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>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>

1 files changed, 11 insertions, 0 deletions

diff --git a/arch/arm64/kernel/kaslr.c b/arch/arm64/kernel/kaslr.c
index 2a11a962e571..53b8a4ee64ff 100644
--- a/arch/arm64/kernel/kaslr.c
+++ b/arch/arm64/kernel/kaslr.c
@@ -120,6 +120,17 @@ u64 __init kaslr_early_init(u64 dt_phys)
 		return 0;
 	}
 
+	/*
+	 * Mix in any entropy obtainable architecturally, open coded
+	 * since this runs extremely early.
+	 */
+	if (__early_cpu_has_rndr()) {
+		unsigned long raw;
+
+		if (__arm64_rndr(&raw))
+			seed ^= raw;
+	}
+
 	if (!seed) {
 		kaslr_status = KASLR_DISABLED_NO_SEED;
 		return 0;