[PATCH] arm64: random: implement arch_get_random_int/_long based on RNDR

[Ard Biesheuvel]

When support for RNDR/RNDRRS was introduced, we elected to only
implement arch_get_random_seed_int/_long(), and back them by RNDR
instead of RNDRRS. This was needed to prevent potential performance
and/or starvation issues resulting from the fact that the /dev/random
driver used to invoke these routines on various hot paths.

These issues have all been addressed now [0] [1], and so we can wire up
this API more straight-forwardly:

- map arch_get_random_int/_long() onto RNDR, which returns the output of
  a DRBG that is reseeded at an implemented defined rate;
- map arch_get_random_seed_int/_long() onto the TRNG firmware service,
  which returns true, conditioned entropy, or onto RNDRRS if the TRNG
  service is unavailable, which returns the output of a DRBG that is
  reseeded every time it is used.

[0] 390596c9959c random: avoid arch_get_random_seed_long() when collecting IRQ randomness
[1] 2ee25b6968b1 random: avoid superfluous call to RDRAND in CRNG extraction

Cc: Andre Przywara <andre.przywara at arm.com>
Cc: Mark Brown <broonie at kernel.org>
Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
 arch/arm64/include/asm/archrandom.h | 45 +++++++++++++++++---
 1 file changed, 39 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/include/asm/archrandom.h b/arch/arm64/include/asm/archrandom.h
index 09e43272ccb0..d1bb5e71df25 100644
--- a/arch/arm64/include/asm/archrandom.h
+++ b/arch/arm64/include/asm/archrandom.h
@@ -42,13 +42,47 @@ static inline bool __arm64_rndr(unsigned long *v)
 	return ok;
 }
 
+static inline bool __arm64_rndrrs(unsigned long *v)
+{
+	bool ok;
+
+	/*
+	 * Reads of RNDRRS set PSTATE.NZCV to 0b0000 on success,
+	 * and set PSTATE.NZCV to 0b0100 otherwise.
+	 */
+	asm volatile(
+		__mrs_s("%0", SYS_RNDRRS_EL0) "\n"
+	"	cset %w1, ne\n"
+	: "=r" (*v), "=r" (ok)
+	:
+	: "cc");
+
+	return ok;
+}
+
 static inline bool __must_check arch_get_random_long(unsigned long *v)
 {
+	/*
+	 * 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) && __arm64_rndr(v))
+		return true;
 	return false;
 }
 
 static inline bool __must_check arch_get_random_int(unsigned int *v)
 {
+	if (cpus_have_const_cap(ARM64_HAS_RNG)) {
+		unsigned long val;
+
+		if (__arm64_rndr(&val)) {
+			*v = val;
+			return true;
+		}
+	}
 	return false;
 }
 
@@ -71,12 +105,11 @@ static inline bool __must_check arch_get_random_seed_long(unsigned long *v)
 	}
 
 	/*
-	 * 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.
+	 * RNDRRS is not backed by an entropy source but by a DRBG that is
+	 * reseeded after each invocation. This is not a 100% fit but good
+	 * enough to implement this API if no other entropy source exists.
 	 */
-	if (cpus_have_const_cap(ARM64_HAS_RNG) && __arm64_rndr(v))
+	if (cpus_have_const_cap(ARM64_HAS_RNG) && __arm64_rndrrs(v))
 		return true;
 
 	return false;
@@ -96,7 +129,7 @@ static inline bool __must_check arch_get_random_seed_int(unsigned int *v)
 	}
 
 	if (cpus_have_const_cap(ARM64_HAS_RNG)) {
-		if (__arm64_rndr(&val)) {
+		if (__arm64_rndrrs(&val)) {
 			*v = val;
 			return true;
 		}
-- 
2.30.2