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 <asm/nospec-branch.h>
   5
   6 static int __init nobp_setup_early(char *str)
   7 {
   8         bool enabled;
   9         int rc;
  10
  11         rc = kstrtobool(str, &enabled);
  12         if (rc)
  13                 return rc;
  14         if (enabled && test_facility(82)) {
  15                 /*
  16                  * The user explicitely requested nobp=1, enable it and
  17                  * disable the expoline support.
  18                  */
  19                 __set_facility(82, S390_lowcore.alt_stfle_fac_list);
  20                 if (IS_ENABLED(CONFIG_EXPOLINE))
  21                         nospec_disable = 1;
  22         } else {
  23                 __clear_facility(82, S390_lowcore.alt_stfle_fac_list);
  24         }
  25         return 0;
  26 }
  27 early_param("nobp", nobp_setup_early);
  28
  29 static int __init nospec_setup_early(char *str)
  30 {
  31         __clear_facility(82, S390_lowcore.alt_stfle_fac_list);
  32         return 0;
  33 }
  34 early_param("nospec", nospec_setup_early);
  35
  36 static int __init nospec_report(void)
  37 {
  38         if (IS_ENABLED(CC_USING_EXPOLINE) && !nospec_disable)
  39                 pr_info("Spectre V2 mitigation: execute trampolines\n");
  40         if (__test_facility(82, S390_lowcore.alt_stfle_fac_list))
  41                 pr_info("Spectre V2 mitigation: limited branch prediction\n");
  42         return 0;
  43 }
  44 arch_initcall(nospec_report);
  45
  46 #ifdef CONFIG_EXPOLINE
  47
  48 int nospec_disable = IS_ENABLED(CONFIG_EXPOLINE_OFF);
  49
  50 static int __init nospectre_v2_setup_early(char *str)
  51 {
  52         nospec_disable = 1;
  53         return 0;
  54 }
  55 early_param("nospectre_v2", nospectre_v2_setup_early);
  56
  57 void __init nospec_auto_detect(void)
  58 {
  59         if (IS_ENABLED(CC_USING_EXPOLINE)) {
  60                 /*
  61                  * The kernel has been compiled with expolines.
  62                  * Keep expolines enabled and disable nobp.
  63                  */
  64                 nospec_disable = 0;
  65                 __clear_facility(82, S390_lowcore.alt_stfle_fac_list);
  66         }
  67         /*
  68          * If the kernel has not been compiled with expolines the
  69          * nobp setting decides what is done, this depends on the
  70          * CONFIG_KERNEL_NP option and the nobp/nospec parameters.
  71          */
  72 }
  73
  74 static int __init spectre_v2_setup_early(char *str)
  75 {
  76         if (str && !strncmp(str, "on", 2)) {
  77                 nospec_disable = 0;
  78                 __clear_facility(82, S390_lowcore.alt_stfle_fac_list);
  79         }
  80         if (str && !strncmp(str, "off", 3))
  81                 nospec_disable = 1;
  82         if (str && !strncmp(str, "auto", 4))
  83                 nospec_auto_detect();
  84         return 0;
  85 }
  86 early_param("spectre_v2", spectre_v2_setup_early);
  87
  88 static void __init_or_module __nospec_revert(s32 *start, s32 *end)
  89 {
  90         enum { BRCL_EXPOLINE, BRASL_EXPOLINE } type;
  91         u8 *instr, *thunk, *br;
  92         u8 insnbuf[6];
  93         s32 *epo;
  94
  95         /* Second part of the instruction replace is always a nop */
  96         for (epo = start; epo < end; epo++) {
  97                 instr = (u8 *) epo + *epo;
  98                 if (instr[0] == 0xc0 && (instr[1] & 0x0f) == 0x04)
  99                         type = BRCL_EXPOLINE;   /* brcl instruction */
 100                 else if (instr[0] == 0xc0 && (instr[1] & 0x0f) == 0x05)
 101                         type = BRASL_EXPOLINE;  /* brasl instruction */
 102                 else
 103                         continue;
 104                 thunk = instr + (*(int *)(instr + 2)) * 2;
 105                 if (thunk[0] == 0xc6 && thunk[1] == 0x00)
 106                         /* exrl %r0,<target-br> */
 107                         br = thunk + (*(int *)(thunk + 2)) * 2;
 108                 else if (thunk[0] == 0xc0 && (thunk[1] & 0x0f) == 0x00 &&
 109                          thunk[6] == 0x44 && thunk[7] == 0x00 &&
 110                          (thunk[8] & 0x0f) == 0x00 && thunk[9] == 0x00 &&
 111                          (thunk[1] & 0xf0) == (thunk[8] & 0xf0))
 112                         /* larl %rx,<target br> + ex %r0,0(%rx) */
 113                         br = thunk + (*(int *)(thunk + 2)) * 2;
 114                 else
 115                         continue;
 116                 /* Check for unconditional branch 0x07f? or 0x47f???? */
 117                 if ((br[0] & 0xbf) != 0x07 || (br[1] & 0xf0) != 0xf0)
 118                         continue;
 119
 120                 memcpy(insnbuf + 2, (char[]) { 0x47, 0x00, 0x07, 0x00 }, 4);
 121                 switch (type) {
 122                 case BRCL_EXPOLINE:
 123                         insnbuf[0] = br[0];
 124                         insnbuf[1] = (instr[1] & 0xf0) | (br[1] & 0x0f);
 125                         if (br[0] == 0x47) {
 126                                 /* brcl to b, replace with bc + nopr */
 127                                 insnbuf[2] = br[2];
 128                                 insnbuf[3] = br[3];
 129                         } else {
 130                                 /* brcl to br, replace with bcr + nop */
 131                         }
 132                         break;
 133                 case BRASL_EXPOLINE:
 134                         insnbuf[1] = (instr[1] & 0xf0) | (br[1] & 0x0f);
 135                         if (br[0] == 0x47) {
 136                                 /* brasl to b, replace with bas + nopr */
 137                                 insnbuf[0] = 0x4d;
 138                                 insnbuf[2] = br[2];
 139                                 insnbuf[3] = br[3];
 140                         } else {
 141                                 /* brasl to br, replace with basr + nop */
 142                                 insnbuf[0] = 0x0d;
 143                         }
 144                         break;
 145                 }
 146
 147                 s390_kernel_write(instr, insnbuf, 6);
 148         }
 149 }
 150
 151 void __init_or_module nospec_revert(s32 *start, s32 *end)
 152 {
 153         if (nospec_disable)
 154                 __nospec_revert(start, end);
 155 }
 156
 157 extern s32 __nospec_call_start[], __nospec_call_end[];
 158 extern s32 __nospec_return_start[], __nospec_return_end[];
 159 void __init nospec_init_branches(void)
 160 {
 161         nospec_revert(__nospec_call_start, __nospec_call_end);
 162         nospec_revert(__nospec_return_start, __nospec_return_end);
 163 }
 164
 165 #endif /* CONFIG_EXPOLINE */