random: only read from /dev/random after its pool has received 128 bits

Immediately after boot, we allow reads from /dev/random before its
entropy pool has been fully initialized.  Fix this so that we don't
allow this until the blocking pool has received 128 bits.

We do this by repurposing the initialized flag in the entropy pool
struct, and use the initialized flag in the blocking pool to indicate
whether it is safe to pull from the blocking pool.

To do this, we needed to rework when we decide to push entropy from the
input pool to the blocking pool, since the initialized flag for the
input pool was used for this purpose.  To simplify things, we no
longer use the initialized flag for that purpose, nor do we use the
entropy_total field any more.

Signed-off-by: Theodore Ts'o <[email protected]>

Author: tytso

drivers/char/random.c

@@ -470,7 +470,6 @@ struct entropy_store {
 	unsigned short add_ptr;
 	unsigned short input_rotate;
 	int entropy_count;
-	int entropy_total;
 	unsigned int initialized:1;
 	unsigned int last_data_init:1;
 	__u8 last_data[EXTRACT_SIZE];
@@ -643,7 +642,7 @@ static void process_random_ready_list(void)
  */
 static void credit_entropy_bits(struct entropy_store *r, int nbits)
 {
-	int entropy_count, orig;
+	int entropy_count, orig, has_initialized = 0;
 	const int pool_size = r->poolinfo->poolfracbits;
 	int nfrac = nbits << ENTROPY_SHIFT;
 
@@ -698,23 +697,25 @@ static void credit_entropy_bits(struct entropy_store *r, int nbits)
 		entropy_count = 0;
 	} else if (entropy_count > pool_size)
 		entropy_count = pool_size;
+	if ((r == &blocking_pool) && !r->initialized &&
+	    (entropy_count >> ENTROPY_SHIFT) > 128)
+		has_initialized = 1;
 	if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)
 		goto retry;
 
-	r->entropy_total += nbits;
-	if (!r->initialized && r->entropy_total > 128) {
+	if (has_initialized)
 		r->initialized = 1;
-		r->entropy_total = 0;
-	}
 
 	trace_credit_entropy_bits(r->name, nbits,
-				  entropy_count >> ENTROPY_SHIFT,
-				  r->entropy_total, _RET_IP_);
+				  entropy_count >> ENTROPY_SHIFT, _RET_IP_);
 
 	if (r == &input_pool) {
 		int entropy_bits = entropy_count >> ENTROPY_SHIFT;
+		struct entropy_store *other = &blocking_pool;
 
-		if (crng_init < 2 && entropy_bits >= 128) {
+		if (crng_init < 2) {
+			if (entropy_bits < 128)
+				return;
 			crng_reseed(&primary_crng, r);
 			entropy_bits = r->entropy_count >> ENTROPY_SHIFT;
 		}
@@ -725,20 +726,14 @@ static void credit_entropy_bits(struct entropy_store *r, int nbits)
 			wake_up_interruptible(&random_read_wait);
 			kill_fasync(&fasync, SIGIO, POLL_IN);
 		}
-		/* If the input pool is getting full, send some
-		 * entropy to the blocking pool until it is 75% full.
+		/* If the input pool is getting full, and the blocking
+		 * pool has room, send some entropy to the blocking
+		 * pool.
 		 */
-		if (entropy_bits > random_write_wakeup_bits &&
-		    r->initialized &&
-		    r->entropy_total >= 2*random_read_wakeup_bits) {
-			struct entropy_store *other = &blocking_pool;
-
-			if (other->entropy_count <=
-			    3 * other->poolinfo->poolfracbits / 4) {
-				schedule_work(&other->push_work);
-				r->entropy_total = 0;
-			}
-		}
+		if (!work_pending(&other->push_work) &&
+		    (ENTROPY_BITS(r) > 6 * r->poolinfo->poolbytes) &&
+		    (ENTROPY_BITS(other) <= 6 * other->poolinfo->poolbytes))
+			schedule_work(&other->push_work);
 	}
 }
 
@@ -1553,6 +1548,11 @@ static ssize_t extract_entropy_user(struct entropy_store *r, void __user *buf,
 	int large_request = (nbytes > 256);
 
 	trace_extract_entropy_user(r->name, nbytes, ENTROPY_BITS(r), _RET_IP_);
+	if (!r->initialized && r->pull) {
+		xfer_secondary_pool(r, ENTROPY_BITS(r->pull)/8);
+		if (!r->initialized)
+			return 0;
+	}
 	xfer_secondary_pool(r, nbytes);
 	nbytes = account(r, nbytes, 0, 0);
 

include/trace/events/random.h

@@ -62,30 +62,27 @@ DEFINE_EVENT(random__mix_pool_bytes, mix_pool_bytes_nolock,
 
 TRACE_EVENT(credit_entropy_bits,
 	TP_PROTO(const char *pool_name, int bits, int entropy_count,
-		 int entropy_total, unsigned long IP),
+		 unsigned long IP),
 
-	TP_ARGS(pool_name, bits, entropy_count, entropy_total, IP),
+	TP_ARGS(pool_name, bits, entropy_count, IP),
 
 	TP_STRUCT__entry(
 		__field( const char *,	pool_name		)
 		__field(	  int,	bits			)
 		__field(	  int,	entropy_count		)
-		__field(	  int,	entropy_total		)
 		__field(unsigned long,	IP			)
 	),
 
 	TP_fast_assign(
 		__entry->pool_name	= pool_name;
 		__entry->bits		= bits;
 		__entry->entropy_count	= entropy_count;
-		__entry->entropy_total	= entropy_total;
 		__entry->IP		= IP;
 	),
 
-	TP_printk("%s pool: bits %d entropy_count %d entropy_total %d "
-		  "caller %pS", __entry->pool_name, __entry->bits,
-		  __entry->entropy_count, __entry->entropy_total,
-		  (void *)__entry->IP)
+	TP_printk("%s pool: bits %d entropy_count %d caller %pS",
+		  __entry->pool_name, __entry->bits,
+		  __entry->entropy_count, (void *)__entry->IP)
 );
 
 TRACE_EVENT(push_to_pool,