x86/CET: Add a setcontext test for CET

  Verify that setcontext works with gaps above and below the newly
allocated shadow stack.

OK for master?

H.J.
---
	* sysdeps/x86/Makefile (tests): Add tst-cet-setcontext-1 if
	CET is enabled.
	(CFLAGS-tst-cet-setcontext-1.c): Add -mshstk.
	* sysdeps/x86/tst-cet-setcontext-1.c: New file.
---
 sysdeps/x86/Makefile               |   5 ++
 sysdeps/x86/tst-cet-setcontext-1.c | 119 +++++++++++++++++++++++++++++
 2 files changed, 124 insertions(+)
 create mode 100644 sysdeps/x86/tst-cet-setcontext-1.c
  

On 07/25/2018 08:22 AM, H.J. Lu wrote:
> Verify that setcontext works with gaps above and below the newly
> allocated shadow stack.
> 
> OK for master?
> 
> H.J.
> ---
> 	* sysdeps/x86/Makefile (tests): Add tst-cet-setcontext-1 if
> 	CET is enabled.
> 	(CFLAGS-tst-cet-setcontext-1.c): Add -mshstk.
> 	* sysdeps/x86/tst-cet-setcontext-1.c: New file.

OK for 2.28 only if you add a paragraph about exactly how the shadow
stacks are being laid out by the calls and why unmapping ctx3 and ctx4 works
to leave ctx1 with gap above and below.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

> ---
>  sysdeps/x86/Makefile               |   5 ++
>  sysdeps/x86/tst-cet-setcontext-1.c | 119 +++++++++++++++++++++++++++++
>  2 files changed, 124 insertions(+)
>  create mode 100644 sysdeps/x86/tst-cet-setcontext-1.c
> 
> diff --git a/sysdeps/x86/Makefile b/sysdeps/x86/Makefile
> index 672bb19489..761d396108 100644
> --- a/sysdeps/x86/Makefile
> +++ b/sysdeps/x86/Makefile
> @@ -92,4 +92,9 @@ $(objpfx)check-cet.out: $(..)sysdeps/x86/check-cet.awk \
>  	$(evaluate-test)
>  generated += check-cet.out
>  endif
> +
> +ifeq ($(subdir),stdlib)
> +tests += tst-cet-setcontext-1
> +CFLAGS-tst-cet-setcontext-1.c += -mshstk
> +endif

OK, still within the CET enable block.

>  endif
> diff --git a/sysdeps/x86/tst-cet-setcontext-1.c b/sysdeps/x86/tst-cet-setcontext-1.c
> new file mode 100644
> index 0000000000..08b7f6378e
> --- /dev/null
> +++ b/sysdeps/x86/tst-cet-setcontext-1.c
> @@ -0,0 +1,119 @@
> +/* Check getcontext and setcontext on the context from makecontext
> +   with shadow stack.

OK.

> +   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/>.  */
> +
> +#include <stdio.h>
> +#include <stdint.h>
> +#include <stdlib.h>
> +#include <ucontext.h>
> +#include <unistd.h>
> +#include <sys/mman.h>
> +#include <stdatomic.h>
> +#include <x86intrin.h>
> +
> +static ucontext_t ctx[5];

OK, 5 contexts.

> +static atomic_int done;
> +
> +static void
> +__attribute__((noinline, noclone))
> +f2 (void)
> +{
> +  printf ("start f2\n");
> +  done++;

Increment done.

> +  if (setcontext (&ctx[2]) != 0)

Go back to ctx[2].

> +    {
> +      printf ("%s: setcontext: %m\n", __FUNCTION__);
> +      exit (EXIT_FAILURE);
> +    }
> +}
> +
> +static void
> +f1 (void)
> +{
> +  printf ("start f1\n");
> +  if (getcontext (&ctx[2]) != 0)
> +    {
> +      printf ("%s: getcontext: %m\n", __FUNCTION__);
> +      exit (EXIT_FAILURE);
> +    }

OK.

> +  if (done)
> +    exit (EXIT_SUCCESS);

Call exit the second time. Having tested a context get/set
within a context that has gaps above and below it.

> +  f2 ();

Calls f2 first time.

> +}
> +
> +static int
> +do_test (void)
> +{
> +  char st1[32768];

OK, large stack block.

> +  puts ("making contexts");
> +  if (getcontext (&ctx[0]) != 0)

OK, create a context, this makes a new shadow stack.

> +    {
> +      printf ("%s: getcontext: %m\n", __FUNCTION__);
> +      exit (EXIT_FAILURE);
> +    }
> +  if (getcontext (&ctx[1]) != 0)

OK, make another one.

> +    {
> +      printf ("%s: getcontext: %m\n", __FUNCTION__);
> +      exit (EXIT_FAILURE);
> +    }
> +
> +  ctx[3].uc_stack.ss_sp = st1;
> +  ctx[3].uc_stack.ss_size = sizeof st1;
> +  ctx[3].uc_link = &ctx[0];
> +  makecontext (&ctx[3], (void (*) (void)) f1, 0);

This is invalid?

ctx[3] must have been initialized by getcontext.

If the point is to force a shadow stack allocation then we should
add a comment here that we are purposely altering an invalid context
to trigger this work.

> +
> +  ctx[1].uc_stack.ss_sp = st1;
> +  ctx[1].uc_stack.ss_size = sizeof st1;
> +  ctx[1].uc_link = &ctx[0];
> +  makecontext (&ctx[1], (void (*) (void)) f1, 0);

OK, adjust stack for ctx[1].

> +
> +  ctx[4].uc_stack.ss_sp = st1;
> +  ctx[4].uc_stack.ss_size = sizeof st1;
> +  ctx[4].uc_link = &ctx[0];
> +  makecontext (&ctx[4], (void (*) (void)) f1, 0);

Also invalid?

At this point we have likely have:

0th - context
3rd - invalid
1st - context (new stack, and f1 function)
4th - invalid

> +
> +  /* Free the unused shadow stacks to create gaps above and below the
> +     shadow stack of CTX1.  */

Needs a big comment explaining the layout and what we are accomplishing here:

> +  if (_get_ssp () != 0)
> +    {
> +      if (ctx[3].__ssp[1] != 0
> +	  && munmap ((void *) (uintptr_t) ctx[3].__ssp[1],
> +		     (size_t) ctx[3].__ssp[2]) != 0)
> +	{
> +	  printf ("%s: munmap: %m\n", __FUNCTION__);
> +	  exit (EXIT_FAILURE);
> +	}
> +
> +      if (ctx[4].__ssp[1] != 0
> +	  && munmap ((void *) (uintptr_t) ctx[4].__ssp[1],
> +		     (size_t) ctx[4].__ssp[2]) != 0)
> +	{
> +	  printf ("%s: munmap: %m\n", __FUNCTION__);
> +	  exit (EXIT_FAILURE);
> +	}
> +    }
> +

0th - context
3rd - invalid (unmapped shadow stack)
1st - context (new stack, and f1 function)
4th - invalid (unmapped shadow stack)

> +  if (setcontext (&ctx[1]) != 0)

OK, jump to f1.

> +    {
> +      printf ("%s: setcontext: %m\n", __FUNCTION__);
> +      exit (EXIT_FAILURE);
> +    }
> +  exit (EXIT_FAILURE);
> +}
> +
> +#include <support/test-driver.c>
>