[PATCH] x86: Use pad in pthread_unwind_buf to preserve shadow stack register

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

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On Thu, Mar 29, 2018 at 1:20 PM, Florian Weimer <fw@deneb.enyo.de> wrote:
> * H. J. Lu:
>
>> On Thu, Mar 29, 2018 at 1:15 PM, Florian Weimer <fw@deneb.enyo.de> wrote:
>>> * H. J. Lu:
>>>
>>>> You need to make a choice.  You either don't introduce a new symbol
>>>> version or don't save shadow stack for thread cancellation.  You
>>>> can't have both.
>>>
>>> I don't understand.  We have room to save the shadow stack pointer in
>>> the existing struct.
>>
>> No, we don't have room in struct pthread_unwind_buf:
>>
>> Note: There is an unused pointer space in pthread_unwind_buf_data.  But
>> it isn't suitable for saving and restoring shadow stack register since
>> x32 is a 64-bit process with 32-bit software pointer and kernel may
>> place x32 shadow stack above 4GB.  We need to save and restore 64-bit
>> shadow stack register for x32.
>
> We have for void * fields.  They are subsequently overwritten by
> __pthread_register_cancel.  But __sigsetjmp can write to them first
> without causing any harm.  We just need a private __longjmp_cancel
> that doesn't restore the shadow stack pointer.

Here is the patch which does that.   Any comments?

Thanks.

-- 
H.J.

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From f222537447f5ec879427f318b7c0396362b7453a Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Sat, 24 Feb 2018 17:23:54 -0800
Subject: [PATCH] x86: Use pad in pthread_unwind_buf to preserve shadow stack
 register

The pad array in struct pthread_unwind_buf is used by setjmp to save
shadow stack register.  We assert that size of struct pthread_unwind_buf
is no less than offset of shadow stack pointer + shadow stack pointer
size.

Since functions, like LIBC_START_MAIN, START_THREAD_DEFN as well as
these with thread cancellation, call setjmp, but never return after
__libc_unwind_longjmp, __libc_unwind_longjmp, which is defined as
__libc_longjmp on x86, doesn't need to restore shadow stack register.
__libc_longjmp, which is a private interface for thread cancellation
implementation in libpthread, is changed to call __longjmp_cancel,
instead of __longjmp.  __longjmp_cancel is a new internal function
in libc, which is similar to __longjmp, but doesn't restore shadow
stack register.

The compatibility longjmp and siglongjmp in libpthread.so are changed
to call __libc_siglongjmp, instead of __libc_longjmp, so that they will
restore shadow stack register.

Tested with build-many-glibcs.py.

	* nptl/pthread_create.c (START_THREAD_DEFN): Clear previous
	handlers after setjmp.
	* setjmp/longjmp.c (__libc_longjmp): Don't define alias if
	defined.
	* sysdeps/unix/sysv/linux/x86/setjmpP.h (_JUMP_BUF_SIGSET_NSIG):
	Changed to 97.
	* sysdeps/x86/Makefile (sysdep_routines): Add __longjmp_cancel.
	* sysdeps/x86/__longjmp_cancel.S: New file.
	* sysdeps/x86/longjmp.c: Likewise.
	* sysdeps/x86/nptl/pt-longjmp.c: Likewise.
---
 nptl/pthread_create.c                 |  9 ++--
 setjmp/longjmp.c                      |  2 +
 sysdeps/unix/sysv/linux/x86/setjmpP.h |  4 +-
 sysdeps/x86/Makefile                  |  4 ++
 sysdeps/x86/__longjmp_cancel.S        | 20 ++++++++
 sysdeps/x86/longjmp.c                 | 45 ++++++++++++++++
 sysdeps/x86/nptl/pt-longjmp.c         | 97 +++++++++++++++++++++++++++++++++++
 7 files changed, 176 insertions(+), 5 deletions(-)
 create mode 100644 sysdeps/x86/__longjmp_cancel.S
 create mode 100644 sysdeps/x86/longjmp.c
 create mode 100644 sysdeps/x86/nptl/pt-longjmp.c

diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
index caaf07c134..1c5b3780c6 100644
--- a/nptl/pthread_create.c
+++ b/nptl/pthread_create.c
@@ -427,12 +427,15 @@ START_THREAD_DEFN
      compilers without that support we do use setjmp.  */
   struct pthread_unwind_buf unwind_buf;
 
-  /* No previous handlers.  */
+  int not_first_call;
+  not_first_call = setjmp ((struct __jmp_buf_tag *) unwind_buf.cancel_jmp_buf);
+
+  /* No previous handlers.  NB: This must be done after setjmp since
+     the same space may be used by setjmp to store extra data which
+     should never be used by __libc_unwind_longjmp.  */
   unwind_buf.priv.data.prev = NULL;
   unwind_buf.priv.data.cleanup = NULL;
 
-  int not_first_call;
-  not_first_call = setjmp ((struct __jmp_buf_tag *) unwind_buf.cancel_jmp_buf);
   if (__glibc_likely (! not_first_call))
     {
       /* Store the new cleanup handler info.  */
diff --git a/setjmp/longjmp.c b/setjmp/longjmp.c
index a2a7065a85..453889e103 100644
--- a/setjmp/longjmp.c
+++ b/setjmp/longjmp.c
@@ -40,9 +40,11 @@ __libc_siglongjmp (sigjmp_buf env, int val)
 }
 
 #ifndef __libc_siglongjmp
+# ifndef __libc_longjmp
 /* __libc_longjmp is a private interface for cancellation implementation
    in libpthread.  */
 strong_alias (__libc_siglongjmp, __libc_longjmp)
+# endif
 weak_alias (__libc_siglongjmp, _longjmp)
 weak_alias (__libc_siglongjmp, longjmp)
 weak_alias (__libc_siglongjmp, siglongjmp)
diff --git a/sysdeps/unix/sysv/linux/x86/setjmpP.h b/sysdeps/unix/sysv/linux/x86/setjmpP.h
index c0ed767a0d..90a6bbcf32 100644
--- a/sysdeps/unix/sysv/linux/x86/setjmpP.h
+++ b/sysdeps/unix/sysv/linux/x86/setjmpP.h
@@ -22,8 +22,8 @@
 #include <bits/types/__sigset_t.h>
 
 /* The biggest signal number + 1.  As of kernel 4.14, x86 _NSIG is 64.
-   Define it to 513 to leave some rooms for future use.  */
-#define _JUMP_BUF_SIGSET_NSIG	513
+   Define it to 97 to leave some rooms for future use.  */
+#define _JUMP_BUF_SIGSET_NSIG	97
 /* Number of longs to hold all signals.  */
 #define _JUMP_BUF_SIGSET_NWORDS \
   ((_JUMP_BUF_SIGSET_NSIG - 1 + 7) / (8 * sizeof (unsigned long int)))
diff --git a/sysdeps/x86/Makefile b/sysdeps/x86/Makefile
index 0d0326c21a..d25d6f0ae4 100644
--- a/sysdeps/x86/Makefile
+++ b/sysdeps/x86/Makefile
@@ -8,3 +8,7 @@ sysdep-dl-routines += dl-get-cpu-features
 tests += tst-get-cpu-features
 tests-static += tst-get-cpu-features-static
 endif
+
+ifeq ($(subdir),setjmp)
+sysdep_routines += __longjmp_cancel
+endif
diff --git a/sysdeps/x86/__longjmp_cancel.S b/sysdeps/x86/__longjmp_cancel.S
new file mode 100644
index 0000000000..b57dbfa376
--- /dev/null
+++ b/sysdeps/x86/__longjmp_cancel.S
@@ -0,0 +1,20 @@
+/* __longjmp_cancel for x86.
+   Copyright (C) 2018 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/>.  */
+
+#define __longjmp __longjmp_cancel
+#include <__longjmp.S>
diff --git a/sysdeps/x86/longjmp.c b/sysdeps/x86/longjmp.c
new file mode 100644
index 0000000000..a53f31e1dd
--- /dev/null
+++ b/sysdeps/x86/longjmp.c
@@ -0,0 +1,45 @@
+/* __libc_siglongjmp for x86.
+   Copyright (C) 2018 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/>.  */
+
+#define __libc_longjmp __redirect___libc_longjmp
+#include <setjmp/longjmp.c>
+#undef __libc_longjmp
+
+extern void __longjmp_cancel (__jmp_buf __env, int __val)
+     __attribute__ ((__noreturn__)) attribute_hidden;
+
+/* Since __libc_longjmp is a private interface for cancellation
+   implementation in libpthread, there is no need to restore shadow
+   stack register.  */
+
+void
+__libc_longjmp (sigjmp_buf env, int val)
+{
+  /* Perform any cleanups needed by the frames being unwound.  */
+  _longjmp_unwind (env, val);
+
+  if (env[0].__mask_was_saved)
+    /* Restore the saved signal mask.  */
+    (void) __sigprocmask (SIG_SETMASK,
+			  (sigset_t *) &env[0].__saved_mask,
+			  (sigset_t *) NULL);
+
+  /* Call the machine-dependent function to restore machine state
+     without shadow stack.  */
+  __longjmp_cancel (env[0].__jmpbuf, val ?: 1);
+}
diff --git a/sysdeps/x86/nptl/pt-longjmp.c b/sysdeps/x86/nptl/pt-longjmp.c
new file mode 100644
index 0000000000..7eb8651cfe
--- /dev/null
+++ b/sysdeps/x86/nptl/pt-longjmp.c
@@ -0,0 +1,97 @@
+/* ABI compatibility for 'longjmp' and 'siglongjmp' symbols in libpthread ABI.
+   X86 version.
+   Copyright (C) 18 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/>.  */
+
+/* <nptl/descr.h> has
+
+struct pthread_unwind_buf
+{
+  struct
+  {
+    __jmp_buf jmp_buf;
+    int mask_was_saved;
+  } cancel_jmp_buf[1];
+
+  union
+  {
+    void *pad[4];
+    struct
+    {
+      struct pthread_unwind_buf *prev;
+      struct _pthread_cleanup_buffer *cleanup;
+      int canceltype;
+    } data;
+  } priv;
+};
+
+  The pad array in struct pthread_unwind_buf is used by setjmp to save
+  shadow stack register.  Assert that size of struct pthread_unwind_buf
+  is no less than offset of shadow stack pointer plus shadow stack
+  pointer size.
+
+  NB: setjmp is called in libpthread to save shadow stack register.  But
+  __libc_unwind_longjmp doesn't restore shadow stack register since they
+  never return after longjmp.  */
+
+#include <pthreadP.h>
+#include <jmp_buf-ssp.h>
+
+#ifdef __x86_64__
+# define SHADOW_STACK_POINTER_SIZE 8
+#else
+# define SHADOW_STACK_POINTER_SIZE 4
+#endif
+
+_Static_assert ((sizeof (struct pthread_unwind_buf)
+		 >= (SHADOW_STACK_POINTER_OFFSET
+		     + SHADOW_STACK_POINTER_SIZE)),
+		"size of struct pthread_unwind_buf < "
+		"(SHADOW_STACK_POINTER_OFFSET + SHADOW_STACK_POINTER_SIZE)");
+
+#include <shlib-compat.h>
+
+/* libpthread once had its own longjmp (and siglongjmp alias), though there
+   was no apparent reason for it.  There is no use in having a separate
+   symbol in libpthread, but the historical ABI requires it.  For static
+   linking, there is no need to provide anything here--the libc version
+   will be linked in.  For shared library ABI compatibility, there must be
+   longjmp and siglongjmp symbols in libpthread.so.
+
+   With an IFUNC resolver, it would be possible to avoid the indirection,
+   but the IFUNC resolver might run before the __libc_longjmp symbol has
+   been relocated, in which case the IFUNC resolver would not be able to
+   provide the correct address.  */
+
+#if SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_22)
+
+static void __attribute__ ((noreturn, used))
+longjmp_compat (jmp_buf env, int val)
+{
+  /* NB: We call __libc_siglongjmp,  instead of __libc_longjmp, since
+     __libc_longjmp is a private interface for cancellation which
+     doesn't restore shadow stack register.  */
+  __libc_siglongjmp (env, val);
+}
+
+strong_alias (longjmp_compat, longjmp_alias)
+compat_symbol (libpthread, longjmp_alias, longjmp, GLIBC_2_0);
+
+strong_alias (longjmp_alias, siglongjmp_alias)
+compat_symbol (libpthread, siglongjmp_alias, siglongjmp, GLIBC_2_0);
+
+#endif
-- 
2.14.3