From c82ed0c15e0e9e47df0b4c22672b72e35f061a9d
From: =?UTF-8?q?Bj=C3=B6rn=20Esser?= <besser82@fedoraproject.org>
Date: Sun, 4 Jul 2021 12:10:11 +0200
Subject: [PATCH] libmisc/salt.c: Use secure system ressources to obtain random
 bytes.
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

In a previous commit we introduced /dev/urandom as a source to obtain
random bytes from.  This may not be available on all systems, or when
operating inside of a chroot.

Almost all systems provide functions to obtain random bytes from
secure system ressources.  Thus we should prefer to use these, and
fall back to /dev/urandom, if there is no such function present, as
a last resort.

Signed-off-by: Björn Esser <besser82@fedoraproject.org>
---
 configure.ac   | 17 +++++++++--------
 libmisc/salt.c | 42 ++++++++++++++++++++++++++++++++++++------
 2 files changed, 45 insertions(+), 14 deletions(-)

Index: shadow-4.8.1/configure.ac
===================================================================
--- shadow-4.8.1.orig/configure.ac
+++ shadow-4.8.1/configure.ac
@@ -44,18 +44,19 @@ AC_HEADER_STDBOOL
 
 AC_CHECK_HEADERS(crypt.h errno.h fcntl.h limits.h unistd.h sys/time.h utmp.h \
 	utmpx.h termios.h termio.h sgtty.h sys/ioctl.h syslog.h paths.h \
-	utime.h ulimit.h sys/capability.h sys/resource.h gshadow.h lastlog.h \
-	locale.h rpc/key_prot.h netdb.h acl/libacl.h attr/libattr.h \
-	attr/error_context.h)
+	utime.h ulimit.h sys/capability.h sys/random.h sys/resource.h \
+	gshadow.h lastlog.h locale.h rpc/key_prot.h netdb.h acl/libacl.h \
+	attr/libattr.h attr/error_context.h)
 
 dnl shadow now uses the libc's shadow implementation
 AC_CHECK_HEADER([shadow.h],,[AC_MSG_ERROR([You need a libc with shadow.h])])
 
-AC_CHECK_FUNCS(l64a fchmod fchown fsync futimes getgroups gethostname getspnam \
-	gettimeofday getusershell getutent initgroups lchown lckpwdf lstat \
-	lutimes memcpy memset setgroups sigaction strchr updwtmp updwtmpx innetgr \
-	getpwnam_r getpwuid_r getgrnam_r getgrgid_r getspnam_r getaddrinfo \
-	ruserok dlopen)
+AC_CHECK_FUNCS(arc4random_buf l64a fchmod fchown fsync futimes getgroups \
+	gethostname getentropy getrandom getspnam gettimeofday getusershell \
+	getutent initgroups lchown lckpwdf lstat lutimes memcpy memset \
+	setgroups sigaction strchr updwtmp updwtmpx innetgr getpwnam_r \
+	getpwuid_r getgrnam_r getgrgid_r getspnam_r getaddrinfo ruserok \
+	dlopen)
 AC_SYS_LARGEFILE
 
 dnl Checks for typedefs, structures, and compiler characteristics.
Index: shadow-4.8.1/libmisc/salt.c
===================================================================
--- shadow-4.8.1.orig/libmisc/salt.c
+++ shadow-4.8.1/libmisc/salt.c
@@ -15,6 +15,9 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#if HAVE_SYS_RANDOM_H
+#include <sys/random.h>
+#endif
 #include "prototypes.h"
 #include "defines.h"
 #include "getdef.h"
@@ -128,19 +131,46 @@ static /*@observer@*/char *l64a (long va
 static long read_random_bytes (void)
 {
 	long randval = 0;
-	FILE *f = fopen ("/dev/urandom", "r");
 
-	if (fread (&randval, sizeof (randval), 1, f) != sizeof (randval))
-	{
-		fprintf (shadow_logfd,
-			 _("Unable to read from /dev/urandom.\n"));
+#ifdef HAVE_ARC4RANDOM_BUF
+	/* arc4random_buf, if it exists, can never fail.  */
+	arc4random_buf (&randval, sizeof (randval));
+	goto end;
+
+#elif defined(HAVE_GETENTROPY)
+	/* getentropy may exist but lack kernel support.  */
+	if (getentropy (&randval, sizeof (randval))) {
+		goto fail;
+	}
+
+	goto end;
+
+#elif defined(HAVE_GETRANDOM)
+	/* Likewise getrandom.  */
+	if ((size_t) getrandom (&randval, sizeof (randval), 0) != sizeof (randval)) {
+		goto fail;
+	}
 
+	goto end;
+
+#else
+	FILE *f = fopen ("/dev/urandom", "r");
+
+	if (fread (&randval, sizeof (randval), 1, f) != sizeof (randval)) {
 		fclose(f);
-		exit (1);
+		goto fail;
 	}
 
 	fclose(f);
+	goto end;
+#endif
+
+fail:
+	fprintf (shadow_logfd,
+		 _("Unable to obtain random bytes.\n"));
+	exit (1);
 
+end:
 	return randval;
 }