x86/CET: Update vfork to prevent child return

Child of vfork should either call _exit or one of the exec family of
functions.  But normally there is nothing to prevent child of vfork from
return of the vfork-calling function.  Simpilfy x86 vfork when shadow
stack is in use to introduce mismatched shadow stack in child of vfork
to trigger SIGSEGV when the child returns from the function in which
vfork was called.

1 files changed, 88 insertions, 0 deletions

diff --git a/sysdeps/unix/sysv/linux/x86/tst-cet-vfork-1.c b/sysdeps/unix/sysv/linux/x86/tst-cet-vfork-1.c
new file mode 100644
index 0000000000..5b9fc8c170
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/x86/tst-cet-vfork-1.c
@@ -0,0 +1,88 @@
+/* Verify that child of the vfork-calling function can't return when
+   shadow stack is in use.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <errno.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <x86intrin.h>
+#include <support/test-driver.h>
+#include <support/xsignal.h>
+#include <support/xunistd.h>
+
+__attribute__ ((noclone, noinline))
+static void
+do_test_1 (void)
+{
+  pid_t p1;
+  int fd[2];
+
+  if (pipe (fd) == -1)
+    {
+      puts ("pipe failed");
+      _exit (EXIT_FAILURE);
+    }
+
+  if ((p1 = vfork ()) == 0)
+    {
+      pid_t p = getpid ();
+      TEMP_FAILURE_RETRY (write (fd[1], &p, sizeof (p)));
+      /* Child return should trigger SIGSEGV.  */
+      return;
+    }
+  else if (p1 == -1)
+    {
+      puts ("vfork failed");
+      _exit (EXIT_FAILURE);
+    }
+
+  pid_t p2 = 0;
+  if (TEMP_FAILURE_RETRY (read (fd[0], &p2, sizeof (pid_t)))
+      != sizeof (pid_t))
+    puts ("pipd read failed");
+  else
+    {
+      int r;
+      if (TEMP_FAILURE_RETRY (waitpid (p1, &r, 0)) != p1)
+	puts ("waitpid failed");
+      else if (r != 0)
+	puts ("pip write in child failed");
+    }
+
+  /* Parent exits immediately so that parent returns without triggering
+     SIGSEGV when shadow stack isn't in use.  */
+  _exit (EXIT_FAILURE);
+}
+
+static int
+do_test (void)
+{
+  /* NB: This test should trigger SIGSEGV with shadow stack enabled.  */
+  if (_get_ssp () == 0)
+    return EXIT_UNSUPPORTED;
+  do_test_1 ();
+  /* Child exits immediately so that child returns without triggering
+     SIGSEGV when shadow stack isn't in use.  */
+  _exit (EXIT_FAILURE);
+}
+
+#define EXPECTED_SIGNAL (_get_ssp () == 0 ? 0 : SIGSEGV)
+#include <support/test-driver.c>