1 // SPDX-License-Identifier: GPL-2.0
2 #include <linux/static_call.h>
3 #include <linux/memory.h>
4 #include <linux/bug.h>
5 #include <asm/text-patching.h>
6
7 enum insn_type {
8 CALL = 0, /* site call */
9 NOP = 1, /* site cond-call */
10 JMP = 2, /* tramp / site tail-call */
11 RET = 3, /* tramp / site cond-tail-call */
12 JCC = 4,
13 };
14
15 /*
16 * ud1 %esp, %ecx - a 3 byte #UD that is unique to trampolines, chosen such
17 * that there is no false-positive trampoline identification while also being a
18 * speculation stop.
19 */
20 static const u8 tramp_ud[] = { 0x0f, 0xb9, 0xcc };
21
22 /*
23 * cs cs cs xorl %eax, %eax - a single 5 byte instruction that clears %[er]ax
24 */
25 static const u8 xor5rax[] = { 0x2e, 0x2e, 0x2e, 0x31, 0xc0 };
26
27 static const u8 retinsn[] = { RET_INSN_OPCODE, 0xcc, 0xcc, 0xcc, 0xcc };
28
29 static u8 __is_Jcc(u8 *insn) /* Jcc.d32 */
30 {
31 u8 ret = 0;
32
33 if (insn[0] == 0x0f) {
34 u8 tmp = insn[1];
35 if ((tmp & 0xf0) == 0x80)
36 ret = tmp;
37 }
38
39 return ret;
40 }
41
42 extern void __static_call_return(void);
43
44 asm (".global __static_call_return\n\t"
45 ".type __static_call_return, @function\n\t"
46 ASM_FUNC_ALIGN "\n\t"
47 "__static_call_return:\n\t"
48 ANNOTATE_NOENDBR
49 ANNOTATE_RETPOLINE_SAFE
50 "ret; int3\n\t"
51 ".size __static_call_return, . - __static_call_return \n\t");
52
53 static void __ref __static_call_transform(void *insn, enum insn_type type,
54 void *func, bool modinit)
55 {
56 const void *emulate = NULL;
57 int size = CALL_INSN_SIZE;
58 const void *code;
59 u8 op, buf[6];
60
61 if ((type == JMP || type == RET) && (op = __is_Jcc(insn)))
62 type = JCC;
63
64 switch (type) {
65 case CALL:
66 func = callthunks_translate_call_dest(func);
67 code = text_gen_insn(CALL_INSN_OPCODE, insn, func);
68 if (func == &__static_call_return0) {
69 emulate = code;
70 code = &xor5rax;
71 }
72
73 break;
74
75 case NOP:
76 code = x86_nops[5];
77 break;
78
79 case JMP:
80 code = text_gen_insn(JMP32_INSN_OPCODE, insn, func);
81 break;
82
83 case RET:
84 if (cpu_feature_enabled(X86_FEATURE_RETHUNK))
85 code = text_gen_insn(JMP32_INSN_OPCODE, insn, x86_return_thunk);
86 else
87 code = &retinsn;
88 break;
89
90 case JCC:
91 if (!func) {
92 func = __static_call_return;
93 if (cpu_feature_enabled(X86_FEATURE_RETHUNK))
94 func = x86_return_thunk;
95 }
96
97 buf[0] = 0x0f;
98 __text_gen_insn(buf+1, op, insn+1, func, 5);
99 code = buf;
100 size = 6;
101
102 break;
103 }
104
105 if (memcmp(insn, code, size) == 0)
106 return;
107
108 if (system_state == SYSTEM_BOOTING || modinit)
109 return text_poke_early(insn, code, size);
110
111 text_poke_bp(insn, code, size, emulate);
112 }
113
114 static void __static_call_validate(u8 *insn, bool tail, bool tramp)
115 {
116 u8 opcode = insn[0];
117
118 if (tramp && memcmp(insn+5, tramp_ud, 3)) {
119 pr_err("trampoline signature fail");
120 BUG();
121 }
122
123 if (tail) {
124 if (opcode == JMP32_INSN_OPCODE ||
125 opcode == RET_INSN_OPCODE ||
126 __is_Jcc(insn))
127 return;
128 } else {
129 if (opcode == CALL_INSN_OPCODE ||
130 !memcmp(insn, x86_nops[5], 5) ||
131 !memcmp(insn, xor5rax, 5))
132 return;
133 }
134
135 /*
136 * If we ever trigger this, our text is corrupt, we'll probably not live long.
137 */
138 pr_err("unexpected static_call insn opcode 0x%x at %pS\n", opcode, insn);
139 BUG();
140 }
141
142 static inline enum insn_type __sc_insn(bool null, bool tail)
143 {
144 /*
145 * Encode the following table without branches:
146 *
147 * tail null insn
148 * -----+-------+------
149 * 0 | 0 | CALL
150 * 0 | 1 | NOP
151 * 1 | 0 | JMP
152 * 1 | 1 | RET
153 */
154 return 2*tail + null;
155 }
156
157 void arch_static_call_transform(void *site, void *tramp, void *func, bool tail)
158 {
159 mutex_lock(&text_mutex);
160
161 if (tramp) {
162 __static_call_validate(tramp, true, true);
163 __static_call_transform(tramp, __sc_insn(!func, true), func, false);
164 }
165
166 if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE) && site) {
167 __static_call_validate(site, tail, false);
168 __static_call_transform(site, __sc_insn(!func, tail), func, false);
169 }
170
171 mutex_unlock(&text_mutex);
172 }
173 EXPORT_SYMBOL_GPL(arch_static_call_transform);
174
175 #ifdef CONFIG_MITIGATION_RETHUNK
176 /*
177 * This is called by apply_returns() to fix up static call trampolines,
178 * specifically ARCH_DEFINE_STATIC_CALL_NULL_TRAMP which is recorded as
179 * having a return trampoline.
180 *
181 * The problem is that static_call() is available before determining
182 * X86_FEATURE_RETHUNK and, by implication, running alternatives.
183 *
184 * This means that __static_call_transform() above can have overwritten the
185 * return trampoline and we now need to fix things up to be consistent.
186 */
187 bool __static_call_fixup(void *tramp, u8 op, void *dest)
188 {
189 unsigned long addr = (unsigned long)tramp;
190 /*
191 * Not all .return_sites are a static_call trampoline (most are not).
192 * Check if the 3 bytes after the return are still kernel text, if not,
193 * then this definitely is not a trampoline and we need not worry
194 * further.
195 *
196 * This avoids the memcmp() below tripping over pagefaults etc..
197 */
198 if (((addr >> PAGE_SHIFT) != ((addr + 7) >> PAGE_SHIFT)) &&
199 !kernel_text_address(addr + 7))
200 return false;
201
202 if (memcmp(tramp+5, tramp_ud, 3)) {
203 /* Not a trampoline site, not our problem. */
204 return false;
205 }
206
207 mutex_lock(&text_mutex);
208 if (op == RET_INSN_OPCODE || dest == &__x86_return_thunk)
209 __static_call_transform(tramp, RET, NULL, true);
210 mutex_unlock(&text_mutex);
211
212 return true;
213 }
214 #endif
215
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.