Re: [PATCH][BZ #13862] Reuse of cached stack can cause bounds overrun of thread DTV

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Thu, Nov 27, 2014 at 04:51:08PM -0500, Carlos O'Donell wrote:
> 
> >> Q: A dlclose() may modify GL(dl_tls_max_dtv_idx) concurrently with another
> >>    thread starting up. Is it safe to read GL(dl_tls_max_dtv_idx) more than
> >>    once in a function since they may differ? Does reading GL(dl_tls_max_dtv_idx)
> >>    require an atomic load? We are trying to be explicit about this in glibc.
> > 
> > There is a race between DTV access and dlclose:
> 
> That doesn't answer my question.
> 
> Alex Oliva and I went through the code and we have come to the conclusion
> that it is *not* safe to read GL(dl_tls_max_dtv_idx) more than once if the
> code doing the reading expects to get the same value. The same value is
> only guaranteed if you hold GL(dl_load_lock). However, it is safe to read
> an old value, since _dl_update_slotinfo will resize the DTV again if you need
> to access a TLS variable of a dlopen that was in-flight during thread creation.
> 
> Therefore, please make the read of GL(dl_tls_max_dtv_idx) an atomic access
> to prevent the compiler from reloading this and to document that it is
> written to by other threads concurrently.

Done.  I used atomic_load_acquire now.

> 
> >> Q: If GL(dl_tls_max_dtv_idx) may change at any time during thread startup
> >>    does it make sense to reallocate the dtv? Why not leave it uninitialized
> >>    and allow _dl_update_slotinfo to reallocate it?
> > 
> > Are you suggesting folding  _dl_allocate_tls_init into _dl_update_slotinfo
> > and remove _dl_allocate_tls_init?  I can give it a try.
> 
> I had not thought of that, but it's a good idea.
> 
> I don't know how you'd make it work though.
> 
> I think _dl_allocate_tls_init needs to do some book keeping, but the
> allocation of the DTV could be deferred to __tls_get_addr time 
> (_dl_update_slotinfo).

I tried and it doesn't work.

> > 
> > There is a race condition. But it is a separate issue.  They should
> > be fixed together.
> 
> You are adding new code. That new code should be correct. This needs
> an atomic load.

Done.  See above.
 
> >> Missing license.
> > 
> > What is the license policy on testcases?
> 
> Same license as the project with the appropriate header.

Added.

> >>> +
> >>> +#define DSO_SHARED_FILES 20
> >>> +#define DSO_OPEN_THREADS 20
> >>> +#define DSO_EXEC_THREADS 2
> >>
> >> Why these particular constants? If you chose them because
> >> they happened to work on a particular system, please comment
> >> that.
> > 
> > They came from:
> > 
> > https://sourceware.org/bugzilla/attachment.cgi?id=6290
> 
> You don't answer the question.
> 
> If you are submitting the patch you have to be ready to defend it.
> 
> If you can't defend it you must at least add a comment saying the
> choices are arbitrary.
> 
> /* The choices of thread count, and file counts are arbitary.
>    The point is simply to run enough threads that an exiting
>    thread has it's stack reused by another thread at the same
>    time as new libraries have been loaded.  */

Thanks for the suggestion.  I copied it to the testcase.

> >> Use /* */.
> > 
> > We have been using // for comments in glibc for a while.  Why not here?
> 
> glibc follows the GNU Coding Standard which says to use /* */?
> 

Done.  Changed to /**/.

> >>
> >> This test needs a verbose, minimum one paragraph, comment explaining
> >> what the test is testing.
> 
> No comment?
 
I added one paragraph for each file.

> >>> +
> >>> +  for (j = 0; j < 100; j++)
> >>
> >> Why 100 times? Please comment.
> 
> Comment stating this is arbitrary.

Done.
 
> >>> +void
> >>> +function (void)
> >>> +{
> >>> +  int i;
> >>> +  for (i = 0; i < 256; i++)
> >>
> >> Why 256 times? Please comment.
> >>> +    var[i] = i;
> >>> +}
> >>>
> > 
> > I don't have answers for any particular values in this
> > testcase.
> 
> Then add a comment saying they are arbitrary. Such that
> future readers know they are not special values.
> 

I added some comments.

Here is the updated patch.  OK to install?

Thanks.


H.J.
---
>From f08bbe49c8820576e2149a5be44af0ce2ddb355b Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Thu, 27 Nov 2014 05:42:23 -0800
Subject: [PATCH] Resize DTV if the current DTV isn't big enough

This patch changes _dl_allocate_tls_init to resize DTV if the current DTV
isn't big enough.  Tested on X86-64, x32 and ia32.

	[BZ #13862]
	* elf/dl-tls.c: Include <atomic.h>.
	(oom): Remove #ifdef SHARED/#endif.
	(_dl_static_dtv, _dl_initial_dtv): Moved before ...
	(_dl_resize_dtv): This.  Extracted from _dl_update_slotinfo.
	(_dl_allocate_tls_init): Resize DTV if the current DTV isn't
	big enough.
	(_dl_update_slotinfo): Call _dl_resize_dtv to resize DTV.
	* nptl/Makefile (tests): Add tst-stack4.
	(modules-names): Add tst-stack4mod.
	($(objpfx)tst-stack4): New.
	(tst-stack4mod.sos): Likewise.
	($(objpfx)tst-stack4.out): Likewise.
	($(tst-stack4mod.sos)): Likewise.
	(clean): Likewise.
	* nptl/tst-stack4.c: New file.
	* nptl/tst-stack4mod.c: Likewise.
---
 ChangeLog            |  20 +++++++
 elf/dl-tls.c         | 102 ++++++++++++++++++++-------------
 nptl/Makefile        |  17 +++++-
 nptl/tst-stack4.c    | 159 +++++++++++++++++++++++++++++++++++++++++++++++++++
 nptl/tst-stack4mod.c |  28 +++++++++
 5 files changed, 283 insertions(+), 43 deletions(-)
 create mode 100644 nptl/tst-stack4.c
 create mode 100644 nptl/tst-stack4mod.c

diff --git a/ChangeLog b/ChangeLog
index be49dba..6a535e7 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,23 @@
+2014-11-27  H.J. Lu  <hongjiu.lu@intel.com>
+
+	[BZ #13862]
+	* elf/dl-tls.c: Include <atomic.h>.
+	(oom): Remove #ifdef SHARED/#endif.
+	(_dl_static_dtv, _dl_initial_dtv): Moved before ...
+	(_dl_resize_dtv): This.  Extracted from _dl_update_slotinfo.
+	(_dl_allocate_tls_init): Resize DTV if the current DTV isn't
+	big enough.
+	(_dl_update_slotinfo): Call _dl_resize_dtv to resize DTV.
+	* nptl/Makefile (tests): Add tst-stack4.
+	(modules-names): Add tst-stack4mod.
+	($(objpfx)tst-stack4): New.
+	(tst-stack4mod.sos): Likewise.
+	($(objpfx)tst-stack4.out): Likewise.
+	($(tst-stack4mod.sos)): Likewise.
+	(clean): Likewise.
+	* nptl/tst-stack4.c: New file.
+	* nptl/tst-stack4mod.c: Likewise.
+
 2014-11-27  J. Brown  <jb999@gmx.de>
 
 	* sysdeps/x86/bits/string.h: Add recent CPUs.
diff --git a/elf/dl-tls.c b/elf/dl-tls.c
index 5204fda..8dd4992 100644
--- a/elf/dl-tls.c
+++ b/elf/dl-tls.c
@@ -23,6 +23,7 @@
 #include <stdlib.h>
 #include <unistd.h>
 #include <sys/param.h>
+#include <atomic.h>
 
 #include <tls.h>
 #include <dl-tls.h>
@@ -34,14 +35,12 @@
 
 
 /* Out-of-memory handler.  */
-#ifdef SHARED
 static void
 __attribute__ ((__noreturn__))
 oom (void)
 {
   _dl_fatal_printf ("cannot allocate memory for thread-local data: ABORT\n");
 }
-#endif
 
 
 size_t
@@ -397,6 +396,53 @@ _dl_allocate_tls_storage (void)
 }
 
 
+#ifndef SHARED
+extern dtv_t _dl_static_dtv[];
+# define _dl_initial_dtv (&_dl_static_dtv[1])
+#endif
+
+static dtv_t *
+_dl_resize_dtv (dtv_t *dtv)
+{
+  /* Resize the dtv.  */
+  dtv_t *newp;
+  /* Load GL(dl_tls_max_dtv_idx) atomically since it may be written to by
+     other threads oncurrently.  */
+  size_t newsize
+    = atomic_load_acquire (&GL(dl_tls_max_dtv_idx)) + DTV_SURPLUS;
+  size_t oldsize = dtv[-1].counter;
+
+  if (dtv == GL(dl_initial_dtv))
+    {
+      /* This is the initial dtv that was either statically allocated in
+	 __libc_setup_tls or allocated during rtld startup using the
+	 dl-minimal.c malloc instead of the real malloc.  We can't free
+	 it, we have to abandon the old storage.  */
+
+      newp = malloc ((2 + newsize) * sizeof (dtv_t));
+      if (newp == NULL)
+	oom ();
+      memcpy (newp, &dtv[-1], (2 + oldsize) * sizeof (dtv_t));
+    }
+  else
+    {
+      newp = realloc (&dtv[-1],
+		      (2 + newsize) * sizeof (dtv_t));
+      if (newp == NULL)
+	oom ();
+    }
+
+  newp[0].counter = newsize;
+
+  /* Clear the newly allocated part.  */
+  memset (newp + 2 + oldsize, '\0',
+	  (newsize - oldsize) * sizeof (dtv_t));
+
+  /* Return the generation counter.  */
+  return &newp[1];
+}
+
+
 void *
 internal_function
 _dl_allocate_tls_init (void *result)
@@ -410,6 +456,16 @@ _dl_allocate_tls_init (void *result)
   size_t total = 0;
   size_t maxgen = 0;
 
+  /* Check if the current dtv is big enough.   */
+  if (dtv[-1].counter < GL(dl_tls_max_dtv_idx))
+    {
+      /* Resize the dtv.  */
+      dtv = _dl_resize_dtv (dtv);
+
+      /* Install this new dtv in the thread data structures.  */
+      INSTALL_DTV (result, &dtv[-1]);
+    }
+
   /* We have to prepare the dtv for all currently loaded modules using
      TLS.  For those which are dynamically loaded we add the values
      indicating deferred allocation.  */
@@ -492,11 +548,6 @@ _dl_allocate_tls (void *mem)
 rtld_hidden_def (_dl_allocate_tls)
 
 
-#ifndef SHARED
-extern dtv_t _dl_static_dtv[];
-# define _dl_initial_dtv (&_dl_static_dtv[1])
-#endif
-
 void
 internal_function
 _dl_deallocate_tls (void *tcb, bool dealloc_tcb)
@@ -645,41 +696,10 @@ _dl_update_slotinfo (unsigned long int req_modid)
 	      assert (total + cnt == modid);
 	      if (dtv[-1].counter < modid)
 		{
-		  /* Reallocate the dtv.  */
-		  dtv_t *newp;
-		  size_t newsize = GL(dl_tls_max_dtv_idx) + DTV_SURPLUS;
-		  size_t oldsize = dtv[-1].counter;
-
-		  assert (map->l_tls_modid <= newsize);
-
-		  if (dtv == GL(dl_initial_dtv))
-		    {
-		      /* This is the initial dtv that was allocated
-			 during rtld startup using the dl-minimal.c
-			 malloc instead of the real malloc.  We can't
-			 free it, we have to abandon the old storage.  */
-
-		      newp = malloc ((2 + newsize) * sizeof (dtv_t));
-		      if (newp == NULL)
-			oom ();
-		      memcpy (newp, &dtv[-1], (2 + oldsize) * sizeof (dtv_t));
-		    }
-		  else
-		    {
-		      newp = realloc (&dtv[-1],
-				      (2 + newsize) * sizeof (dtv_t));
-		      if (newp == NULL)
-			oom ();
-		    }
-
-		  newp[0].counter = newsize;
-
-		  /* Clear the newly allocated part.  */
-		  memset (newp + 2 + oldsize, '\0',
-			  (newsize - oldsize) * sizeof (dtv_t));
+		  /* Resize the dtv.  */
+		  dtv = _dl_resize_dtv (dtv);
 
-		  /* Point dtv to the generation counter.  */
-		  dtv = &newp[1];
+		  assert (modid <= dtv[-1].counter);
 
 		  /* Install this new dtv in the thread data
 		     structures.  */
diff --git a/nptl/Makefile b/nptl/Makefile
index 86a1b0c..ac76596 100644
--- a/nptl/Makefile
+++ b/nptl/Makefile
@@ -254,7 +254,7 @@ tests = tst-typesizes \
 	tst-exec1 tst-exec2 tst-exec3 tst-exec4 \
 	tst-exit1 tst-exit2 tst-exit3 \
 	tst-stdio1 tst-stdio2 \
-	tst-stack1 tst-stack2 tst-stack3 tst-pthread-getattr \
+	tst-stack1 tst-stack2 tst-stack3 tst-stack4 tst-pthread-getattr \
 	tst-pthread-attr-affinity \
 	tst-unload \
 	tst-dlsym1 \
@@ -311,7 +311,7 @@ endif
 
 modules-names = tst-atfork2mod tst-tls3mod tst-tls4moda tst-tls4modb \
 		tst-tls5mod tst-tls5moda tst-tls5modb tst-tls5modc \
-		tst-tls5modd tst-tls5mode tst-tls5modf \
+		tst-tls5modd tst-tls5mode tst-tls5modf tst-stack4mod \
 		tst-_res1mod1 tst-_res1mod2 tst-execstack-mod tst-fini1mod
 extra-test-objs += $(addsuffix .os,$(strip $(modules-names))) tst-cleanup4aux.o
 test-extras += $(modules-names) tst-cleanup4aux
@@ -479,6 +479,19 @@ $(objpfx)tst-stack3-mem.out: $(objpfx)tst-stack3.out
 	$(evaluate-test)
 generated += tst-stack3-mem.out tst-stack3.mtrace
 
+$(objpfx)tst-stack4: $(libdl) $(shared-thread-library)
+tst-stack4mod.sos=$(shell for i in 0 1 2 3 4 5 6 7 8 9 10 \
+				   11 12 13 14 15 16 17 18 19; do \
+			    for j in 0 1 2 3 4 5 6 7 8 9 10 \
+				     11 12 13 14 15 16 17 18 19; do \
+			      echo $(objpfx)tst-stack4mod-$$i-$$j.so; \
+			    done; done)
+$(objpfx)tst-stack4.out: $(tst-stack4mod.sos)
+$(tst-stack4mod.sos): $(objpfx)tst-stack4mod.so
+	cp -f $< $@
+clean:
+	rm -f $(tst-stack4mod.sos)
+
 $(objpfx)tst-cleanup4: $(objpfx)tst-cleanup4aux.o $(shared-thread-library)
 $(objpfx)tst-cleanupx4: $(objpfx)tst-cleanup4aux.o $(shared-thread-library)
 
diff --git a/nptl/tst-stack4.c b/nptl/tst-stack4.c
new file mode 100644
index 0000000..344c658
--- /dev/null
+++ b/nptl/tst-stack4.c
@@ -0,0 +1,159 @@
+/* Test DTV size overflow with pthread_create and TLS in dlopened shared
+   object.
+   Copyright (C) 2014 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 <stdio.h>
+#include <stdint.h>
+#include <dlfcn.h>
+#include <assert.h>
+#include <pthread.h>
+
+/* The choices of thread count, and file counts are arbitary.
+   The point is simply to run enough threads that an exiting
+   thread has it's stack reused by another thread at the same
+   time as new libraries have been loaded.  */
+#define DSO_SHARED_FILES 20
+#define DSO_OPEN_THREADS 20
+#define DSO_EXEC_THREADS 2
+
+/* Used to make sure that only one thread is calling dlopen and dlclose
+   at a time.  */
+pthread_mutex_t g_lock;
+
+typedef void (*function) (void);
+
+void *
+dso_invoke(void *dso_fun)
+{
+  function *fun_vec = (function *) dso_fun;
+  int dso;
+
+  for (dso = 0; dso < DSO_SHARED_FILES; dso++)
+    (*fun_vec[dso]) ();
+
+  pthread_exit (NULL);
+}
+
+void *
+dso_process (void * p)
+{
+  void *handle[DSO_SHARED_FILES];
+  function fun_vec[DSO_SHARED_FILES];
+  char dso_path[DSO_SHARED_FILES][100];
+  int dso;
+  uintptr_t t = (uintptr_t) p;
+
+  /* Open DSOs and get a function.  */
+  for (dso = 0; dso < DSO_SHARED_FILES; dso++)
+    {
+      sprintf (dso_path[dso], "tst-stack4mod-%i-%i.so", t, dso);
+
+      pthread_mutex_lock (&g_lock);
+
+      handle[dso] = dlopen (dso_path[dso], RTLD_NOW);
+      assert (handle[dso]);
+
+      fun_vec[dso] = (function) dlsym (handle[dso], "function");
+      assert (fun_vec[dso]);
+
+      pthread_mutex_unlock (&g_lock);
+    }
+
+  /* Spawn workers.  */
+  pthread_t thread[DSO_EXEC_THREADS];
+  int i, ret;
+  uintptr_t result = 0;
+  for (i = 0; i < DSO_EXEC_THREADS; i++)
+    {
+      pthread_mutex_lock (&g_lock);
+      ret = pthread_create (&thread[i], NULL, dso_invoke, (void *) fun_vec);
+      if (ret != 0)
+	{
+	  printf ("pthread_create failed: %d\n", ret);
+	  result = 1;
+	}
+      pthread_mutex_unlock (&g_lock);
+    }
+
+  if (!result) 
+    for (i = 0; i < DSO_EXEC_THREADS; i++)
+      {
+	ret = pthread_join (thread[i], NULL);
+	if (ret != 0)
+	  {
+	    printf ("pthread_join failed: %d\n", ret);
+	    result = 1;
+	  }
+      }
+
+  /* Close all DSOs.  */
+  for (dso = 0; dso < DSO_SHARED_FILES; dso++)
+    {
+      pthread_mutex_lock (&g_lock);
+      dlclose (handle[dso]);
+      pthread_mutex_unlock (&g_lock);
+    }
+
+  /* Exit.  */
+  pthread_exit ((void *) result);
+}
+
+static int
+do_test (void)
+{
+  pthread_t thread[DSO_OPEN_THREADS];
+  int i,j;
+  int ret;
+  int result = 0;
+
+  pthread_mutex_init (&g_lock, NULL);
+
+  /* 100 is arbitrary here and is known to trigger PR 13862.  */
+  for (j = 0; j < 100; j++)
+    {
+      for (i = 0; i < DSO_OPEN_THREADS; i++)
+	{
+	  ret = pthread_create (&thread[i], NULL, dso_process,
+				(void *) (uintptr_t) i);
+	  if (ret != 0)
+	    {
+	      printf ("pthread_create failed: %d\n", ret);
+	      result = 1;
+	    }
+	}
+
+      if (result)
+	break;
+
+      for (i = 0; i < DSO_OPEN_THREADS; i++)
+	{
+	  ret = pthread_join (thread[i], NULL);
+	  if (ret != 0)
+	    {
+	      printf ("pthread_join failed: %d\n", ret);
+	      result = 1;
+	    }
+	}
+    }
+
+  return result;
+}
+
+#define TEST_FUNCTION do_test ()
+#define TIMEOUT 100
+#include "../test-skeleton.c"
diff --git a/nptl/tst-stack4mod.c b/nptl/tst-stack4mod.c
new file mode 100644
index 0000000..a75aa09
--- /dev/null
+++ b/nptl/tst-stack4mod.c
@@ -0,0 +1,28 @@
+/* This tests DTV usage with TLS in dlopened shared object.
+   Copyright (C) 2014 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/>.  */
+
+/* 256 is arbitrary here and is known to trigger PR 13862.  */
+__thread int var[256] attribute_hidden = {0};
+
+void
+function (void)
+{
+  int i;
+  for (i = 0; i < sizeof (var) / sizeof (int); i++)
+    var[i] = i;
+}
-- 
1.9.3