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