arm64: Use v8.5-RNG entropy for KASLR seed

mainline inclusion
from mainline-v5.5-rc3
commit 2e8e1ea8
category:feature
bugzilla:NA
CVE:NA

-------------------

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: NMark Brown <broonie@kernel.org>
Reviewed-by: NMark Rutland <mark.rutland@arm.com>
Reviewed-by: NArd Biesheuvel <ardb@kernel.org>
Signed-off-by: NWill Deacon <will@kernel.org>
Signed-off-by: NChen Jun <chenjun102@huawei.com>
Reviewed-by: NXie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

arch/arm64/include/asm/archrandom.h

@@ -59,9 +59,17 @@ static inline bool __must_check arch_get_random_seed_int(unsigned int *v)
 	return ok;
 }
 
+static inline bool __init __early_cpu_has_rndr(void)
+{
+	/* Open code as we run prior to the first call to cpufeature. */
+	unsigned long ftr = read_sysreg_s(SYS_ID_AA64ISAR0_EL1);
+	return (ftr >> ID_AA64ISAR0_RNDR_SHIFT) & 0xf;
+}
+
 #else
 
 static inline bool __arm64_rndr(unsigned long *v) { return false; }
+static inline bool __init __early_cpu_has_rndr(void) { return false; }
 
 #endif /* CONFIG_ARCH_RANDOM */
 #endif /* _ASM_ARCHRANDOM_H */

arch/arm64/kernel/kaslr.c

@@ -126,6 +126,17 @@ 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;