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/facility.h>
   6 #include <asm/nospec-branch.h>
   7
   8 static int __init nobp_setup_early(char *str)
   9 {
  10         bool enabled;
  11         int rc;
  12
  13         rc = kstrtobool(str, &enabled);
  14         if (rc)
  15                 return rc;
  16         if (enabled && test_facility(82)) {
  17                 /*
  18                  * The user explicitely requested nobp=1, enable it and
  19                  * disable the expoline support.
  20                  */
  21                 __set_facility(82, S390_lowcore.alt_stfle_fac_list);
  22                 if (IS_ENABLED(CONFIG_EXPOLINE))
  23                         nospec_disable = 1;
  24         } else {
  25                 __clear_facility(82, S390_lowcore.alt_stfle_fac_list);
  26         }
  27         return 0;
  28 }
  29 early_param("nobp", nobp_setup_early);
  30
  31 static int __init nospec_setup_early(char *str)
  32 {
  33         __clear_facility(82, S390_lowcore.alt_stfle_fac_list);
  34         return 0;
  35 }
  36 early_param("nospec", nospec_setup_early);
  37
  38 static int __init nospec_report(void)
  39 {
  40         if (IS_ENABLED(CC_USING_EXPOLINE) && !nospec_disable)
  41                 pr_info("Spectre V2 mitigation: execute trampolines.\n");
  42         if (__test_facility(82, S390_lowcore.alt_stfle_fac_list))
  43                 pr_info("Spectre V2 mitigation: limited branch prediction.\n");
  44         return 0;
  45 }
  46 arch_initcall(nospec_report);
  47
  48 #ifdef CONFIG_EXPOLINE
  49
  50 int nospec_disable = IS_ENABLED(CONFIG_EXPOLINE_OFF);
  51
  52 static int __init nospectre_v2_setup_early(char *str)
  53 {
  54         nospec_disable = 1;
  55         return 0;
  56 }
  57 early_param("nospectre_v2", nospectre_v2_setup_early);
  58
  59
  60 void __init nospec_auto_detect(void)
  61 {
  62         if (cpu_mitigations_off()) {
  63                 /*
  64                  * Disable expolines and disable nobp.
  65                  */
  66                 if (IS_ENABLED(CC_USING_EXPOLINE))
  67                         nospec_disable = 1;
  68                 __clear_facility(82, S390_lowcore.alt_stfle_fac_list);
  69         }
  70         if (IS_ENABLED(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, S390_lowcore.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, S390_lowcore.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         u8 *instr, *thunk, *br;
 103         u8 insnbuf[6];
 104         s32 *epo;
 105
 106         /* Second part of the instruction replace is always a nop */
 107         for (epo = start; epo < end; epo++) {
 108                 instr = (u8 *) epo + *epo;
 109                 if (instr[0] == 0xc0 && (instr[1] & 0x0f) == 0x04)
 110                         type = BRCL_EXPOLINE;   /* brcl instruction */
 111                 else if (instr[0] == 0xc0 && (instr[1] & 0x0f) == 0x05)
 112                         type = BRASL_EXPOLINE;  /* brasl instruction */
 113                 else
 114                         continue;
 115                 thunk = instr + (*(int *)(instr + 2)) * 2;
 116                 if (thunk[0] == 0xc6 && thunk[1] == 0x00)
 117                         /* exrl %r0,<target-br> */
 118                         br = thunk + (*(int *)(thunk + 2)) * 2;
 119                 else if (thunk[0] == 0xc0 && (thunk[1] & 0x0f) == 0x00 &&
 120                          thunk[6] == 0x44 && thunk[7] == 0x00 &&
 121                          (thunk[8] & 0x0f) == 0x00 && thunk[9] == 0x00 &&
 122                          (thunk[1] & 0xf0) == (thunk[8] & 0xf0))
 123                         /* larl %rx,<target br> + ex %r0,0(%rx) */
 124                         br = thunk + (*(int *)(thunk + 2)) * 2;
 125                 else
 126                         continue;
 127                 /* Check for unconditional branch 0x07f? or 0x47f???? */
 128                 if ((br[0] & 0xbf) != 0x07 || (br[1] & 0xf0) != 0xf0)
 129                         continue;
 130
 131                 memcpy(insnbuf + 2, (char[]) { 0x47, 0x00, 0x07, 0x00 }, 4);
 132                 switch (type) {
 133                 case BRCL_EXPOLINE:
 134                         insnbuf[0] = br[0];
 135                         insnbuf[1] = (instr[1] & 0xf0) | (br[1] & 0x0f);
 136                         if (br[0] == 0x47) {
 137                                 /* brcl to b, replace with bc + nopr */
 138                                 insnbuf[2] = br[2];
 139                                 insnbuf[3] = br[3];
 140                         } else {
 141                                 /* brcl to br, replace with bcr + nop */
 142                         }
 143                         break;
 144                 case BRASL_EXPOLINE:
 145                         insnbuf[1] = (instr[1] & 0xf0) | (br[1] & 0x0f);
 146                         if (br[0] == 0x47) {
 147                                 /* brasl to b, replace with bas + nopr */
 148                                 insnbuf[0] = 0x4d;
 149                                 insnbuf[2] = br[2];
 150                                 insnbuf[3] = br[3];
 151                         } else {
 152                                 /* brasl to br, replace with basr + nop */
 153                                 insnbuf[0] = 0x0d;
 154                         }
 155                         break;
 156                 }
 157
 158                 s390_kernel_write(instr, insnbuf, 6);
 159         }
 160 }
 161
 162 void __init_or_module nospec_revert(s32 *start, s32 *end)
 163 {
 164         if (nospec_disable)
 165                 __nospec_revert(start, end);
 166 }
 167
 168 extern s32 __nospec_call_start[], __nospec_call_end[];
 169 extern s32 __nospec_return_start[], __nospec_return_end[];
 170 void __init nospec_init_branches(void)
 171 {
 172         nospec_revert(__nospec_call_start, __nospec_call_end);
 173         nospec_revert(__nospec_return_start, __nospec_return_end);
 174 }
 175
 176 #endif /* CONFIG_EXPOLINE */