arm64: Add support for SMCCC TRNG entropy source

The ARM architected TRNG firmware interface, described in ARM spec
DEN0098, defines an ARM SMCCC based interface to a true random number
generator, provided by firmware.
This can be discovered via the SMCCC >=v1.1 interface, and provides
up to 192 bits of entropy per call.

Hook this SMC call into arm64's arch_get_random_*() implementation,
coming to the rescue when the CPU does not implement the ARM v8.5 RNG
system registers.

For the detection, we piggy back on the PSCI/SMCCC discovery (which gives
us the conduit to use (hvc/smc)), then try to call the
ARM_SMCCC_TRNG_VERSION function, which returns -1 if this interface is
not implemented.

Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/arch/arm64/include/asm/archrandom.h b/arch/arm64/include/asm/archrandom.h
index abe07c2..09e4327 100644 (file)
--- a/arch/arm64/include/asm/archrandom.h
+++ b/arch/arm64/include/asm/archrandom.h
@@ -4,13 +4,24 @@
 
 #ifdef CONFIG_ARCH_RANDOM
 
+#include <linux/arm-smccc.h>
 #include <linux/bug.h>
 #include <linux/kernel.h>
 #include <asm/cpufeature.h>
 
+#define ARM_SMCCC_TRNG_MIN_VERSION     0x10000UL
+
+extern bool smccc_trng_available;
+
 static inline bool __init smccc_probe_trng(void)
 {
-       return false;
+       struct arm_smccc_res res;
+
+       arm_smccc_1_1_invoke(ARM_SMCCC_TRNG_VERSION, &res);
+       if ((s32)res.a0 < 0)
+               return false;
+
+       return res.a0 >= ARM_SMCCC_TRNG_MIN_VERSION;
 }
 
 static inline bool __arm64_rndr(unsigned long *v)
@@ -43,26 +54,55 @@ static inline bool __must_check arch_get_random_int(unsigned int *v)
 
 static inline bool __must_check arch_get_random_seed_long(unsigned long *v)
 {
+       struct arm_smccc_res res;
+
+       /*
+        * We prefer the SMCCC call, since its semantics (return actual
+        * hardware backed entropy) is closer to the idea behind this
+        * function here than what even the RNDRSS register provides
+        * (the output of a pseudo RNG freshly seeded by a TRNG).
+        */
+       if (smccc_trng_available) {
+               arm_smccc_1_1_invoke(ARM_SMCCC_TRNG_RND64, 64, &res);
+               if ((int)res.a0 >= 0) {
+                       *v = res.a3;
+                       return true;
+               }
+       }
+
        /*
         * Only support the generic interface after we have detected
         * the system wide capability, avoiding complexity with the
         * cpufeature code and with potential scheduling between CPUs
         * with and without the feature.
         */
-       if (!cpus_have_const_cap(ARM64_HAS_RNG))
-               return false;
+       if (cpus_have_const_cap(ARM64_HAS_RNG) && __arm64_rndr(v))
+               return true;
 
-       return __arm64_rndr(v);
+       return false;
 }
 
-
 static inline bool __must_check arch_get_random_seed_int(unsigned int *v)
 {
+       struct arm_smccc_res res;
        unsigned long val;
-       bool ok = arch_get_random_seed_long(&val);
 
-       *v = val;
-       return ok;
+       if (smccc_trng_available) {
+               arm_smccc_1_1_invoke(ARM_SMCCC_TRNG_RND64, 32, &res);
+               if ((int)res.a0 >= 0) {
+                       *v = res.a3 & GENMASK(31, 0);
+                       return true;
+               }
+       }
+
+       if (cpus_have_const_cap(ARM64_HAS_RNG)) {
+               if (__arm64_rndr(&val)) {
+                       *v = val;
+                       return true;
+               }
+       }
+
+       return false;
 }
 
 static inline bool __init __early_cpu_has_rndr(void)
@@ -77,10 +117,20 @@ arch_get_random_seed_long_early(unsigned long *v)
 {
        WARN_ON(system_state != SYSTEM_BOOTING);
 
-       if (!__early_cpu_has_rndr())
-               return false;
+       if (smccc_trng_available) {
+               struct arm_smccc_res res;
+
+               arm_smccc_1_1_invoke(ARM_SMCCC_TRNG_RND64, 64, &res);
+               if ((int)res.a0 >= 0) {
+                       *v = res.a3;
+                       return true;
+               }
+       }
 
-       return __arm64_rndr(v);
+       if (__early_cpu_has_rndr() && __arm64_rndr(v))
+               return true;
+
+       return false;
 }
 #define arch_get_random_seed_long_early arch_get_random_seed_long_early