s390/kernel/nospec-branch.c

0001 // SPDX-License-Identifier: GPL-2.0
0002 #include <linux/module.h>
0003 #include <linux/device.h>
0004 #include <linux/cpu.h>
0005 #include <asm/nospec-branch.h>
0006
0007 static int __init nobp_setup_early(char *str)
0008 {
0009     bool enabled;
0010     int rc;
0011
0012     rc = kstrtobool(str, &enabled);
0013     if (rc)
0014         return rc;
0015     if (enabled && test_facility(82)) {
0016         /*
0017          * The user explicitely requested nobp=1, enable it and
0018          * disable the expoline support.
0019          */
0020         __set_facility(82, alt_stfle_fac_list);
0021         if (IS_ENABLED(CONFIG_EXPOLINE))
0022             nospec_disable = 1;
0023     } else {
0024         __clear_facility(82, alt_stfle_fac_list);
0025     }
0026     return 0;
0027 }
0028 early_param("nobp", nobp_setup_early);
0029
0030 static int __init nospec_setup_early(char *str)
0031 {
0032     __clear_facility(82, alt_stfle_fac_list);
0033     return 0;
0034 }
0035 early_param("nospec", nospec_setup_early);
0036
0037 static int __init nospec_report(void)
0038 {
0039     if (test_facility(156))
0040         pr_info("Spectre V2 mitigation: etokens\n");
0041     if (nospec_uses_trampoline())
0042         pr_info("Spectre V2 mitigation: execute trampolines\n");
0043     if (__test_facility(82, alt_stfle_fac_list))
0044         pr_info("Spectre V2 mitigation: limited branch prediction\n");
0045     return 0;
0046 }
0047 arch_initcall(nospec_report);
0048
0049 #ifdef CONFIG_EXPOLINE
0050
0051 int nospec_disable = IS_ENABLED(CONFIG_EXPOLINE_OFF);
0052
0053 static int __init nospectre_v2_setup_early(char *str)
0054 {
0055     nospec_disable = 1;
0056     return 0;
0057 }
0058 early_param("nospectre_v2", nospectre_v2_setup_early);
0059
0060 void __init nospec_auto_detect(void)
0061 {
0062     if (test_facility(156) || cpu_mitigations_off()) {
0063         /*
0064          * The machine supports etokens.
0065          * Disable expolines and disable nobp.
0066          */
0067         if (__is_defined(CC_USING_EXPOLINE))
0068             nospec_disable = 1;
0069         __clear_facility(82, alt_stfle_fac_list);
0070     } else if (__is_defined(CC_USING_EXPOLINE)) {
0071         /*
0072          * The kernel has been compiled with expolines.
0073          * Keep expolines enabled and disable nobp.
0074          */
0075         nospec_disable = 0;
0076         __clear_facility(82, alt_stfle_fac_list);
0077     }
0078     /*
0079      * If the kernel has not been compiled with expolines the
0080      * nobp setting decides what is done, this depends on the
0081      * CONFIG_KERNEL_NP option and the nobp/nospec parameters.
0082      */
0083 }
0084
0085 static int __init spectre_v2_setup_early(char *str)
0086 {
0087     if (str && !strncmp(str, "on", 2)) {
0088         nospec_disable = 0;
0089         __clear_facility(82, alt_stfle_fac_list);
0090     }
0091     if (str && !strncmp(str, "off", 3))
0092         nospec_disable = 1;
0093     if (str && !strncmp(str, "auto", 4))
0094         nospec_auto_detect();
0095     return 0;
0096 }
0097 early_param("spectre_v2", spectre_v2_setup_early);
0098
0099 static void __init_or_module __nospec_revert(s32 *start, s32 *end)
0100 {
0101     enum { BRCL_EXPOLINE, BRASL_EXPOLINE } type;
0102     static const u8 branch[] = { 0x47, 0x00, 0x07, 0x00 };
0103     u8 *instr, *thunk, *br;
0104     u8 insnbuf[6];
0105     s32 *epo;
0106
0107     /* Second part of the instruction replace is always a nop */
0108     memcpy(insnbuf + 2, branch, sizeof(branch));
0109     for (epo = start; epo < end; epo++) {
0110         instr = (u8 *) epo + *epo;
0111         if (instr[0] == 0xc0 && (instr[1] & 0x0f) == 0x04)
0112             type = BRCL_EXPOLINE;   /* brcl instruction */
0113         else if (instr[0] == 0xc0 && (instr[1] & 0x0f) == 0x05)
0114             type = BRASL_EXPOLINE;  /* brasl instruction */
0115         else
0116             continue;
0117         thunk = instr + (*(int *)(instr + 2)) * 2;
0118         if (thunk[0] == 0xc6 && thunk[1] == 0x00)
0119             /* exrl %r0,<target-br> */
0120             br = thunk + (*(int *)(thunk + 2)) * 2;
0121         else
0122             continue;
0123         if (br[0] != 0x07 || (br[1] & 0xf0) != 0xf0)
0124             continue;
0125         switch (type) {
0126         case BRCL_EXPOLINE:
0127             /* brcl to thunk, replace with br + nop */
0128             insnbuf[0] = br[0];
0129             insnbuf[1] = (instr[1] & 0xf0) | (br[1] & 0x0f);
0130             break;
0131         case BRASL_EXPOLINE:
0132             /* brasl to thunk, replace with basr + nop */
0133             insnbuf[0] = 0x0d;
0134             insnbuf[1] = (instr[1] & 0xf0) | (br[1] & 0x0f);
0135             break;
0136         }
0137
0138         s390_kernel_write(instr, insnbuf, 6);
0139     }
0140 }
0141
0142 void __init_or_module nospec_revert(s32 *start, s32 *end)
0143 {
0144     if (nospec_disable)
0145         __nospec_revert(start, end);
0146 }
0147
0148 extern s32 __nospec_call_start[], __nospec_call_end[];
0149 extern s32 __nospec_return_start[], __nospec_return_end[];
0150 void __init nospec_init_branches(void)
0151 {
0152     nospec_revert(__nospec_call_start, __nospec_call_end);
0153     nospec_revert(__nospec_return_start, __nospec_return_end);
0154 }
0155
0156 #endif /* CONFIG_EXPOLINE */