
 arch/arm64/kernel/kaslr.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/kaslr.c b/arch/arm64/kernel/kaslr.c
index 2a11a962e571..fc38c4c25eb6 100644
--- a/arch/arm64/kernel/kaslr.c
+++ b/arch/arm64/kernel/kaslr.c

@@ -81,7 +81,8 @@ static __init const u8 *kaslr_get_cmdline(void *fdt)
 u64 __init kaslr_early_init(u64 dt_phys)
 {
 	void *fdt;
-	u64 seed, offset, mask, module_range;
+	u64 seed, offset, mask, module_range, ftr;
+	unsigned long raw;
 	const u8 *cmdline, *str;
 	int size;
 

@@ -120,6 +121,18 @@ u64 __init kaslr_early_init(u64 dt_phys)
 		return 0;
 	}
 
+	/*
+	 * Mix in any entropy obtainable architecturally, open coded
+	 * since this runs extremely early.
+	 */
+	if (IS_ENABLED(CONFIG_ARCH_RANDOM)) {
+		ftr = read_sysreg_s(SYS_ID_AA64ISAR0_EL1);
+		if ((ftr >> ID_AA64ISAR0_RNDR_SHIFT) & 0xf) {
+			if (__arm64_rndr(&raw))
+				seed += raw;
+		}
+	}