Re: [PATCH 08/22] Add Intel CET support for EH in libgcc.

[Jeff Law <law at redhat dot com>]

On 10/12/2017 01:56 PM, Tsimbalist, Igor V wrote:
> Control-flow Enforcement Technology (CET), published by Intel, Introduces
> the Shadow Stack feature, which ensures a return from a function is done
> to exactly the same location from where the function was called. When EH
> is present the control-flow transfer may skip some stack frames and the
> shadow stack has to be adjusted not to signal a violation of a
> control-flow transfer. It's done by counting a number of skipping frames
> and adjusting shadow stack pointer by this number.
> 
> gcc/
> 	* config/i386/i386.c (ix86_expand_epilogue): Change simple
> 	return to indirect jump for EH return. Change explicit 'false'
> 	argument in pro_epilogue_adjust_stack with a value of
> 	flag_cf_protection.
> 	* config/i386/i386.md (simple_return_indirect_internal): Remove
> 	SImode restriction to support 64-bit.
> 
> libgcc/
> 	* config/i386/linux-unwind.h: Include
> 	config/i386/shadow-stack-unwind.h.
> 	* config/i386/shadow-stack-unwind.h: New file.
> 	* unwind-dw2.c: (uw_install_context): Add a FRAMES argument and
> 	pass it to _Unwind_Frames_Extra.
> 	* unwind-generic.h (FRAMES_P_DECL): New.
> 	(FRAMES_VAR): Likewise.
> 	(FRAMES_VAR_P): Likewise.
> 	(FRAMES_VAR_DECL): Likewise.
> 	(FRAMES_VAR_DECL_1): Likewise.
> 	(FRAMES_VAR_INC): Likewise.
> 	(FRAMES_P_UPDATE): Likewise.
> 	(_Unwind_Frames_Extra): Likewise.
> 	* unwind.inc (_Unwind_RaiseException_Phase2): Use FRAMES_P_DECL,
> 	FRAMES_VAR_DECL_1, FRAMES_VAR_INC and FRAMES_P_UPDATE.
> 	(_Unwind_RaiseException): Use FRAMES_VAR_DECL, FRAMES_VAR_P and
> 	FRAMES_VAR.
> 	(_Unwind_ForcedUnwind_Phase2): Use FRAMES_P_DECL,
> 	FRAMES_VAR_DECL_1, FRAMES_VAR_INC, FRAMES_P_UPDATE.
> 	(_Unwind_ForcedUnwind): Use FRAMES_VAR_DECL, FRAMES_VAR_P and
> 	FRAMES_VAR.
> 	(_Unwind_Resume): Use FRAMES_VAR_DECL, FRAMES_VAR_P and
> 	FRAMES_VAR.
> 	(_Unwind_Resume_or_Rethrow): Use FRAMES_VAR_DECL, FRAMES_VAR_P
> 	and FRAMES_VAR. 
> 
> Igor
> 
> 
> 
> 0008-Add-Intel-CET-support-for-EH-in-libgcc.patch
> 
> 
> From 16eb1d0d9239e039fba28f1ae71762f19061b157 Mon Sep 17 00:00:00 2001
> From: Igor Tsimbalist <igor.v.tsimbalist@intel.com>
> Date: Wed, 19 Jul 2017 03:04:46 +0300
> Subject: [PATCH 08/22] Add Intel CET support for EH in libgcc.
> 
> Control-flow Enforcement Technology (CET), published by Intel,
> introduces
> the Shadow Stack feature, which ensures a return from a function is done
> to exactly the same location from where the function was called. When EH
> is present the control-flow transfer may skip some stack frames and the
> shadow stack has to be adjusted not to signal a violation of a
> control-flow transfer. It's done by counting a number of skiping frames
> and adjasting shadow stack pointer by this number.
> 
> Having new semantic of the 'ret' instruction if CET is supported in HW
> the 'ret' instruction cannot be generated in ix86_expand_epilogue when
> we are returning after EH is processed. Added a code in
> ix86_expand_epilogue to adjust Shadow Stack pointer and to generate an
> indirect jump instead of 'ret'. As sp register is used during this
> adjustment thus the argument in pro_epilogue_adjust_stack is changed
> to update cfa_reg based on whether control-flow instrumentation is set.
> Without updating the cfa_reg field there is an assert later in dwarf2
> pass related to mismatch the stack register and cfa_reg value.
> 
> gcc/
> 	* config/i386/i386.c (ix86_expand_epilogue): Change simple
> 	return to indirect jump for EH return. Change explicit 'false'
> 	argument in pro_epilogue_adjust_stack with a value of
> 	flag_cf_protection.
> 	* config/i386/i386.md (simple_return_indirect_internal): Remove
> 	SImode restriction to support 64-bit.
> 
> libgcc/
> 	* config/i386/linux-unwind.h: Include
> 	config/i386/shadow-stack-unwind.h.
> 	* config/i386/shadow-stack-unwind.h: New file.
> 	* unwind-dw2.c: (uw_install_context): Add a FRAMES argument and
> 	pass it to _Unwind_Frames_Extra.
> 	* unwind-generic.h (FRAMES_P_DECL): New.
> 	(FRAMES_VAR): Likewise.
> 	(FRAMES_VAR_P): Likewise.
> 	(FRAMES_VAR_DECL): Likewise.
> 	(FRAMES_VAR_DECL_1): Likewise.
> 	(FRAMES_VAR_INC): Likewise.
> 	(FRAMES_P_UPDATE): Likewise.
> 	(_Unwind_Frames_Extra): Likewise.
> 	* unwind.inc (_Unwind_RaiseException_Phase2): Use FRAMES_P_DECL,
> 	FRAMES_VAR_DECL_1, FRAMES_VAR_INC and FRAMES_P_UPDATE.
> 	(_Unwind_RaiseException): Use FRAMES_VAR_DECL, FRAMES_VAR_P and
> 	FRAMES_VAR.
> 	(_Unwind_ForcedUnwind_Phase2): Use FRAMES_P_DECL,
> 	FRAMES_VAR_DECL_1, FRAMES_VAR_INC, FRAMES_P_UPDATE.
> 	(_Unwind_ForcedUnwind): Use FRAMES_VAR_DECL, FRAMES_VAR_P and
> 	FRAMES_VAR.
> 	(_Unwind_Resume): Use FRAMES_VAR_DECL, FRAMES_VAR_P and
> 	FRAMES_VAR.
> 	(_Unwind_Resume_or_Rethrow): Use FRAMES_VAR_DECL, FRAMES_VAR_P
> 	and FRAMES_VAR.
> 
> 


> diff --git a/libgcc/config/i386/shadow-stack-unwind.h b/libgcc/config/i386/shadow-stack-unwind.h
> new file mode 100644
> index 0000000..c5d0d56
> --- /dev/null
> +++ b/libgcc/config/i386/shadow-stack-unwind.h
> @@ -0,0 +1,66 @@
> +/* _Unwind_Frames_Extra with shadow stack for x86-64 and x86.
> +   Copyright (C) 2017 Free Software Foundation, Inc.
> +
> +This file is part of GCC.
> +
> +GCC is free software; you can redistribute it and/or modify
> +it under the terms of the GNU General Public License as published by
> +the Free Software Foundation; either version 3, or (at your option)
> +any later version.
> +
> +GCC 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 General Public License for more details.
> +
> +Under Section 7 of GPL version 3, you are granted additional
> +permissions described in the GCC Runtime Library Exception, version
> +3.1, as published by the Free Software Foundation.
> +
> +You should have received a copy of the GNU General Public License and
> +a copy of the GCC Runtime Library Exception along with this program;
> +see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
> +<http://www.gnu.org/licenses/>.  */
> +
> +#ifdef __x86_64__
> +# define incssp(x) __builtin_ia32_incsspq ((x))
> +# define rdssp(x) __builtin_ia32_rdsspq (x)
> +#else
> +# define incssp(x) __builtin_ia32_incsspd ((x))
> +# define rdssp(x) __builtin_ia32_rdsspd (x)
> +#endif
> +
> +/* Unwind the shadow stack for EH.  */
> +#undef _Unwind_Frames_Extra
> +#define _Unwind_Frames_Extra(x)			\
> +  do						\
> +    {						\
> +      unsigned long ssp = 0;			\
> +      ssp = rdssp (ssp);			\
> +      if (ssp != 0)				\
> +	{					\
> +	  unsigned long tmp = (x);		\
> +	  while (tmp > 255)			\
> +	    {					\
> +	      incssp (tmp);			\
> +	      tmp -= 255;			\
> +	    }					\
> +	  incssp (tmp);				\
> +	}					\
> +    }						\
> +    while (0)
> +
> +#undef FRAMES_P_DECL
> +#undef FRAMES_VAR
> +#undef FRAMES_VAR_P
> +#undef FRAMES_VAR_DECL
> +#undef FRAMES_VAR_DECL_1
> +#undef FRAMES_VAR_INC
> +#undef FRAMES_P_UPDATE
> +#define FRAMES_P_DECL		, unsigned long *frames_p
> +#define FRAMES_VAR		frames
> +#define FRAMES_VAR_P		, &frames
> +#define FRAMES_VAR_DECL		unsigned long frames
> +#define FRAMES_VAR_DECL_1	unsigned long frames = 1
> +#define FRAMES_VAR_INC		frames++
> +#define FRAMES_P_UPDATE		*frames_p = frames
> diff --git a/libgcc/unwind-dw2.c b/libgcc/unwind-dw2.c
> index 3f26eaf..7e9d26b 100644
> --- a/libgcc/unwind-dw2.c
> +++ b/libgcc/unwind-dw2.c
> @@ -1646,12 +1646,13 @@ uw_frob_return_addr (struct _Unwind_Context *current
>     macro because __builtin_eh_return must be invoked in the context of
>     our caller.  */
>  
> -#define uw_install_context(CURRENT, TARGET)				\
> +#define uw_install_context(CURRENT, TARGET, FRAMES)			\
Comment likely needs updating to describe the new parameter.

I must say that I'm not happy with all the macro games we're playing in
this patch.   Is there no cleaner way to get the desired behavior?

Are there any ABI/API implications of this patch?  Ie, does the
signature of any exported function change?

Has this been tested anywhere other than x86/x86_64 linux?


Jeff