arch/s390/kernel/nospec-branch.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 #include <linux/module.h>
   3 #include <linux/device.h>
   4 #include <linux/cpu.h>
   5 #include <asm/nospec-branch.h>
   6
   7 static int __init nobp_setup_early(char *str)
   8 {
   9         bool enabled;
  10         int rc;
  11
  12         rc = kstrtobool(str, &enabled);
  13         if (rc)
  14                 return rc;
  15         if (enabled && test_facility(82)) {
  16                 /*
  17                  * The user explicitely requested nobp=1, enable it and
  18                  * disable the expoline support.
  19                  */
  20                 __set_facility(82, alt_stfle_fac_list);
  21                 if (IS_ENABLED(CONFIG_EXPOLINE))
  22                         nospec_disable = 1;
  23         } else {
  24                 __clear_facility(82, alt_stfle_fac_list);
  25         }
  26         return 0;
  27 }
  28 early_param("nobp", nobp_setup_early);
  29
  30 static int __init nospec_setup_early(char *str)
  31 {
  32         __clear_facility(82, alt_stfle_fac_list);
  33         return 0;
  34 }
  35 early_param("nospec", nospec_setup_early);
  36
  37 static int __init nospec_report(void)
  38 {
  39         if (test_facility(156))
  40                 pr_info("Spectre V2 mitigation: etokens\n");
  41         if (__is_defined(CC_USING_EXPOLINE) && !nospec_disable)
  42                 pr_info("Spectre V2 mitigation: execute trampolines\n");
  43         if (__test_facility(82, alt_stfle_fac_list))
  44                 pr_info("Spectre V2 mitigation: limited branch prediction\n");
  45         return 0;
  46 }
  47 arch_initcall(nospec_report);
  48
  49 #ifdef CONFIG_EXPOLINE
  50
  51 int nospec_disable = IS_ENABLED(CONFIG_EXPOLINE_OFF);
  52
  53 static int __init nospectre_v2_setup_early(char *str)
  54 {
  55         nospec_disable = 1;
  56         return 0;
  57 }
  58 early_param("nospectre_v2", nospectre_v2_setup_early);
  59
  60 void __init nospec_auto_detect(void)
  61 {
  62         if (test_facility(156) || cpu_mitigations_off()) {
  63                 /*
  64                  * The machine supports etokens.
  65                  * Disable expolines and disable nobp.
  66                  */
  67                 if (__is_defined(CC_USING_EXPOLINE))
  68                         nospec_disable = 1;
  69                 __clear_facility(82, alt_stfle_fac_list);
  70         } else if (__is_defined(CC_USING_EXPOLINE)) {
  71                 /*
  72                  * The kernel has been compiled with expolines.
  73                  * Keep expolines enabled and disable nobp.
  74                  */
  75                 nospec_disable = 0;
  76                 __clear_facility(82, alt_stfle_fac_list);
  77         }
  78         /*
  79          * If the kernel has not been compiled with expolines the
  80          * nobp setting decides what is done, this depends on the
  81          * CONFIG_KERNEL_NP option and the nobp/nospec parameters.
  82          */
  83 }
  84
  85 static int __init spectre_v2_setup_early(char *str)
  86 {
  87         if (str && !strncmp(str, "on", 2)) {
  88                 nospec_disable = 0;
  89                 __clear_facility(82, alt_stfle_fac_list);
  90         }
  91         if (str && !strncmp(str, "off", 3))
  92                 nospec_disable = 1;
  93         if (str && !strncmp(str, "auto", 4))
  94                 nospec_auto_detect();
  95         return 0;
  96 }
  97 early_param("spectre_v2", spectre_v2_setup_early);
  98
  99 static void __init_or_module __nospec_revert(s32 *start, s32 *end)
 100 {
 101         enum { BRCL_EXPOLINE, BRASL_EXPOLINE } type;
 102         static const u8 branch[] = { 0x47, 0x00, 0x07, 0x00 };
 103         u8 *instr, *thunk, *br;
 104         u8 insnbuf[6];
 105         s32 *epo;
 106
 107         /* Second part of the instruction replace is always a nop */
 108         for (epo = start; epo < end; epo++) {
 109                 instr = (u8 *) epo + *epo;
 110                 if (instr[0] == 0xc0 && (instr[1] & 0x0f) == 0x04)
 111                         type = BRCL_EXPOLINE;   /* brcl instruction */
 112                 else if (instr[0] == 0xc0 && (instr[1] & 0x0f) == 0x05)
 113                         type = BRASL_EXPOLINE;  /* brasl instruction */
 114                 else
 115                         continue;
 116                 thunk = instr + (*(int *)(instr + 2)) * 2;
 117                 if (thunk[0] == 0xc6 && thunk[1] == 0x00)
 118                         /* exrl %r0,<target-br> */
 119                         br = thunk + (*(int *)(thunk + 2)) * 2;
 120                 else if (thunk[0] == 0xc0 && (thunk[1] & 0x0f) == 0x00 &&
 121                          thunk[6] == 0x44 && thunk[7] == 0x00 &&
 122                          (thunk[8] & 0x0f) == 0x00 && thunk[9] == 0x00 &&
 123                          (thunk[1] & 0xf0) == (thunk[8] & 0xf0))
 124                         /* larl %rx,<target br> + ex %r0,0(%rx) */
 125                         br = thunk + (*(int *)(thunk + 2)) * 2;
 126                 else
 127                         continue;
 128                 /* Check for unconditional branch 0x07f? or 0x47f???? */
 129                 if ((br[0] & 0xbf) != 0x07 || (br[1] & 0xf0) != 0xf0)
 130                         continue;
 131
 132                 memcpy(insnbuf + 2, branch, sizeof(branch));
 133                 switch (type) {
 134                 case BRCL_EXPOLINE:
 135                         insnbuf[0] = br[0];
 136                         insnbuf[1] = (instr[1] & 0xf0) | (br[1] & 0x0f);
 137                         if (br[0] == 0x47) {
 138                                 /* brcl to b, replace with bc + nopr */
 139                                 insnbuf[2] = br[2];
 140                                 insnbuf[3] = br[3];
 141                         } else {
 142                                 /* brcl to br, replace with bcr + nop */
 143                         }
 144                         break;
 145                 case BRASL_EXPOLINE:
 146                         insnbuf[1] = (instr[1] & 0xf0) | (br[1] & 0x0f);
 147                         if (br[0] == 0x47) {
 148                                 /* brasl to b, replace with bas + nopr */
 149                                 insnbuf[0] = 0x4d;
 150                                 insnbuf[2] = br[2];
 151                                 insnbuf[3] = br[3];
 152                         } else {
 153                                 /* brasl to br, replace with basr + nop */
 154                                 insnbuf[0] = 0x0d;
 155                         }
 156                         break;
 157                 }
 158
 159                 s390_kernel_write(instr, insnbuf, 6);
 160         }
 161 }
 162
 163 void __init_or_module nospec_revert(s32 *start, s32 *end)
 164 {
 165         if (nospec_disable)
 166                 __nospec_revert(start, end);
 167 }
 168
 169 extern s32 __nospec_call_start[], __nospec_call_end[];
 170 extern s32 __nospec_return_start[], __nospec_return_end[];
 171 void __init nospec_init_branches(void)
 172 {
 173         nospec_revert(__nospec_call_start, __nospec_call_end);
 174         nospec_revert(__nospec_return_start, __nospec_return_end);
 175 }
 176
 177 #endif /* CONFIG_EXPOLINE */