Re: [PATCH] Linux/x86: Support shadow stack pointer in setjmp/longjmp

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

Resent to glibc.  Sorry if you got this email twice.

On Mon, Dec 18, 2017 at 9:54 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Mon, Dec 18, 2017 at 9:44 AM, Joseph Myers <joseph@codesourcery.com> wrote:
>> On Mon, 18 Dec 2017, H.J. Lu wrote:
>>
>>>       * sysdeps/unix/sysv/linux/i386/__longjmp.S: New file.
>>>       * sysdeps/unix/sysv/linux/i386/bsd-_setjmp.S: Likewise.
>>>       * sysdeps/unix/sysv/linux/i386/bsd-setjmp.S: Likewise.
>>>       * sysdeps/unix/sysv/linux/i386/setjmp.S: Likewise.
>>>       * sysdeps/unix/sysv/linux/x86_64/__longjmp.S: Likewise.
>>>       * sysdeps/unix/sysv/linux/x86_64/setjmp.S: Likewise.
>>
>> Why are all these files Linux-specific?  ____longjmp_chk is Linux-specific
>> because it does a sysaltstack syscall, but I don't see anything
>> OS-specific in these files.  Why shouldn't shadow stack support be
>> available for all OSes on these architectures?
>
> Shadow stack support needs CET support in OS kernel.  We are updating
> Linux arch_prctl syscall to support CET.
>
>> Is support for the relevant instructions available in all binutils
>> versions supported for building glibc?  If not, does __SHSTK__ being
>> defined guarantee that GCC was built with a binutils version with the
>> required support, or do we need additional configure checks for binutils
>> support?
>
> We check if binutils and GCC support CET before we enable CET:
>
> https://sourceware.org/git/?p=glibc.git;a=commit;h=d977bdb7caa1a0795687b1ea88cd24183231a37e
>
> diff --git a/sysdeps/unix/sysv/linux/x86/configure.ac
> b/sysdeps/unix/sysv/linux/x86/configure.ac
> new file mode 100644
> index 0000000..7ef9fbe
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/x86/configure.ac
> @@ -0,0 +1,48 @@
> +GLIBC_PROVIDES dnl See aclocal.m4 in the top level source directory.
> +# Local configure fragment for sysdeps/unix/sysv/linux/x86.
> +
> +if test x"$enable_cet" = xdefault || test x"$enable_cet" = xyes; then
> +  # Check if CET can be enabled.
> +  AC_CACHE_CHECK(whether CET can be enabled,
> + libc_cv_x86_cet_available, [dnl
> +cat > conftest.c <<EOF
> +#if !defined __IBT__ || !defined __SHSTK__
> +# error CET isn't available.
> +#endif
> +EOF
> + if AC_TRY_COMMAND(${CC-cc} -c $CFLAGS -fcf-protection -mcet -include
> cet.h conftest.c 1>&AS_MESSAGE_LOG_FD); then
> +    libc_cv_x86_cet_available=yes
> + else
> +    libc_cv_x86_cet_available=no
> + fi
> + rm -rf conftest*])
> +  if test $libc_cv_x86_cet_available = yes; then
> +    enable_cet=yes
> +  else
> +    if test x"$enable_cet" = xdefault; then
> +      enable_cet=no
> +    else
> +      AC_MSG_ERROR([$CC doesn't support CET])
> +    fi
> +  fi
> +fi
> +if test $enable_cet = yes; then
> +  # Check if assembler supports CET.
> +  AC_CACHE_CHECK(whether $AS supports CET,
> + libc_cv_x86_cet_as, [dnl
> +cat > conftest.s <<EOF
> + incsspd %ecx
> +EOF
> + if AC_TRY_COMMAND(${CC-cc} -c $CFLAGS conftest.s -o conftest.o
> 1>&AS_MESSAGE_LOG_FD); then
> +    libc_cv_x86_cet_as=yes
> + else
> +    libc_cv_x86_cet_as=no
> + fi
> + rm -rf conftest*])
> +  if test $libc_cv_x86_cet_as = no; then
> +    AC_MSG_ERROR([$AS doesn't support CET])
> +  fi
> +  AC_DEFINE(ENABLE_CET, 1,
> +     [Enable Intel Control-flow Enforcement Technology (CET)])
> +fi
> +LIBC_CONFIG_VAR([enable-cet], [$enable_cet])
> --
>
> H.J.



-- 
H.J.