x86: Add --enable-cet=permissive

When CET is enabled, it is an error to dlopen a non CET enabled shared
library in CET enabled application.  It may be desirable to make CET
permissive, that is disable CET when dlopening a non CET enabled shared
library.  With the new --enable-cet=permissive configure option, CET is
disabled when dlopening a non CET enabled shared library.

Add DEFAULT_DL_X86_CET_CONTROL to config.h.in:

 /* The default value of x86 CET control.  */
 #define DEFAULT_DL_X86_CET_CONTROL cet_elf_property

which enables CET features based on ELF property note.

--enable-cet=permissive it to

 /* The default value of x86 CET control.  */
 #define DEFAULT_DL_X86_CET_CONTROL cet_permissive

which enables CET features permissively.

Update tst-cet-legacy-5a, tst-cet-legacy-5b, tst-cet-legacy-6a and
tst-cet-legacy-6b to check --enable-cet and --enable-cet=permissive.

INSTALL

@@ -123,20 +123,24 @@ if 'CFLAGS' is specified it must enable optimization.  For example:
      executables (PIE) by default.
 
 '--enable-cet'
+'--enable-cet=permissive'
      Enable Intel Control-flow Enforcement Technology (CET) support.
-     When the GNU C Library is built with '--enable-cet', the resulting
+     When the GNU C Library is built with '--enable-cet' or
-     library is protected with indirect branch tracking (IBT) and shadow
+     '--enable-cet=permissive', the resulting library is protected with
-     stack (SHSTK).  When CET is enabled, the GNU C Library is
+     indirect branch tracking (IBT) and shadow stack (SHSTK).  When CET
-     compatible with all existing executables and shared libraries.
+     is enabled, the GNU C Library is compatible with all existing
-     This feature is currently supported on i386, x86_64 and x32 with
+     executables and shared libraries.  This feature is currently
-     GCC 8 and binutils 2.29 or later.  Note that when CET is enabled,
+     supported on i386, x86_64 and x32 with GCC 8 and binutils 2.29 or
-     the GNU C Library requires CPUs capable of multi-byte NOPs, like
+     later.  Note that when CET is enabled, the GNU C Library requires
-     x86-64 processors as well as Intel Pentium Pro or newer.
+     CPUs capable of multi-byte NOPs, like x86-64 processors as well as
+     Intel Pentium Pro or newer.  With '--enable-cet', it is an error to
+     dlopen a non CET enabled shared library in CET enabled application.
+     With '--enable-cet=permissive', CET is disabled when dlopening a
+     non CET enabled shared library in CET enabled application.
 
      NOTE: '--enable-cet' has been tested for i686, x86_64 and x32 on
-     non-CET processors.  '--enable-cet' has been tested for x86_64 and
+     non-CET processors.  '--enable-cet' has been tested for i686,
-     x32 on CET SDVs, but Intel CET support hasn't been validated for
+     x86_64 and x32 on CET processors.
-     i686.
 
 '--disable-profile'
      Don't build libraries with profiling information.  You may want to

config.h.in

@@ -259,4 +259,7 @@
    in i386 6 argument syscall issue).  */
 #define CAN_USE_REGISTER_ASM_EBP 0
 
+/* The default value of x86 CET control.  */
+#define DEFAULT_DL_X86_CET_CONTROL cet_elf_property
+
 #endif

manual/install.texi

@@ -152,20 +152,24 @@ PIE.  This option also implies that glibc programs and tests are created
 as dynamic position independent executables (PIE) by default.
 
 @item --enable-cet
+@itemx --enable-cet=permissive
 Enable Intel Control-flow Enforcement Technology (CET) support.  When
-@theglibc{} is built with @option{--enable-cet}, the resulting library
+@theglibc{} is built with @option{--enable-cet} or
+@option{--enable-cet=permissive}, the resulting library
 is protected with indirect branch tracking (IBT) and shadow stack
 (SHSTK)@.  When CET is enabled, @theglibc{} is compatible with all
 existing executables and shared libraries.  This feature is currently
 supported on i386, x86_64 and x32 with GCC 8 and binutils 2.29 or later.
 Note that when CET is enabled, @theglibc{} requires CPUs capable of
 multi-byte NOPs, like x86-64 processors as well as Intel Pentium Pro or
-newer.
+newer.  With @option{--enable-cet}, it is an error to dlopen a non CET
+enabled shared library in CET enabled application.  With
+@option{--enable-cet=permissive}, CET is disabled when dlopening a
+non CET enabled shared library in CET enabled application.
 
 NOTE: @option{--enable-cet} has been tested for i686, x86_64 and x32
 on non-CET processors.  @option{--enable-cet} has been tested for
-x86_64 and x32 on CET SDVs, but Intel CET support hasn't been validated
+i686, x86_64 and x32 on CET processors.
-for i686.
 
 @item --disable-profile
 Don't build libraries with profiling information.  You may want to use

sysdeps/unix/sysv/linux/x86/Makefile

@@ -24,7 +24,7 @@ ifeq ($(subdir),setjmp)
 tests += tst-saved_mask-1
 endif
 
-ifeq ($(enable-cet),yes)
+ifneq ($(enable-cet),no)
 ifeq ($(subdir),elf)
 tests += tst-cet-property-1 tst-cet-property-2
 

sysdeps/x86/Makefile

@@ -14,7 +14,7 @@ gen-as-const-headers += jmp_buf-ssp.sym
 sysdep_routines += __longjmp_cancel
 endif
 
-ifeq ($(enable-cet),yes)
+ifneq ($(enable-cet),no)
 ifeq ($(subdir),elf)
 sysdep-dl-routines += dl-cet
 
@@ -42,13 +42,21 @@ CFLAGS-tst-cet-legacy-4.c += -fcf-protection=branch
 CFLAGS-tst-cet-legacy-4a.c += -fcf-protection
 CFLAGS-tst-cet-legacy-4b.c += -fcf-protection
 CFLAGS-tst-cet-legacy-mod-4.c += -fcf-protection=none
-CFLAGS-tst-cet-legacy-5a.c += -fcf-protection
+CFLAGS-tst-cet-legacy-5a.c += -fcf-protection -mshstk
-CFLAGS-tst-cet-legacy-5b.c += -fcf-protection
+ifeq ($(enable-cet),permissive)
+CPPFLAGS-tst-cet-legacy-5a.c += -DCET_IS_PERMISSIVE=1
+endif
+CFLAGS-tst-cet-legacy-5b.c += -fcf-protection -mshstk
+CPPFLAGS-tst-cet-legacy-5b.c += -DCET_DISABLED_BY_ENV=1
 CFLAGS-tst-cet-legacy-mod-5a.c += -fcf-protection=branch
 CFLAGS-tst-cet-legacy-mod-5b.c += -fcf-protection
 CFLAGS-tst-cet-legacy-mod-5c.c += -fcf-protection
-CFLAGS-tst-cet-legacy-6a.c += -fcf-protection
+CFLAGS-tst-cet-legacy-6a.c += -fcf-protection -mshstk
-CFLAGS-tst-cet-legacy-6b.c += -fcf-protection
+ifeq ($(enable-cet),permissive)
+CPPFLAGS-tst-cet-legacy-6a.c += -DCET_IS_PERMISSIVE=1
+endif
+CFLAGS-tst-cet-legacy-6b.c += -fcf-protection -mshstk
+CPPFLAGS-tst-cet-legacy-6b.c += -DCET_DISABLED_BY_ENV=1
 CFLAGS-tst-cet-legacy-mod-6a.c += -fcf-protection=branch
 CFLAGS-tst-cet-legacy-mod-6b.c += -fcf-protection
 CFLAGS-tst-cet-legacy-mod-6c.c += -fcf-protection

sysdeps/x86/configure

@@ -1,7 +1,7 @@
 # This file is generated from configure.ac by Autoconf.  DO NOT EDIT!
  # Local configure fragment for sysdeps/x86.
 
-if test x"$enable_cet" = xyes; then
+if test $enable_cet != no; then
   # Check if CET can be enabled.
   { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether CET can be enabled" >&5
 $as_echo_n "checking whether CET can be enabled... " >&6; }
@@ -27,17 +27,11 @@ EOF
 fi
 { $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_x86_cet_available" >&5
 $as_echo "$libc_cv_x86_cet_available" >&6; }
-  if test $libc_cv_x86_cet_available = yes; then
+  if test $libc_cv_x86_cet_available != yes; then
-    enable_cet=yes
+    as_fn_error $? "$CC doesn't support CET" "$LINENO" 5
-  else
-    if test x"$enable_cet" = xdefault; then
-      enable_cet=no
-    else
-      as_fn_error $? "$CC doesn't support CET" "$LINENO" 5
-    fi
   fi
 fi
-if test $enable_cet = yes; then
+if test $enable_cet != no; then
   # Check if assembler supports CET.
   { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $AS supports CET" >&5
 $as_echo_n "checking whether $AS supports CET... " >&6; }
@@ -65,5 +59,12 @@ $as_echo "$libc_cv_x86_cet_as" >&6; }
     as_fn_error $? "$AS doesn't support CET" "$LINENO" 5
   fi
 fi
+if test $enable_cet = yes; then
+  $as_echo "#define DEFAULT_DL_X86_CET_CONTROL cet_elf_property" >>confdefs.h
+
+elif test $enable_cet = permissive; then
+  $as_echo "#define DEFAULT_DL_X86_CET_CONTROL cet_permissive" >>confdefs.h
+
+fi
 config_vars="$config_vars
 enable-cet = $enable_cet"

sysdeps/x86/configure.ac

@@ -1,7 +1,7 @@
 GLIBC_PROVIDES dnl See aclocal.m4 in the top level source directory.
 # Local configure fragment for sysdeps/x86.
 
-if test x"$enable_cet" = xyes; then
+if test $enable_cet != no; then
   # Check if CET can be enabled.
   AC_CACHE_CHECK(whether CET can be enabled,
 		 libc_cv_x86_cet_available, [dnl
@@ -16,17 +16,11 @@ EOF
 		   libc_cv_x86_cet_available=no
 		 fi
 		 rm -rf conftest*])
-  if test $libc_cv_x86_cet_available = yes; then
+  if test $libc_cv_x86_cet_available != yes; then
-    enable_cet=yes
+    AC_MSG_ERROR([$CC doesn't support CET])
-  else
-    if test x"$enable_cet" = xdefault; then
-      enable_cet=no
-    else
-      AC_MSG_ERROR([$CC doesn't support CET])
-    fi
   fi
 fi
-if test $enable_cet = yes; then
+if test $enable_cet != no; then
   # Check if assembler supports CET.
   AC_CACHE_CHECK(whether $AS supports CET,
 		 libc_cv_x86_cet_as, [dnl
@@ -43,4 +37,9 @@ EOF
     AC_MSG_ERROR([$AS doesn't support CET])
   fi
 fi
+if test $enable_cet = yes; then
+  AC_DEFINE(DEFAULT_DL_X86_CET_CONTROL, cet_elf_property)
+elif test $enable_cet = permissive; then
+  AC_DEFINE(DEFAULT_DL_X86_CET_CONTROL, cet_permissive)
+fi
 LIBC_CONFIG_VAR([enable-cet], [$enable_cet])

sysdeps/x86/dl-procruntime.c

@@ -65,8 +65,8 @@ PROCINFO_CLASS struct dl_x86_feature_control _dl_x86_feature_control
 # endif
 # ifndef PROCINFO_DECL
 = {
-    .ibt = cet_elf_property,
+    .ibt = DEFAULT_DL_X86_CET_CONTROL,
-    .shstk = cet_elf_property
+    .shstk = DEFAULT_DL_X86_CET_CONTROL,
   }
 # endif
 # if !defined SHARED || defined PROCINFO_DECL

sysdeps/x86/tst-cet-legacy-5.c

@@ -22,6 +22,14 @@
 #include <stdlib.h>
 #include <stdbool.h>
 #include <string.h>
+#include <x86intrin.h>
+#include <support/check.h>
+
+#if defined CET_IS_PERMISSIVE || defined CET_DISABLED_BY_ENV
+# define CET_MAYBE_DISABLED 1
+#else
+# define CET_MAYBE_DISABLED 0
+#endif
 
 static void
 do_test_1 (const char *modname, bool fail)
@@ -32,24 +40,25 @@ do_test_1 (const char *modname, bool fail)
   h = dlopen (modname, RTLD_LAZY);
   if (h == NULL)
     {
+      const char *err = dlerror ();
       if (fail)
 	{
-	  const char *err = dlerror ();
 	  if (strstr (err, "rebuild shared object with SHSTK support enabled")
 	      == NULL)
-	    {
+	    FAIL_EXIT1 ("incorrect dlopen '%s' error: %s\n", modname, err);
-	      printf ("incorrect dlopen '%s' error: %s\n", modname,
-		      err);
-	      exit (1);
-	    }
 
 	  return;
 	}
 
-      printf ("cannot open '%s': %s\n", modname, dlerror ());
+      FAIL_EXIT1 ("cannot open '%s': %s\n", modname, err);
-      exit (1);
     }
 
+  /* NB: dlopen should never fail on non-CET platforms.  If SHSTK is
+     disabled, assuming IBT is also disabled.  */
+  bool cet_enabled = _get_ssp () != 0 && !CET_MAYBE_DISABLED;
+  if (fail && cet_enabled)
+    FAIL_EXIT1 ("dlopen should have failed\n");
+
   fp = dlsym (h, "test");
   if (fp == NULL)
     {

sysdeps/x86/tst-cet-legacy-6.c

@@ -22,6 +22,14 @@
 #include <stdlib.h>
 #include <stdbool.h>
 #include <string.h>
+#include <x86intrin.h>
+#include <support/check.h>
+
+#if defined CET_IS_PERMISSIVE || defined CET_DISABLED_BY_ENV
+# define CET_MAYBE_DISABLED 1
+#else
+# define CET_MAYBE_DISABLED 0
+#endif
 
 static void
 do_test_1 (const char *modname, bool fail)
@@ -32,24 +40,25 @@ do_test_1 (const char *modname, bool fail)
   h = dlopen (modname, RTLD_LAZY);
   if (h == NULL)
     {
+      const char *err = dlerror ();
       if (fail)
 	{
-	  const char *err = dlerror ();
 	  if (strstr (err, "rebuild shared object with SHSTK support enabled")
 	      == NULL)
-	    {
+	    FAIL_EXIT1 ("incorrect dlopen '%s' error: %s\n", modname, err);
-	      printf ("incorrect dlopen '%s' error: %s\n", modname,
-		      err);
-	      exit (1);
-	    }
 
 	  return;
 	}
 
-      printf ("cannot open '%s': %s\n", modname, dlerror ());
+      FAIL_EXIT1 ("cannot open '%s': %s\n", modname, err);
-      exit (1);
     }
 
+  /* NB: dlopen should never fail on non-CET platforms.  If SHSTK is
+     disabled, assuming IBT is also disabled.  */
+  bool cet_enabled = _get_ssp () != 0 && !CET_MAYBE_DISABLED;
+  if (fail && cet_enabled)
+    FAIL_EXIT1 ("dlopen should have failed\n");
+
   fp = dlsym (h, "test");
   if (fp == NULL)
     {