xorg-x11-server/u_Use-better-fallbacks-to-generate-cookies-if-arc4rand.patch

From 44a643812ce3c07cd38972abfa9dbd163529c192 Mon Sep 17 00:00:00 2001
From: Matthias Gerstner <mgerstner@suse.de>
Date: Thu, 13 Jul 2017 14:58:04 +0200
Subject: [PATCH] Use better fallbacks to generate cookies if arc4random_buf(3)
 is unavailable
References: bsc#1025084

If arc4random_buf() is not available for generating cookies:

- use getentropy(), if available (which was only recently added to
  glibc)
- use getrandom() via syscall(), if available (there was no glibc
  wrapper for this syscall for a long time)
- if all else fails, directly read from /dev/urandom as before, but
  employ O_CLOEXEC, do an OsAbort() in case the random data couldn't be
  read to avoid unsecure situations. Don't know if that's too hard a
  measure but it shouldn't actually occur except on maximum number of
  FDs reached

Reviewed-by: Stefan Dirsch <sndirsch@suse.de>
---
 configure.ac            |   4 +-
 include/dix-config.h.in |   6 +++
 os/auth.c               | 138 ++++++++++++++++++++++++++++++++++++++++++++++--
 3 files changed, 141 insertions(+), 7 deletions(-)

Index: xorg-server-1.19.3/configure.ac
===================================================================
--- xorg-server-1.19.3.orig/configure.ac
+++ xorg-server-1.19.3/configure.ac
@@ -134,7 +134,7 @@ AM_CONDITIONAL(SPECIAL_DTRACE_OBJECTS, [
 AC_HEADER_DIRENT
 AC_HEADER_STDC
 AC_CHECK_HEADERS([fcntl.h stdlib.h string.h unistd.h dlfcn.h stropts.h \
- fnmatch.h sys/mkdev.h sys/sysmacros.h sys/utsname.h])
+ fnmatch.h sys/mkdev.h sys/sysmacros.h sys/utsname.h sys/syscall.h])
 
 dnl Checks for typedefs, structures, and compiler characteristics.
 AC_C_CONST
@@ -226,7 +226,7 @@ AC_REPLACE_FUNCS([reallocarray strcasecm
 AM_CONDITIONAL(POLL, [test "x$ac_cv_func_poll" = "xyes"])
 
 AC_CHECK_LIB([bsd], [arc4random_buf])
-AC_CHECK_FUNCS([arc4random_buf])
+AC_CHECK_FUNCS([arc4random_buf getentropy])
 
 AC_CHECK_DECLS([program_invocation_short_name], [], [], [[#include <errno.h>]])
 
Index: xorg-server-1.19.3/include/dix-config.h.in
===================================================================
--- xorg-server-1.19.3.orig/include/dix-config.h.in
+++ xorg-server-1.19.3/include/dix-config.h.in
@@ -167,6 +167,9 @@
 /* Define to 1 if you have the `arc4random_buf' function. */
 #undef HAVE_ARC4RANDOM_BUF
 
+/* Define to 1 if you have the `getentropy' function. */
+#undef HAVE_GETENTROPY
+
 /* Define to use libc SHA1 functions */
 #undef HAVE_SHA1_IN_LIBC
 
@@ -244,6 +247,9 @@
 /* Define to 1 if you have the <sys/utsname.h> header file. */
 #undef HAVE_SYS_UTSNAME_H
 
+/* Define to 1 if you have the <sys/syscall.h> header file. */
+#undef HAVE_SYS_SYSCALL_H
+
 /* Define to 1 if you have the `timingsafe_memcmp' function. */
 #undef HAVE_TIMINGSAFE_MEMCMP
 
Index: xorg-server-1.19.3/os/auth.c
===================================================================
--- xorg-server-1.19.3.orig/os/auth.c
+++ xorg-server-1.19.3/os/auth.c
@@ -48,6 +48,10 @@ from The Open Group.
 #ifdef HAVE_LIBBSD
 #include   <bsd/stdlib.h>       /* for arc4random_buf() */
 #endif
+#include   <errno.h>
+#ifdef HAVE_SYS_SYSCALL_H
+#include   <syscall.h>
+#endif
 
 struct protocol {
     unsigned short name_length;
@@ -302,18 +306,142 @@ GenerateAuthorization(unsigned name_leng
     return -1;
 }
 
+#if ! defined(HAVE_ARC4RANDOM_BUF)
+
+// fallback function to get random data directly from /dev/urandom
+
+static int
+GetUrandom ( char *buffer, size_t length )
+{
+    int random_fd = -1;
+    int res = -1;
+    size_t filled = 0;
+
+    // larger requests are typically rejected by getentropy() / getrandom()
+    // because they could block or return partially filled buffers
+    if( length > 256 ) {
+        errno = EIO;
+        return -1;
+    }
+
+    random_fd = open("/dev/urandom", O_RDONLY | O_CLOEXEC);
+
+    if( random_fd == -1 ) {
+        return -1;
+    }
+
+    while( filled < length ) {
+        res = read(random_fd, (char*)buffer + filled, length - filled);
+
+        if( res == -1 ) {
+            // shouldn't actually happen acc. to man(4) random,
+            // but you never know
+            if( errno == EINTR ) {
+                continue;
+            }
+
+            res = errno;
+            close(random_fd);
+            errno = res;
+            return -1;
+        }
+        else if( res == 0 ) {
+            close(random_fd);
+            // no more bytes available? should not happen
+            errno = EIO;
+            return -1;
+        }
+
+        filled += res;
+    }
+
+    close(random_fd);
+
+    return 0;
+}
+
+#endif // ! defined(HAVE_ARC4RANDOM_BUF)
+
+#if !defined(HAVE_GETENTROPY) && defined(HAVE_SYS_SYSCALL_H) && defined(SYS_getrandom)
+#    define TRY_GETRANDOM
+#endif
+
+#ifdef TRY_GETRANDOM
+
+/*
+ * wrapper for the getrandom() syscall which was for a long time implemented
+ * in the Linux kernel, but not wrapped in glibc
+ */
+static int
+GetRandom ( char *buffer, size_t length )
+{
+    int res;
+    size_t filled = 0;
+
+    // larger requests are typically rejected by getentropy() / getrandom()
+    // because they could block or return partially filled buffers
+    if( length > 256 )
+    {
+        errno = EIO;
+        return -1;
+    }
+
+    while( filled < length )
+    {
+        /*
+         * glibc does not contain a syscall wrapper for this in older
+         * versions
+         */
+        res = syscall(SYS_getrandom, (char*)buffer + filled, length - filled, 0);
+
+        if( res == -1 )
+        {
+            if( errno == EINTR ) {
+                continue;
+            }
+
+            return -1;
+        }
+        else if( res == 0 )
+        {
+            // no more bytes available? should not happen
+            errno = EIO;
+            return -1;
+        }
+
+        filled += res;
+    }
+
+    return 0;
+}
+
+#endif /* TRY_GETRANDOM */
+
 void
 GenerateRandomData(int len, char *buf)
 {
 #ifdef HAVE_ARC4RANDOM_BUF
     arc4random_buf(buf, len);
 #else
-    int fd;
+    int ret = -1;
+#   ifdef HAVE_GETENTROPY
+    /* use getentropy instead */
+    ret = getentropy (buf, len);
+#   elif defined(TRY_GETRANDOM)
+    /* try getrandom() wrapper */
+    ret = GetRandom(buf, len);
+#   endif
+
+    if( ret == -1 ) {
+        // fallback to manual reading of /dev/urandom
+        ret = GetUrandom(buf, len);
+    }
 
-    fd = open("/dev/urandom", O_RDONLY);
-    read(fd, buf, len);
-    close(fd);
-#endif
+    if( ret == -1 ) {
+        // no error return possible, rather abort than have security problems
+        OsAbort();
+    }
+#endif // HAVE_ARC4RANDOM_BUF
 }
 
 #endif                          /* XCSECURITY */