Bug 20116: Fix use after free in pthread_create()

The commit documents the ownership rules around 'struct pthread' and
when a thread can read or write to the descriptor. With those ownership
rules in place it becomes obvious that pd->stopped_start should not be
touched in several of the paths during thread startup, particularly so
for detached threads. In the case of detached threads, between the time
the thread is created by the OS kernel and the creating thread checks
pd->stopped_start, the detached thread might have already exited and the
memory for pd unmapped. As a regression test we add a simple test which
exercises this exact case by quickly creating detached threads with
large enough stacks to ensure the thread stack cache is bypassed and the
stacks are unmapped. Before the fix the testcase segfaults, after the
fix it works correctly and completes without issue.

For a detailed discussion see:
https://www.sourceware.org/ml/libc-alpha/2017-01/msg00505.html

1 files changed, 26 insertions, 0 deletions

diff --git a/support/xpthread_attr_destroy.c b/support/xpthread_attr_destroy.c
new file mode 100644
index 0000000000..664c809e9f
--- /dev/null
+++ b/support/xpthread_attr_destroy.c
@@ -0,0 +1,26 @@
+/* pthread_attr_destroy with error checking.
+   Copyright (C) 2017 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
+   <http://www.gnu.org/licenses/>.  */
+
+#include <support/xthread.h>
+
+void
+xpthread_attr_destroy (pthread_attr_t *attr)
+{
+  xpthread_check_return ("pthread_attr_destroy",
+			 pthread_attr_destroy (attr));
+}