drivers/misc/lkdtm/cfi.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * This is for all the tests relating directly to Control Flow Integrity.
   4  */
   5 #include "lkdtm.h"
   6 #include <asm/page.h>
   7
   8 static int called_count;
   9
  10 /* Function taking one argument, without a return value. */
  11 static noinline void lkdtm_increment_void(int *counter)
  12 {
  13         (*counter)++;
  14 }
  15
  16 /* Function taking one argument, returning int. */
  17 static noinline int lkdtm_increment_int(int *counter)
  18 {
  19         (*counter)++;
  20
  21         return *counter;
  22 }
  23
  24 /* Don't allow the compiler to inline the calls. */
  25 static noinline void lkdtm_indirect_call(void (*func)(int *))
  26 {
  27         func(&called_count);
  28 }
  29
  30 /*
  31  * This tries to call an indirect function with a mismatched prototype.
  32  */
  33 static void lkdtm_CFI_FORWARD_PROTO(void)
  34 {
  35         /*
  36          * Matches lkdtm_increment_void()'s prototype, but not
  37          * lkdtm_increment_int()'s prototype.
  38          */
  39         pr_info("Calling matched prototype ...\n");
  40         lkdtm_indirect_call(lkdtm_increment_void);
  41
  42         pr_info("Calling mismatched prototype ...\n");
  43         lkdtm_indirect_call((void *)lkdtm_increment_int);
  44
  45         pr_err("FAIL: survived mismatched prototype function call!\n");
  46         pr_expected_config(CONFIG_CFI_CLANG);
  47 }
  48
  49 /*
  50  * This can stay local to LKDTM, as there should not be a production reason
  51  * to disable PAC && SCS.
  52  */
  53 #ifdef CONFIG_ARM64_PTR_AUTH_KERNEL
  54 # ifdef CONFIG_ARM64_BTI_KERNEL
  55 #  define __no_pac             "branch-protection=bti"
  56 # else
  57 #  ifdef CONFIG_CC_HAS_BRANCH_PROT_PAC_RET
  58 #   define __no_pac            "branch-protection=none"
  59 #  else
  60 #   define __no_pac            "sign-return-address=none"
  61 #  endif
  62 # endif
  63 # define __no_ret_protection   __noscs __attribute__((__target__(__no_pac)))
  64 #else
  65 # define __no_ret_protection   __noscs
  66 #endif
  67
  68 #define no_pac_addr(addr)      \
  69         ((__force __typeof__(addr))((uintptr_t)(addr) | PAGE_OFFSET))
  70
  71 /* The ultimate ROP gadget. */
  72 static noinline __no_ret_protection
  73 void set_return_addr_unchecked(unsigned long *expected, unsigned long *addr)
  74 {
  75         /* Use of volatile is to make sure final write isn't seen as a dead store. */
  76         unsigned long * volatile *ret_addr = (unsigned long **)__builtin_frame_address(0) + 1;
  77
  78         /* Make sure we've found the right place on the stack before writing it. */
  79         if (no_pac_addr(*ret_addr) == expected)
  80                 *ret_addr = (addr);
  81         else
  82                 /* Check architecture, stack layout, or compiler behavior... */
  83                 pr_warn("Eek: return address mismatch! %px != %px\n",
  84                         *ret_addr, addr);
  85 }
  86
  87 static noinline
  88 void set_return_addr(unsigned long *expected, unsigned long *addr)
  89 {
  90         /* Use of volatile is to make sure final write isn't seen as a dead store. */
  91         unsigned long * volatile *ret_addr = (unsigned long **)__builtin_frame_address(0) + 1;
  92
  93         /* Make sure we've found the right place on the stack before writing it. */
  94         if (no_pac_addr(*ret_addr) == expected)
  95                 *ret_addr = (addr);
  96         else
  97                 /* Check architecture, stack layout, or compiler behavior... */
  98                 pr_warn("Eek: return address mismatch! %px != %px\n",
  99                         *ret_addr, addr);
 100 }
 101
 102 static volatile int force_check;
 103
 104 static void lkdtm_CFI_BACKWARD(void)
 105 {
 106         /* Use calculated gotos to keep labels addressable. */
 107         void *labels[] = { NULL, &&normal, &&redirected, &&check_normal, &&check_redirected };
 108
 109         pr_info("Attempting unchecked stack return address redirection ...\n");
 110
 111         /* Always false */
 112         if (force_check) {
 113                 /*
 114                  * Prepare to call with NULLs to avoid parameters being treated as
 115                  * constants in -02.
 116                  */
 117                 set_return_addr_unchecked(NULL, NULL);
 118                 set_return_addr(NULL, NULL);
 119                 if (force_check)
 120                         goto *labels[1];
 121                 if (force_check)
 122                         goto *labels[2];
 123                 if (force_check)
 124                         goto *labels[3];
 125                 if (force_check)
 126                         goto *labels[4];
 127                 return;
 128         }
 129
 130         /*
 131          * Use fallthrough switch case to keep basic block ordering between
 132          * set_return_addr*() and the label after it.
 133          */
 134         switch (force_check) {
 135         case 0:
 136                 set_return_addr_unchecked(&&normal, &&redirected);
 137                 fallthrough;
 138         case 1:
 139 normal:
 140                 /* Always true */
 141                 if (!force_check) {
 142                         pr_err("FAIL: stack return address manipulation failed!\n");
 143                         /* If we can't redirect "normally", we can't test mitigations. */
 144                         return;
 145                 }
 146                 break;
 147         default:
 148 redirected:
 149                 pr_info("ok: redirected stack return address.\n");
 150                 break;
 151         }
 152
 153         pr_info("Attempting checked stack return address redirection ...\n");
 154
 155         switch (force_check) {
 156         case 0:
 157                 set_return_addr(&&check_normal, &&check_redirected);
 158                 fallthrough;
 159         case 1:
 160 check_normal:
 161                 /* Always true */
 162                 if (!force_check) {
 163                         pr_info("ok: control flow unchanged.\n");
 164                         return;
 165                 }
 166
 167 check_redirected:
 168                 pr_err("FAIL: stack return address was redirected!\n");
 169                 break;
 170         }
 171
 172         if (IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL)) {
 173                 pr_expected_config(CONFIG_ARM64_PTR_AUTH_KERNEL);
 174                 return;
 175         }
 176         if (IS_ENABLED(CONFIG_SHADOW_CALL_STACK)) {
 177                 pr_expected_config(CONFIG_SHADOW_CALL_STACK);
 178                 return;
 179         }
 180         pr_warn("This is probably expected, since this %s was built *without* %s=y nor %s=y\n",
 181                 lkdtm_kernel_info,
 182                 "CONFIG_ARM64_PTR_AUTH_KERNEL", "CONFIG_SHADOW_CALL_STACK");
 183 }
 184
 185 static struct crashtype crashtypes[] = {
 186         CRASHTYPE(CFI_FORWARD_PROTO),
 187         CRASHTYPE(CFI_BACKWARD),
 188 };
 189
 190 struct crashtype_category cfi_crashtypes = {
 191         .crashtypes = crashtypes,
 192         .len        = ARRAY_SIZE(crashtypes),
 193 };