xref: /openbmc/linux/arch/s390/kernel/nospec-branch.c (revision b4a6aaea)
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 (nospec_uses_trampoline())
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 */
178