[PATCH] s390/archrandom: simplify back to earlier design

[Jason A. Donenfeld]

s390x appears to present two RNG interfaces:
- a "TRNG" that gathers entropy using some hardware function; and
- a "DRBG" that takes in a seed and expands it.

Previously, the TRNG was wired up to arch_get_random_{long,int}(), but
it was observed that this was being called really frequently, resulting
in high overhead. So it was changed to be wired up to arch_get_random_
seed_{long,int}(), which was a reasonable decision. Later on, the DRBG
was then wired up to arch_get_random_{long,int}(), with a complicated
buffer filling thread, to control overhead and rate.

Fortunately, none of the performance issues matter much now. The RNG
always attempts to use arch_get_random_seed_{long,int}() first, which
means a complicated implementation of arch_get_random_{long,int}() isn't
really valuable or useful to have around. And it's only used when
reseeding, which means it won't hit the high throughput complications
that were faced before.

So this commit returns to an earlier design of just calling the TRNG in
arch_get_random_seed_{long,int}(), and returning false in arch_get_
random_{long,int}().

Cc: stable@xxxxxxxxxxxxxxx
Cc: Harald Freudenberger <freude@xxxxxxxxxxxxx>
Cc: Ingo Franzki <ifranzki@xxxxxxxxxxxxx>
Cc: Juergen Christ <jchrist@xxxxxxxxxxxxx>
Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
---
Harald - please let me know if my assumptions about how the s390
machinery works are correct. I might have misunderstood the history of
these changes and their motivations. -Jason

 arch/s390/crypto/arch_random.c     | 211 +----------------------------
 arch/s390/include/asm/archrandom.h |  14 +-
 2 files changed, 9 insertions(+), 216 deletions(-)

diff --git a/arch/s390/crypto/arch_random.c b/arch/s390/crypto/arch_random.c
index 56007c763902..55c35d4c03b2 100644
--- a/arch/s390/crypto/arch_random.c
+++ b/arch/s390/crypto/arch_random.c
@@ -4,36 +4,12 @@
  *
  * Copyright IBM Corp. 2017, 2020
  * Author(s): Harald Freudenberger
- *
- * The s390_arch_random_generate() function may be called from random.c
- * in interrupt context. So this implementation does the best to be very
- * fast. There is a buffer of random data which is asynchronously checked
- * and filled by a workqueue thread.
- * If there are enough bytes in the buffer the s390_arch_random_generate()
- * just delivers these bytes. Otherwise false is returned until the
- * worker thread refills the buffer.
- * The worker fills the rng buffer by pulling fresh entropy from the
- * high quality (but slow) true hardware random generator. This entropy
- * is then spread over the buffer with an pseudo random generator PRNG.
- * As the arch_get_random_seed_long() fetches 8 bytes and the calling
- * function add_interrupt_randomness() counts this as 1 bit entropy the
- * distribution needs to make sure there is in fact 1 bit entropy contained
- * in 8 bytes of the buffer. The current values pull 32 byte entropy
- * and scatter this into a 2048 byte buffer. So 8 byte in the buffer
- * will contain 1 bit of entropy.
- * The worker thread is rescheduled based on the charge level of the
- * buffer but at least with 500 ms delay to avoid too much CPU consumption.
- * So the max. amount of rng data delivered via arch_get_random_seed is
- * limited to 4k bytes per second.
  */
 
 #include <linux/kernel.h>
 #include <linux/atomic.h>
 #include <linux/random.h>
-#include <linux/slab.h>
 #include <linux/static_key.h>
-#include <linux/workqueue.h>
-#include <linux/moduleparam.h>
 #include <asm/cpacf.h>
 
 DEFINE_STATIC_KEY_FALSE(s390_arch_random_available);
@@ -41,194 +17,11 @@ DEFINE_STATIC_KEY_FALSE(s390_arch_random_available);
 atomic64_t s390_arch_random_counter = ATOMIC64_INIT(0);
 EXPORT_SYMBOL(s390_arch_random_counter);
 
-#define ARCH_REFILL_TICKS (HZ/2)
-#define ARCH_PRNG_SEED_SIZE 32
-#define ARCH_RNG_BUF_SIZE 2048
-
-static DEFINE_SPINLOCK(arch_rng_lock);
-static u8 *arch_rng_buf;
-static unsigned int arch_rng_buf_idx;
-
-static void arch_rng_refill_buffer(struct work_struct *);
-static DECLARE_DELAYED_WORK(arch_rng_work, arch_rng_refill_buffer);
-
-bool s390_arch_random_generate(u8 *buf, unsigned int nbytes)
-{
-	/* max hunk is ARCH_RNG_BUF_SIZE */
-	if (nbytes > ARCH_RNG_BUF_SIZE)
-		return false;
-
-	/* lock rng buffer */
-	if (!spin_trylock(&arch_rng_lock))
-		return false;
-
-	/* try to resolve the requested amount of bytes from the buffer */
-	arch_rng_buf_idx -= nbytes;
-	if (arch_rng_buf_idx < ARCH_RNG_BUF_SIZE) {
-		memcpy(buf, arch_rng_buf + arch_rng_buf_idx, nbytes);
-		atomic64_add(nbytes, &s390_arch_random_counter);
-		spin_unlock(&arch_rng_lock);
-		return true;
-	}
-
-	/* not enough bytes in rng buffer, refill is done asynchronously */
-	spin_unlock(&arch_rng_lock);
-
-	return false;
-}
-EXPORT_SYMBOL(s390_arch_random_generate);
-
-static void arch_rng_refill_buffer(struct work_struct *unused)
-{
-	unsigned int delay = ARCH_REFILL_TICKS;
-
-	spin_lock(&arch_rng_lock);
-	if (arch_rng_buf_idx > ARCH_RNG_BUF_SIZE) {
-		/* buffer is exhausted and needs refill */
-		u8 seed[ARCH_PRNG_SEED_SIZE];
-		u8 prng_wa[240];
-		/* fetch ARCH_PRNG_SEED_SIZE bytes of entropy */
-		cpacf_trng(NULL, 0, seed, sizeof(seed));
-		/* blow this entropy up to ARCH_RNG_BUF_SIZE with PRNG */
-		memset(prng_wa, 0, sizeof(prng_wa));
-		cpacf_prno(CPACF_PRNO_SHA512_DRNG_SEED,
-			   &prng_wa, NULL, 0, seed, sizeof(seed));
-		cpacf_prno(CPACF_PRNO_SHA512_DRNG_GEN,
-			   &prng_wa, arch_rng_buf, ARCH_RNG_BUF_SIZE, NULL, 0);
-		arch_rng_buf_idx = ARCH_RNG_BUF_SIZE;
-	}
-	delay += (ARCH_REFILL_TICKS * arch_rng_buf_idx) / ARCH_RNG_BUF_SIZE;
-	spin_unlock(&arch_rng_lock);
-
-	/* kick next check */
-	queue_delayed_work(system_long_wq, &arch_rng_work, delay);
-}
-
-/*
- * Here follows the implementation of s390_arch_get_random_long().
- *
- * The random longs to be pulled by arch_get_random_long() are
- * prepared in an 4K buffer which is filled from the NIST 800-90
- * compliant s390 drbg. By default the random long buffer is refilled
- * 256 times before the drbg itself needs a reseed. The reseed of the
- * drbg is done with 32 bytes fetched from the high quality (but slow)
- * trng which is assumed to deliver 100% entropy. So the 32 * 8 = 256
- * bits of entropy are spread over 256 * 4KB = 1MB serving 131072
- * arch_get_random_long() invocations before reseeded.
- *
- * How often the 4K random long buffer is refilled with the drbg
- * before the drbg is reseeded can be adjusted. There is a module
- * parameter 's390_arch_rnd_long_drbg_reseed' accessible via
- *   /sys/module/arch_random/parameters/rndlong_drbg_reseed
- * or as kernel command line parameter
- *   arch_random.rndlong_drbg_reseed=<value>
- * This parameter tells how often the drbg fills the 4K buffer before
- * it is re-seeded by fresh entropy from the trng.
- * A value of 16 results in reseeding the drbg at every 16 * 4 KB = 64
- * KB with 32 bytes of fresh entropy pulled from the trng. So a value
- * of 16 would result in 256 bits entropy per 64 KB.
- * A value of 256 results in 1MB of drbg output before a reseed of the
- * drbg is done. So this would spread the 256 bits of entropy among 1MB.
- * Setting this parameter to 0 forces the reseed to take place every
- * time the 4K buffer is depleted, so the entropy rises to 256 bits
- * entropy per 4K or 0.5 bit entropy per arch_get_random_long().  With
- * setting this parameter to negative values all this effort is
- * disabled, arch_get_random long() returns false and thus indicating
- * that the arch_get_random_long() feature is disabled at all.
- */
-
-static unsigned long rndlong_buf[512];
-static DEFINE_SPINLOCK(rndlong_lock);
-static int rndlong_buf_index;
-
-static int rndlong_drbg_reseed = 256;
-module_param_named(rndlong_drbg_reseed, rndlong_drbg_reseed, int, 0600);
-MODULE_PARM_DESC(rndlong_drbg_reseed, "s390 arch_get_random_long() drbg reseed");
-
-static inline void refill_rndlong_buf(void)
-{
-	static u8 prng_ws[240];
-	static int drbg_counter;
-
-	if (--drbg_counter < 0) {
-		/* need to re-seed the drbg */
-		u8 seed[32];
-
-		/* fetch seed from trng */
-		cpacf_trng(NULL, 0, seed, sizeof(seed));
-		/* seed drbg */
-		memset(prng_ws, 0, sizeof(prng_ws));
-		cpacf_prno(CPACF_PRNO_SHA512_DRNG_SEED,
-			   &prng_ws, NULL, 0, seed, sizeof(seed));
-		/* re-init counter for drbg */
-		drbg_counter = rndlong_drbg_reseed;
-	}
-
-	/* fill the arch_get_random_long buffer from drbg */
-	cpacf_prno(CPACF_PRNO_SHA512_DRNG_GEN, &prng_ws,
-		   (u8 *) rndlong_buf, sizeof(rndlong_buf),
-		   NULL, 0);
-}
-
-bool s390_arch_get_random_long(unsigned long *v)
-{
-	bool rc = false;
-	unsigned long flags;
-
-	/* arch_get_random_long() disabled ? */
-	if (rndlong_drbg_reseed < 0)
-		return false;
-
-	/* try to lock the random long lock */
-	if (!spin_trylock_irqsave(&rndlong_lock, flags))
-		return false;
-
-	if (--rndlong_buf_index >= 0) {
-		/* deliver next long value from the buffer */
-		*v = rndlong_buf[rndlong_buf_index];
-		rc = true;
-		goto out;
-	}
-
-	/* buffer is depleted and needs refill */
-	if (in_interrupt()) {
-		/* delay refill in interrupt context to next caller */
-		rndlong_buf_index = 0;
-		goto out;
-	}
-
-	/* refill random long buffer */
-	refill_rndlong_buf();
-	rndlong_buf_index = ARRAY_SIZE(rndlong_buf);
-
-	/* and provide one random long */
-	*v = rndlong_buf[--rndlong_buf_index];
-	rc = true;
-
-out:
-	spin_unlock_irqrestore(&rndlong_lock, flags);
-	return rc;
-}
-EXPORT_SYMBOL(s390_arch_get_random_long);
-
 static int __init s390_arch_random_init(void)
 {
-	/* all the needed PRNO subfunctions available ? */
-	if (cpacf_query_func(CPACF_PRNO, CPACF_PRNO_TRNG) &&
-	    cpacf_query_func(CPACF_PRNO, CPACF_PRNO_SHA512_DRNG_GEN)) {
-
-		/* alloc arch random working buffer */
-		arch_rng_buf = kmalloc(ARCH_RNG_BUF_SIZE, GFP_KERNEL);
-		if (!arch_rng_buf)
-			return -ENOMEM;
-
-		/* kick worker queue job to fill the random buffer */
-		queue_delayed_work(system_long_wq,
-				   &arch_rng_work, ARCH_REFILL_TICKS);
-
-		/* enable arch random to the outside world */
+	/* check if subfunction CPACF_PRNO_TRNG is available */
+	if (cpacf_query_func(CPACF_PRNO, CPACF_PRNO_TRNG))
 		static_branch_enable(&s390_arch_random_available);
-	}
 
 	return 0;
 }
diff --git a/arch/s390/include/asm/archrandom.h b/arch/s390/include/asm/archrandom.h
index 5dc712fde3c7..2c6e1c6ecbe7 100644
--- a/arch/s390/include/asm/archrandom.h
+++ b/arch/s390/include/asm/archrandom.h
@@ -15,17 +15,13 @@
 
 #include <linux/static_key.h>
 #include <linux/atomic.h>
+#include <asm/cpacf.h>
 
 DECLARE_STATIC_KEY_FALSE(s390_arch_random_available);
 extern atomic64_t s390_arch_random_counter;
 
-bool s390_arch_get_random_long(unsigned long *v);
-bool s390_arch_random_generate(u8 *buf, unsigned int nbytes);
-
 static inline bool __must_check arch_get_random_long(unsigned long *v)
 {
-	if (static_branch_likely(&s390_arch_random_available))
-		return s390_arch_get_random_long(v);
 	return false;
 }
 
@@ -37,7 +33,9 @@ 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)
 {
 	if (static_branch_likely(&s390_arch_random_available)) {
-		return s390_arch_random_generate((u8 *)v, sizeof(*v));
+		cpacf_trng(NULL, 0, (u8 *)v, sizeof(*v));
+		atomic64_add(sizeof(*v), &s390_arch_random_counter);
+		return true;
 	}
 	return false;
 }
@@ -45,7 +43,9 @@ static inline bool __must_check arch_get_random_seed_long(unsigned long *v)
 static inline bool __must_check arch_get_random_seed_int(unsigned int *v)
 {
 	if (static_branch_likely(&s390_arch_random_available)) {
-		return s390_arch_random_generate((u8 *)v, sizeof(*v));
+		cpacf_trng(NULL, 0, (u8 *)v, sizeof(*v));
+		atomic64_add(sizeof(*v), &s390_arch_random_counter);
+		return true;
 	}
 	return false;
 }
-- 
2.35.1