libgcc/config/i386/shadow-stack-unwind.h

   1 /* _Unwind_Frames_Extra with shadow stack for x86-64 and x86.
   2    Copyright (C) 2017-2024 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