[PATCH v5 2/5] random: Add and use arch_get_rng_seed

[Andy Lutomirski <luto@amacapital.net>]

Currently, init_std_data contains its own logic for using arch
random sources.  This replaces that logic with a generic function
arch_get_rng_seed that allows arch code to supply its own logic.
The default implementation tries arch_get_random_seed_long and
arch_get_random_long individually.

The only functional change here is that random_get_entropy() is used
unconditionally instead of being used only when the arch sources
fail.  This may add a tiny amount of security.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
 drivers/char/random.c  | 14 +++++++++++---
 include/linux/random.h | 40 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+), 3 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 0a7ac0a..be7a94e 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1236,6 +1236,10 @@ void get_random_bytes_arch(void *buf, int nbytes)
 }
 EXPORT_SYMBOL(get_random_bytes_arch);
 
+static void seed_entropy_store(void *ctx, u32 data)
+{
+	mix_pool_bytes((struct entropy_store *)ctx, &data, sizeof(data), NULL);
+}
 
 /*
  * init_std_data - initialize pool with system data
@@ -1251,15 +1255,19 @@ static void init_std_data(struct entropy_store *r)
 	int i;
 	ktime_t now = ktime_get_real();
 	unsigned long rv;
+	char log_prefix[128];
 
 	r->last_pulled = jiffies;
 	mix_pool_bytes(r, &now, sizeof(now), NULL);
 	for (i = r->poolinfo->poolbytes; i > 0; i -= sizeof(rv)) {
-		if (!arch_get_random_seed_long(&rv) &&
-		    !arch_get_random_long(&rv))
-			rv = random_get_entropy();
+		rv = random_get_entropy();
 		mix_pool_bytes(r, &rv, sizeof(rv), NULL);
 	}
+
+	sprintf(log_prefix, "random: seeded %s pool", r->name);
+	arch_get_rng_seed(r, seed_entropy_store, 8 * r->poolinfo->poolbytes,
+			  log_prefix);
+
 	mix_pool_bytes(r, utsname(), sizeof(*(utsname())), NULL);
 }
 
diff --git a/include/linux/random.h b/include/linux/random.h
index 57fbbff..81a6145 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -106,6 +106,46 @@ static inline int arch_has_random_seed(void)
 }
 #endif
 
+#ifndef __HAVE_ARCH_GET_RNG_SEED
+
+/**
+ * arch_get_rng_seed() - get architectural rng seed data
+ * @ctx: context for the seed function
+ * @seed: function to call for each u32 obtained
+ * @bits_per_source: number of bits from each source to try to use
+ * @log_prefix: beginning of log output (may be NULL)
+ *
+ * Synchronously load some architectural entropy or other best-effort
+ * random seed data.  An arch-specific implementation should be no worse
+ * than this generic implementation.  If the arch code does something
+ * interesting, it may log something of the form "log_prefix with
+ * 8 bits of stuff".
+ *
+ * No arch-specific implementation should be any worse than the generic
+ * implementation.
+ */
+static inline void arch_get_rng_seed(void *ctx,
+				     void (*seed)(void *ctx, u32 data),
+				     int bits_per_source,
+				     const char *log_prefix)
+{
+	int i;
+
+	for (i = 0; i < bits_per_source; i += 8 * sizeof(long)) {
+		unsigned long rv;
+
+		if (arch_get_random_seed_long(&rv) ||
+		    arch_get_random_long(&rv)) {
+			seed(ctx, (u32)rv);
+#if BITS_PER_LONG > 32
+			seed(ctx, (u32)(rv >> 32));
+#endif
+		}
+	}
+}
+
+#endif /* __HAVE_ARCH_GET_RNG_SEED */
+
 /* Pseudo random number generator from numerical recipes. */
 static inline u32 next_pseudo_random32(u32 seed)
 {
-- 
1.9.3