1  /* _Unwind_Frames_Extra with shadow stack for x86-64 and x86.
       2     Copyright (C) 2017-2023 Free Software Foundation, Inc.
       3  
       4  This file is part of GCC.
       5  
       6  GCC is free software; you can redistribute it and/or modify
       7  it under the terms of the GNU General Public License as published by
       8  the Free Software Foundation; either version 3, or (at your option)
       9  any later version.
      10  
      11  GCC is distributed in the hope that it will be useful,
      12  but WITHOUT ANY WARRANTY; without even the implied warranty of
      13  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
      14  GNU General Public License for more details.
      15  
      16  Under Section 7 of GPL version 3, you are granted additional
      17  permissions described in the GCC Runtime Library Exception, version
      18  3.1, as published by the Free Software Foundation.
      19  
      20  You should have received a copy of the GNU General Public License and
      21  a copy of the GCC Runtime Library Exception along with this program;
      22  see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
      23  <http://www.gnu.org/licenses/>.  */
      24  
      25  #include <x86gprintrin.h>
      26  
      27  /* Unwind the shadow stack for EH.  */
      28  #undef _Unwind_Frames_Extra
      29  #define _Unwind_Frames_Extra(x)			\
      30    do						\
      31      {						\
      32        _Unwind_Word ssp = _get_ssp ();		\
      33        if (ssp != 0)				\
      34  	{					\
      35  	  _Unwind_Word tmp = (x);		\
      36  	  while (tmp > 255)			\
      37  	    {					\
      38  	      _inc_ssp (255);			\
      39  	      tmp -= 255;			\
      40  	    }					\
      41  	  _inc_ssp (tmp);			\
      42  	}					\
      43      }						\
      44      while (0)
      45  
      46  /* Linux CET kernel places a restore token on shadow stack for signal
      47     handler to enhance security.  The restore token is 8 byte and aligned
      48     to 8 bytes.  It is usually transparent to user programs since kernel
      49     will pop the restore token when signal handler returns.  But when an
      50     exception is thrown from a signal handler, now we need to pop the
      51     restore token from shadow stack.  For x86-64, we just need to treat
      52     the signal frame as normal frame.  For i386, we need to search for
      53     the restore token to check if the original shadow stack is 8 byte
      54     aligned.  If the original shadow stack is 8 byte aligned, we just
      55     need to pop 2 slots, one restore token, from shadow stack.  Otherwise,
      56     we need to pop 3 slots, one restore token + 4 byte padding, from
      57     shadow stack.
      58  
      59     When popping a stack frame, we compare the return address on normal
      60     stack against the return address on shadow stack.  If they don't match,
      61     return _URC_FATAL_PHASE2_ERROR for the corrupted return address on
      62     normal stack.  Don't check the return address for
      63     1. Non-catchable exception where exception_class == 0.  Process will
      64        be terminated.
      65     2. Zero return address which marks the outermost stack frame.
      66     3. Signal stack frame since kernel puts a restore token on shadow
      67        stack.
      68   */
      69  #undef _Unwind_Frames_Increment
      70  #ifdef __x86_64__
      71  #define _Unwind_Frames_Increment(exc, context, frames)	\
      72      {							\
      73        frames++;						\
      74        if (exc->exception_class != 0			\
      75  	  && _Unwind_GetIP (context) != 0		\
      76  	  && !_Unwind_IsSignalFrame (context))		\
      77  	{						\
      78  	  _Unwind_Word ssp = _get_ssp ();		\
      79  	  if (ssp != 0)					\
      80  	    {						\
      81  	      ssp += 8 * frames;			\
      82  	      _Unwind_Word ra = *(_Unwind_Word *) ssp;	\
      83  	      if (ra != _Unwind_GetIP (context))	\
      84  		return _URC_FATAL_PHASE2_ERROR;		\
      85  	    }						\
      86  	}						\
      87      }
      88  #else
      89  #define _Unwind_Frames_Increment(exc, context, frames)	\
      90    if (_Unwind_IsSignalFrame (context))			\
      91      do							\
      92        {							\
      93  	_Unwind_Word ssp, prev_ssp, token;		\
      94  	ssp = _get_ssp ();				\
      95  	if (ssp != 0)					\
      96  	  {						\
      97  	    /* Align shadow stack pointer to the next	\
      98  	       8 byte aligned boundary.  */		\
      99  	    ssp = (ssp + 4) & ~7;			\
     100  	    do						\
     101  	      {						\
     102  		/* Look for a restore token.  */	\
     103  		token = (*(_Unwind_Word *) (ssp - 8));	\
     104  		prev_ssp = token & ~7;			\
     105  		if (prev_ssp == ssp)			\
     106  		  break;				\
     107  		ssp += 8;				\
     108  	      }						\
     109  	    while (1);					\
     110  	    frames += (token & 0x4) ? 3 : 2;		\
     111  	  }						\
     112        }							\
     113      while (0);						\
     114    else							\
     115      {							\
     116        frames++;						\
     117        if (exc->exception_class != 0			\
     118  	  && _Unwind_GetIP (context) != 0)		\
     119  	{						\
     120  	  _Unwind_Word ssp = _get_ssp ();		\
     121  	  if (ssp != 0)					\
     122  	    {						\
     123  	      ssp += 4 * frames;			\
     124  	      _Unwind_Word ra = *(_Unwind_Word *) ssp;	\
     125  	      if (ra != _Unwind_GetIP (context))	\
     126  		return _URC_FATAL_PHASE2_ERROR;		\
     127  	    }						\
     128  	}						\
     129      }
     130  #endif