Further reduce diffs with OpenBSD's arc4random. The main functional

change here is to ensure that when a process forks after arc4random
is seeded, the parent and child don't observe the same random sequence.
OpenBSD's fix introduces some additional overhead in the form of a
getpid() call.  This could be improved upon, e.g., by setting a flag
in fork(), if it proves to be a problem.

This was discussed with secteam (simon, csjp, rwatson) in 2008, shortly
prior to my going out of town and forgetting all about it.  The conclusion
was that the problem with forks is worrisome, but it doesn't appear to
have introduced an actual vulnerability for any known programs.

The only significant remaining difference between our arc4random and
OpenBSD's is in how we seed the generator in arc4_stir().

1 files changed, 30 insertions, 33 deletions

diff --git a/lib/libc/gen/arc4random.c b/lib/libc/gen/arc4random.c
index 02f7814..a2b235f 100644
--- a/lib/libc/gen/arc4random.c
+++ b/lib/libc/gen/arc4random.c
@@ -33,11 +33,13 @@
 __FBSDID("$FreeBSD$");
 
 #include "namespace.h"
-#include <sys/types.h>
-#include <sys/time.h>
-#include <stdlib.h>
 #include <fcntl.h>
+#include <limits.h>
+#include <stdlib.h>
 #include <unistd.h>
+#include <sys/types.h>
+#include <sys/param.h>
+#include <sys/time.h>
 #include <pthread.h>
 
 #include "libc_private.h"
@@ -71,9 +73,9 @@ static pthread_mutex_t	arc4random_mtx = PTHREAD_MUTEX_INITIALIZER;
 			_pthread_mutex_unlock(&arc4random_mtx);	\
 	} while (0)
 
-static struct arc4_stream rs;
 static int rs_initialized;
-static int rs_stired;
+static struct arc4_stream rs;
+static pid_t arc4_stir_pid;
 static int arc4_count;
 
 static inline u_int8_t arc4_getbyte(void);
@@ -117,6 +119,10 @@ arc4_stir(void)
 		u_char	 	rnd[KEYSIZE];
 	} rdat;
 
+	if (!rs_initialized) {
+		arc4_init();
+		rs_initialized = 1;
+	}
 	fd = _open(RANDOMDEV, O_RDONLY, 0);
 	done = 0;
 	if (fd >= 0) {
@@ -141,6 +147,18 @@ arc4_stir(void)
 	arc4_count = 1600000;
 }
 
+static void
+arc4_stir_if_needed(void)
+{
+	pid_t pid = getpid();
+
+	if (arc4_count <= 0 || !rs_initialized || arc4_stir_pid != pid)
+	{
+		arc4_stir_pid = pid;
+		arc4_stir();
+	}
+}
+
 static inline u_int8_t
 arc4_getbyte(void)
 {
@@ -166,31 +184,11 @@ arc4_getword(void)
 	return val;
 }
 
-static void
-arc4_check_init(void)
-{
-	if (!rs_initialized) {
-		arc4_init();
-		rs_initialized = 1;
-	}
-}
-
-static inline void
-arc4_check_stir(void)
-{
-	if (!rs_stired || arc4_count <= 0) {
-		arc4_stir();
-		rs_stired = 1;
-	}
-}
-
 void
 arc4random_stir(void)
 {
 	_ARC4_LOCK();
-	arc4_check_init();
 	arc4_stir();
-	rs_stired = 1;
 	_ARC4_UNLOCK();
 }
 
@@ -198,8 +196,8 @@ void
 arc4random_addrandom(u_char *dat, int datlen)
 {
 	_ARC4_LOCK();
-	arc4_check_init();
-	arc4_check_stir();
+	if (!rs_initialized)
+		arc4_stir();
 	arc4_addrandom(dat, datlen);
 	_ARC4_UNLOCK();
 }
@@ -209,10 +207,9 @@ arc4random(void)
 {
 	u_int32_t val;
 	_ARC4_LOCK();
-	arc4_check_init();
-	arc4_check_stir();
-	val = arc4_getword();
 	arc4_count -= 4;
+	arc4_stir_if_needed();
+	val = arc4_getword();
 	_ARC4_UNLOCK();
 	return val;
 }
@@ -222,11 +219,11 @@ arc4random_buf(void *_buf, size_t n)
 {
 	u_char *buf = (u_char *)_buf;
 	_ARC4_LOCK();
-	arc4_check_init();
+	arc4_stir_if_needed();
 	while (n--) {
-		arc4_check_stir();
+		if (--arc4_count <= 0)
+			arc4_stir();
 		buf[n] = arc4_getbyte();
-		arc4_count--;
 	}
 	_ARC4_UNLOCK();
 }