Patchwork [v4,4/4] i386: Fix i386 sigaction sa_restorer initialization (BZ#21269)

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Submitter Adhemerval Zanella Netto
Date Feb. 12, 2018, 12:42 p.m.
Message ID <1518439345-6013-4-git-send-email-adhemerval.zanella@linaro.org>
Download mbox | patch
Permalink /patch/25914/
State New
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Comments

Adhemerval Zanella Netto - Feb. 12, 2018, 12:42 p.m.
This patch fixes the i386 sa_restorer field initialization for sigaction
syscall for kernel with vDSO.  As described in bug report, x86_32 Linux
(and compat on x86_64) interprets SA_RESTORER clear with nonzero
sa_restorer as a request for stack switching if the SS segment is 'funny'
This means that anything that tries to mix glibc's signal handling with
segmentation (for instance through modify_ldt syscall) is randomly broken
depending on what values lands in sa_restorer.

The testcase added  is based on Linux test tools/testing/selftests/x86/ldt_gdt.c,
more specifically in do_multicpu_tests function.  The main changes are:

  - C11 atomics instead of plain access.

  - Remove x86_64 support which simplifies the syscall handling and fallbacks.

  - Replicate only the test required to trigger the issue.

(I added some comments of my understanding of how the testcase is triggering
the issue, so if someone with more x86 knowledge could check if I get this
right I will be grateful)

Checked on i686-linux-gnu.

	[BZ #21269]
	* sysdeps/unix/sysv/linux/i386/Makefile (tests): Add tst-bz21269.
	* sysdeps/unix/sysv/linux/i386/sigaction.c (SET_SA_RESTORER): Clear
	sa_restorer for vDSO case.
	* sysdeps/unix/sysv/linux/i386/tst-bz21269.c: New file.

Signed-off-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
---
 ChangeLog                                  |   6 +
 sysdeps/unix/sysv/linux/i386/Makefile      |   3 +
 sysdeps/unix/sysv/linux/i386/sigaction.c   |   2 +
 sysdeps/unix/sysv/linux/i386/tst-bz21269.c | 233 +++++++++++++++++++++++++++++
 4 files changed, 244 insertions(+)
 create mode 100644 sysdeps/unix/sysv/linux/i386/tst-bz21269.c
Adhemerval Zanella Netto - Feb. 19, 2018, 5:50 p.m.
Ping.

On 12/02/2018 10:42, Adhemerval Zanella wrote:
> This patch fixes the i386 sa_restorer field initialization for sigaction
> syscall for kernel with vDSO.  As described in bug report, x86_32 Linux
> (and compat on x86_64) interprets SA_RESTORER clear with nonzero
> sa_restorer as a request for stack switching if the SS segment is 'funny'
> This means that anything that tries to mix glibc's signal handling with
> segmentation (for instance through modify_ldt syscall) is randomly broken
> depending on what values lands in sa_restorer.
> 
> The testcase added  is based on Linux test tools/testing/selftests/x86/ldt_gdt.c,
> more specifically in do_multicpu_tests function.  The main changes are:
> 
>   - C11 atomics instead of plain access.
> 
>   - Remove x86_64 support which simplifies the syscall handling and fallbacks.
> 
>   - Replicate only the test required to trigger the issue.
> 
> (I added some comments of my understanding of how the testcase is triggering
> the issue, so if someone with more x86 knowledge could check if I get this
> right I will be grateful)
> 
> Checked on i686-linux-gnu.
> 
> 	[BZ #21269]
> 	* sysdeps/unix/sysv/linux/i386/Makefile (tests): Add tst-bz21269.
> 	* sysdeps/unix/sysv/linux/i386/sigaction.c (SET_SA_RESTORER): Clear
> 	sa_restorer for vDSO case.
> 	* sysdeps/unix/sysv/linux/i386/tst-bz21269.c: New file.
> 
> Signed-off-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
> ---
>  ChangeLog                                  |   6 +
>  sysdeps/unix/sysv/linux/i386/Makefile      |   3 +
>  sysdeps/unix/sysv/linux/i386/sigaction.c   |   2 +
>  sysdeps/unix/sysv/linux/i386/tst-bz21269.c | 233 +++++++++++++++++++++++++++++
>  4 files changed, 244 insertions(+)
>  create mode 100644 sysdeps/unix/sysv/linux/i386/tst-bz21269.c
> 
> diff --git a/sysdeps/unix/sysv/linux/i386/Makefile b/sysdeps/unix/sysv/linux/i386/Makefile
> index 4080b8c..da716e2 100644
> --- a/sysdeps/unix/sysv/linux/i386/Makefile
> +++ b/sysdeps/unix/sysv/linux/i386/Makefile
> @@ -3,6 +3,9 @@ default-abi := 32
>  
>  ifeq ($(subdir),misc)
>  sysdep_routines += ioperm iopl vm86
> +
> +tests += tst-bz21269
> +$(objpfx)tst-bz21269: $(shared-thread-library)
>  endif
>  
>  ifeq ($(subdir),elf)
> diff --git a/sysdeps/unix/sysv/linux/i386/sigaction.c b/sysdeps/unix/sysv/linux/i386/sigaction.c
> index 137c73b..df9fa0c 100644
> --- a/sysdeps/unix/sysv/linux/i386/sigaction.c
> +++ b/sysdeps/unix/sysv/linux/i386/sigaction.c
> @@ -32,6 +32,8 @@ extern void restore (void) asm ("__restore") attribute_hidden;
>           (kact)->sa_restorer = (((act)->sa_flags & SA_SIGINFO)	\
>  			       ? &restore_rt : &restore);	\
>         }							\
> +     else							\
> +       (kact)->sa_restorer = NULL;				\
>    })
>  
>  #define RESET_SA_RESTORER(act, kact) \
> diff --git a/sysdeps/unix/sysv/linux/i386/tst-bz21269.c b/sysdeps/unix/sysv/linux/i386/tst-bz21269.c
> new file mode 100644
> index 0000000..353e365
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/i386/tst-bz21269.c
> @@ -0,0 +1,233 @@
> +/* Test for i386 sigaction sa_restorer handling (BZ#21269)
> +   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/>.  */
> +
> +/* This is based on Linux test tools/testing/selftests/x86/ldt_gdt.c,
> +   more specifically in do_multicpu_tests function.  The main changes
> +   are:
> +
> +   - C11 atomics instead of plain access.
> +   - Remove x86_64 support which simplifies the syscall handling
> +     and fallbacks.
> +   - Replicate only the test required to trigger the issue for the
> +     BZ#21269.  */
> +
> +#include <stdatomic.h>
> +
> +#include <asm/ldt.h>
> +#include <linux/futex.h>
> +
> +#include <setjmp.h>
> +#include <signal.h>
> +#include <errno.h>
> +#include <sys/syscall.h>
> +#include <sys/mman.h>
> +
> +#include <support/xunistd.h>
> +#include <support/check.h>
> +#include <support/xthread.h>
> +
> +static int
> +xset_thread_area (struct user_desc *u_info)
> +{
> +  long ret = syscall (SYS_set_thread_area, u_info);
> +  TEST_VERIFY_EXIT (ret == 0);
> +  return ret;
> +}
> +
> +static void
> +xmodify_ldt (int func, const void *ptr, unsigned long bytecount)
> +{
> +  TEST_VERIFY_EXIT (syscall (SYS_modify_ldt, 1, ptr, bytecount) == 0);
> +}
> +
> +static int
> +futex (int *uaddr, int futex_op, int val, void *timeout, int *uaddr2,
> +	int val3)
> +{
> +  return syscall (SYS_futex, uaddr, futex_op, val, timeout, uaddr2, val3);
> +}
> +
> +static void
> +xsethandler (int sig, void (*handler)(int, siginfo_t *, void *), int flags)
> +{
> +  struct sigaction sa = { 0 };
> +  sa.sa_sigaction = handler;
> +  sa.sa_flags = SA_SIGINFO | flags;
> +  TEST_VERIFY_EXIT (sigemptyset (&sa.sa_mask) == 0);
> +  TEST_VERIFY_EXIT (sigaction (sig, &sa, 0) == 0);
> +}
> +
> +static jmp_buf jmpbuf;
> +
> +static void
> +sigsegv_handler (int sig, siginfo_t *info, void *ctx_void)
> +{
> +  siglongjmp (jmpbuf, 1);
> +}
> +
> +/* Points to an array of 1024 ints, each holding its own index.  */
> +static const unsigned int *counter_page;
> +static struct user_desc *low_user_desc;
> +static struct user_desc *low_user_desc_clear; /* Used to delete GDT entry.  */
> +static int gdt_entry_num;
> +
> +static void
> +setup_counter_page (void)
> +{
> +  long page_size = sysconf (_SC_PAGE_SIZE);
> +  TEST_VERIFY_EXIT (page_size > 0);
> +  unsigned int *page = xmmap (NULL, page_size, PROT_READ | PROT_WRITE,
> +			      MAP_ANONYMOUS | MAP_PRIVATE | MAP_32BIT, -1);
> +  for (int i = 0; i < (page_size / sizeof (unsigned int)); i++)
> +    page[i] = i;
> +  counter_page = page;
> +}
> +
> +static void
> +setup_low_user_desc (void)
> +{
> +  low_user_desc = xmmap (NULL, 2 * sizeof (struct user_desc),
> +			 PROT_READ | PROT_WRITE,
> +			 MAP_ANONYMOUS | MAP_PRIVATE | MAP_32BIT, -1);
> +
> +  low_user_desc->entry_number    = -1;
> +  low_user_desc->base_addr       = (unsigned long) &counter_page[1];
> +  low_user_desc->limit           = 0xffff;
> +  low_user_desc->seg_32bit       = 1;
> +  low_user_desc->contents        = 0;
> +  low_user_desc->read_exec_only  = 0;
> +  low_user_desc->limit_in_pages  = 1;
> +  low_user_desc->seg_not_present = 0;
> +  low_user_desc->useable         = 0;
> +
> +  xset_thread_area (low_user_desc);
> +
> +  low_user_desc_clear = low_user_desc + 1;
> +  low_user_desc_clear->entry_number = gdt_entry_num;
> +  low_user_desc_clear->read_exec_only = 1;
> +  low_user_desc_clear->seg_not_present = 1;
> +}
> +
> +/* Possible values of futex:
> +   0: thread is idle.
> +   1: thread armed.
> +   2: thread should clear LDT entry 0.
> +   3: thread should exit.  */
> +static atomic_uint ftx;
> +
> +static void *
> +threadproc (void *ctx)
> +{
> +  while (1)
> +    {
> +      futex ((int *) &ftx, FUTEX_WAIT, 1, NULL, NULL, 0);
> +      while (atomic_load (&ftx) != 2)
> +	{
> +	  if (atomic_load (&ftx) >= 3)
> +	    return NULL;
> +	}
> +
> +      /* clear LDT entry 0.  */
> +      const struct user_desc desc = { 0 };
> +      xmodify_ldt (1, &desc, sizeof (desc));
> +
> +      /* If ftx == 2, set it to zero,  If ftx == 100, quit.  */
> +      if (atomic_fetch_add (&ftx, -2) != 2)
> +	return NULL;
> +    }
> +}
> +
> +
> +/* As described in testcase, for historical reasons x86_32 Linux (and compat
> +   on x86_64) interprets SA_RESTORER clear with nonzero sa_restorer as a
> +   request for stack switching if the SS segment is 'funny' (this is default
> +   scenario for vDSO system).  This means that anything that tries to mix
> +   signal handling with segmentation should explicit clear the sa_restorer.
> +
> +   This testcase check if sigaction in fact does it by changing the local
> +   descriptor table (LDT) through the modify_ldt syscall and triggering
> +   a synchronous segfault on iret fault by trying to install an invalid
> +   segment.  With a correct zeroed sa_restorer it should not trigger an
> +   'real' SEGSEGV and allows the siglongjmp in signal handler.  */
> +
> +static int
> +do_test (void)
> +{
> +  setup_counter_page ();
> +  setup_low_user_desc ();
> +
> +  pthread_t thread;
> +  unsigned short orig_ss;
> +
> +  xsethandler (SIGSEGV, sigsegv_handler, 0);
> +  /* 32-bit kernels send SIGILL instead of SIGSEGV on IRET faults.  */
> +  xsethandler (SIGILL, sigsegv_handler, 0);
> +
> +  thread = xpthread_create (0, threadproc, 0);
> +
> +  asm volatile ("mov %%ss, %0" : "=rm" (orig_ss));
> +
> +  for (int i = 0; i < 5; i++)
> +    {
> +      if (sigsetjmp (jmpbuf, 1) != 0)
> +	continue;
> +
> +      /* Make sure the thread is ready after the last test. */
> +      while (atomic_load (&ftx) != 0)
> +	;
> +
> +      struct user_desc desc = {
> +	.entry_number       = 0,
> +	.base_addr          = 0,
> +	.limit              = 0xffff,
> +	.seg_32bit          = 1,
> +	.contents           = 0,
> +	.read_exec_only     = 0,
> +	.limit_in_pages     = 1,
> +	.seg_not_present    = 0,
> +	.useable            = 0
> +      };
> +
> +      xmodify_ldt (0x11, &desc, sizeof (desc));
> +
> +      /* Arm the thread.  */
> +      ftx = 1;
> +      futex ((int*) &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
> +
> +      asm volatile ("mov %0, %%ss" : : "r" (0x7));
> +
> +      /* Fire up thread modify_ldt call.  */
> +      atomic_store (&ftx, 2);
> +
> +      while (atomic_load (&ftx) != 0)
> +	;
> +
> +      /* On success, modify_ldt will segfault us synchronously and we will
> +	 escape via siglongjmp.  */
> +      support_record_failure ();
> +    }
> +
> +  atomic_store (&ftx, 100);
> +  futex ((int*) &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
> +
> +  xpthread_join (thread);
> +
> +  return 0;
> +}
> +
> +#include <support/test-driver.c>
>
H.J. Lu - Feb. 19, 2018, 5:54 p.m.
On Mon, Feb 12, 2018 at 4:42 AM, Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
> This patch fixes the i386 sa_restorer field initialization for sigaction
> syscall for kernel with vDSO.  As described in bug report, x86_32 Linux
> (and compat on x86_64) interprets SA_RESTORER clear with nonzero
> sa_restorer as a request for stack switching if the SS segment is 'funny'
> This means that anything that tries to mix glibc's signal handling with
> segmentation (for instance through modify_ldt syscall) is randomly broken
> depending on what values lands in sa_restorer.
>
> The testcase added  is based on Linux test tools/testing/selftests/x86/ldt_gdt.c,
> more specifically in do_multicpu_tests function.  The main changes are:
>
>   - C11 atomics instead of plain access.
>
>   - Remove x86_64 support which simplifies the syscall handling and fallbacks.
>
>   - Replicate only the test required to trigger the issue.
>
> (I added some comments of my understanding of how the testcase is triggering
> the issue, so if someone with more x86 knowledge could check if I get this
> right I will be grateful)
>
> Checked on i686-linux-gnu.
>
>         [BZ #21269]
>         * sysdeps/unix/sysv/linux/i386/Makefile (tests): Add tst-bz21269.
>         * sysdeps/unix/sysv/linux/i386/sigaction.c (SET_SA_RESTORER): Clear
>         sa_restorer for vDSO case.
>         * sysdeps/unix/sysv/linux/i386/tst-bz21269.c: New file.
>
> Signed-off-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

LGTM.

Thanks.

Patch

diff --git a/sysdeps/unix/sysv/linux/i386/Makefile b/sysdeps/unix/sysv/linux/i386/Makefile
index 4080b8c..da716e2 100644
--- a/sysdeps/unix/sysv/linux/i386/Makefile
+++ b/sysdeps/unix/sysv/linux/i386/Makefile
@@ -3,6 +3,9 @@  default-abi := 32
 
 ifeq ($(subdir),misc)
 sysdep_routines += ioperm iopl vm86
+
+tests += tst-bz21269
+$(objpfx)tst-bz21269: $(shared-thread-library)
 endif
 
 ifeq ($(subdir),elf)
diff --git a/sysdeps/unix/sysv/linux/i386/sigaction.c b/sysdeps/unix/sysv/linux/i386/sigaction.c
index 137c73b..df9fa0c 100644
--- a/sysdeps/unix/sysv/linux/i386/sigaction.c
+++ b/sysdeps/unix/sysv/linux/i386/sigaction.c
@@ -32,6 +32,8 @@  extern void restore (void) asm ("__restore") attribute_hidden;
          (kact)->sa_restorer = (((act)->sa_flags & SA_SIGINFO)	\
 			       ? &restore_rt : &restore);	\
        }							\
+     else							\
+       (kact)->sa_restorer = NULL;				\
   })
 
 #define RESET_SA_RESTORER(act, kact) \
diff --git a/sysdeps/unix/sysv/linux/i386/tst-bz21269.c b/sysdeps/unix/sysv/linux/i386/tst-bz21269.c
new file mode 100644
index 0000000..353e365
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/i386/tst-bz21269.c
@@ -0,0 +1,233 @@ 
+/* Test for i386 sigaction sa_restorer handling (BZ#21269)
+   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/>.  */
+
+/* This is based on Linux test tools/testing/selftests/x86/ldt_gdt.c,
+   more specifically in do_multicpu_tests function.  The main changes
+   are:
+
+   - C11 atomics instead of plain access.
+   - Remove x86_64 support which simplifies the syscall handling
+     and fallbacks.
+   - Replicate only the test required to trigger the issue for the
+     BZ#21269.  */
+
+#include <stdatomic.h>
+
+#include <asm/ldt.h>
+#include <linux/futex.h>
+
+#include <setjmp.h>
+#include <signal.h>
+#include <errno.h>
+#include <sys/syscall.h>
+#include <sys/mman.h>
+
+#include <support/xunistd.h>
+#include <support/check.h>
+#include <support/xthread.h>
+
+static int
+xset_thread_area (struct user_desc *u_info)
+{
+  long ret = syscall (SYS_set_thread_area, u_info);
+  TEST_VERIFY_EXIT (ret == 0);
+  return ret;
+}
+
+static void
+xmodify_ldt (int func, const void *ptr, unsigned long bytecount)
+{
+  TEST_VERIFY_EXIT (syscall (SYS_modify_ldt, 1, ptr, bytecount) == 0);
+}
+
+static int
+futex (int *uaddr, int futex_op, int val, void *timeout, int *uaddr2,
+	int val3)
+{
+  return syscall (SYS_futex, uaddr, futex_op, val, timeout, uaddr2, val3);
+}
+
+static void
+xsethandler (int sig, void (*handler)(int, siginfo_t *, void *), int flags)
+{
+  struct sigaction sa = { 0 };
+  sa.sa_sigaction = handler;
+  sa.sa_flags = SA_SIGINFO | flags;
+  TEST_VERIFY_EXIT (sigemptyset (&sa.sa_mask) == 0);
+  TEST_VERIFY_EXIT (sigaction (sig, &sa, 0) == 0);
+}
+
+static jmp_buf jmpbuf;
+
+static void
+sigsegv_handler (int sig, siginfo_t *info, void *ctx_void)
+{
+  siglongjmp (jmpbuf, 1);
+}
+
+/* Points to an array of 1024 ints, each holding its own index.  */
+static const unsigned int *counter_page;
+static struct user_desc *low_user_desc;
+static struct user_desc *low_user_desc_clear; /* Used to delete GDT entry.  */
+static int gdt_entry_num;
+
+static void
+setup_counter_page (void)
+{
+  long page_size = sysconf (_SC_PAGE_SIZE);
+  TEST_VERIFY_EXIT (page_size > 0);
+  unsigned int *page = xmmap (NULL, page_size, PROT_READ | PROT_WRITE,
+			      MAP_ANONYMOUS | MAP_PRIVATE | MAP_32BIT, -1);
+  for (int i = 0; i < (page_size / sizeof (unsigned int)); i++)
+    page[i] = i;
+  counter_page = page;
+}
+
+static void
+setup_low_user_desc (void)
+{
+  low_user_desc = xmmap (NULL, 2 * sizeof (struct user_desc),
+			 PROT_READ | PROT_WRITE,
+			 MAP_ANONYMOUS | MAP_PRIVATE | MAP_32BIT, -1);
+
+  low_user_desc->entry_number    = -1;
+  low_user_desc->base_addr       = (unsigned long) &counter_page[1];
+  low_user_desc->limit           = 0xffff;
+  low_user_desc->seg_32bit       = 1;
+  low_user_desc->contents        = 0;
+  low_user_desc->read_exec_only  = 0;
+  low_user_desc->limit_in_pages  = 1;
+  low_user_desc->seg_not_present = 0;
+  low_user_desc->useable         = 0;
+
+  xset_thread_area (low_user_desc);
+
+  low_user_desc_clear = low_user_desc + 1;
+  low_user_desc_clear->entry_number = gdt_entry_num;
+  low_user_desc_clear->read_exec_only = 1;
+  low_user_desc_clear->seg_not_present = 1;
+}
+
+/* Possible values of futex:
+   0: thread is idle.
+   1: thread armed.
+   2: thread should clear LDT entry 0.
+   3: thread should exit.  */
+static atomic_uint ftx;
+
+static void *
+threadproc (void *ctx)
+{
+  while (1)
+    {
+      futex ((int *) &ftx, FUTEX_WAIT, 1, NULL, NULL, 0);
+      while (atomic_load (&ftx) != 2)
+	{
+	  if (atomic_load (&ftx) >= 3)
+	    return NULL;
+	}
+
+      /* clear LDT entry 0.  */
+      const struct user_desc desc = { 0 };
+      xmodify_ldt (1, &desc, sizeof (desc));
+
+      /* If ftx == 2, set it to zero,  If ftx == 100, quit.  */
+      if (atomic_fetch_add (&ftx, -2) != 2)
+	return NULL;
+    }
+}
+
+
+/* As described in testcase, for historical reasons x86_32 Linux (and compat
+   on x86_64) interprets SA_RESTORER clear with nonzero sa_restorer as a
+   request for stack switching if the SS segment is 'funny' (this is default
+   scenario for vDSO system).  This means that anything that tries to mix
+   signal handling with segmentation should explicit clear the sa_restorer.
+
+   This testcase check if sigaction in fact does it by changing the local
+   descriptor table (LDT) through the modify_ldt syscall and triggering
+   a synchronous segfault on iret fault by trying to install an invalid
+   segment.  With a correct zeroed sa_restorer it should not trigger an
+   'real' SEGSEGV and allows the siglongjmp in signal handler.  */
+
+static int
+do_test (void)
+{
+  setup_counter_page ();
+  setup_low_user_desc ();
+
+  pthread_t thread;
+  unsigned short orig_ss;
+
+  xsethandler (SIGSEGV, sigsegv_handler, 0);
+  /* 32-bit kernels send SIGILL instead of SIGSEGV on IRET faults.  */
+  xsethandler (SIGILL, sigsegv_handler, 0);
+
+  thread = xpthread_create (0, threadproc, 0);
+
+  asm volatile ("mov %%ss, %0" : "=rm" (orig_ss));
+
+  for (int i = 0; i < 5; i++)
+    {
+      if (sigsetjmp (jmpbuf, 1) != 0)
+	continue;
+
+      /* Make sure the thread is ready after the last test. */
+      while (atomic_load (&ftx) != 0)
+	;
+
+      struct user_desc desc = {
+	.entry_number       = 0,
+	.base_addr          = 0,
+	.limit              = 0xffff,
+	.seg_32bit          = 1,
+	.contents           = 0,
+	.read_exec_only     = 0,
+	.limit_in_pages     = 1,
+	.seg_not_present    = 0,
+	.useable            = 0
+      };
+
+      xmodify_ldt (0x11, &desc, sizeof (desc));
+
+      /* Arm the thread.  */
+      ftx = 1;
+      futex ((int*) &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
+
+      asm volatile ("mov %0, %%ss" : : "r" (0x7));
+
+      /* Fire up thread modify_ldt call.  */
+      atomic_store (&ftx, 2);
+
+      while (atomic_load (&ftx) != 0)
+	;
+
+      /* On success, modify_ldt will segfault us synchronously and we will
+	 escape via siglongjmp.  */
+      support_record_failure ();
+    }
+
+  atomic_store (&ftx, 100);
+  futex ((int*) &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
+
+  xpthread_join (thread);
+
+  return 0;
+}
+
+#include <support/test-driver.c>