1 /* 2 * umip.c Emulation for instruction protected by the User-Mode Instruction 3 * Prevention feature 4 * 5 * Copyright (c) 2017, Intel Corporation. 6 * Ricardo Neri <ricardo.neri-calderon@linux.intel.com> 7 */ 8 9 #include <linux/uaccess.h> 10 #include <asm/umip.h> 11 #include <asm/traps.h> 12 #include <asm/insn.h> 13 #include <asm/insn-eval.h> 14 #include <linux/ratelimit.h> 15 16 #undef pr_fmt 17 #define pr_fmt(fmt) "umip: " fmt 18 19 /** DOC: Emulation for User-Mode Instruction Prevention (UMIP) 20 * 21 * User-Mode Instruction Prevention is a security feature present in recent 22 * x86 processors that, when enabled, prevents a group of instructions (SGDT, 23 * SIDT, SLDT, SMSW and STR) from being run in user mode by issuing a general 24 * protection fault if the instruction is executed with CPL > 0. 25 * 26 * Rather than relaying to the user space the general protection fault caused by 27 * the UMIP-protected instructions (in the form of a SIGSEGV signal), it can be 28 * trapped and emulate the result of such instructions to provide dummy values. 29 * This allows to both conserve the current kernel behavior and not reveal the 30 * system resources that UMIP intends to protect (i.e., the locations of the 31 * global descriptor and interrupt descriptor tables, the segment selectors of 32 * the local descriptor table, the value of the task state register and the 33 * contents of the CR0 register). 34 * 35 * This emulation is needed because certain applications (e.g., WineHQ and 36 * DOSEMU2) rely on this subset of instructions to function. 37 * 38 * The instructions protected by UMIP can be split in two groups. Those which 39 * return a kernel memory address (SGDT and SIDT) and those which return a 40 * value (SLDT, STR and SMSW). 41 * 42 * For the instructions that return a kernel memory address, applications 43 * such as WineHQ rely on the result being located in the kernel memory space, 44 * not the actual location of the table. The result is emulated as a hard-coded 45 * value that, lies close to the top of the kernel memory. The limit for the GDT 46 * and the IDT are set to zero. 47 * 48 * The instruction SMSW is emulated to return the value that the register CR0 49 * has at boot time as set in the head_32. 50 * SLDT and STR are emulated to return the values that the kernel programmatically 51 * assigns: 52 * - SLDT returns (GDT_ENTRY_LDT * 8) if an LDT has been set, 0 if not. 53 * - STR returns (GDT_ENTRY_TSS * 8). 54 * 55 * Emulation is provided for both 32-bit and 64-bit processes. 56 * 57 * Care is taken to appropriately emulate the results when segmentation is 58 * used. That is, rather than relying on USER_DS and USER_CS, the function 59 * insn_get_addr_ref() inspects the segment descriptor pointed by the 60 * registers in pt_regs. This ensures that we correctly obtain the segment 61 * base address and the address and operand sizes even if the user space 62 * application uses a local descriptor table. 63 */ 64 65 #define UMIP_DUMMY_GDT_BASE 0xfffffffffffe0000ULL 66 #define UMIP_DUMMY_IDT_BASE 0xffffffffffff0000ULL 67 68 /* 69 * The SGDT and SIDT instructions store the contents of the global descriptor 70 * table and interrupt table registers, respectively. The destination is a 71 * memory operand of X+2 bytes. X bytes are used to store the base address of 72 * the table and 2 bytes are used to store the limit. In 32-bit processes X 73 * has a value of 4, in 64-bit processes X has a value of 8. 74 */ 75 #define UMIP_GDT_IDT_BASE_SIZE_64BIT 8 76 #define UMIP_GDT_IDT_BASE_SIZE_32BIT 4 77 #define UMIP_GDT_IDT_LIMIT_SIZE 2 78 79 #define UMIP_INST_SGDT 0 /* 0F 01 /0 */ 80 #define UMIP_INST_SIDT 1 /* 0F 01 /1 */ 81 #define UMIP_INST_SMSW 2 /* 0F 01 /4 */ 82 #define UMIP_INST_SLDT 3 /* 0F 00 /0 */ 83 #define UMIP_INST_STR 4 /* 0F 00 /1 */ 84 85 static const char * const umip_insns[5] = { 86 [UMIP_INST_SGDT] = "SGDT", 87 [UMIP_INST_SIDT] = "SIDT", 88 [UMIP_INST_SMSW] = "SMSW", 89 [UMIP_INST_SLDT] = "SLDT", 90 [UMIP_INST_STR] = "STR", 91 }; 92 93 #define umip_pr_err(regs, fmt, ...) \ 94 umip_printk(regs, KERN_ERR, fmt, ##__VA_ARGS__) 95 #define umip_pr_warn(regs, fmt, ...) \ 96 umip_printk(regs, KERN_WARNING, fmt, ##__VA_ARGS__) 97 98 /** 99 * umip_printk() - Print a rate-limited message 100 * @regs: Register set with the context in which the warning is printed 101 * @log_level: Kernel log level to print the message 102 * @fmt: The text string to print 103 * 104 * Print the text contained in @fmt. The print rate is limited to bursts of 5 105 * messages every two minutes. The purpose of this customized version of 106 * printk() is to print messages when user space processes use any of the 107 * UMIP-protected instructions. Thus, the printed text is prepended with the 108 * task name and process ID number of the current task as well as the 109 * instruction and stack pointers in @regs as seen when entering kernel mode. 110 * 111 * Returns: 112 * 113 * None. 114 */ 115 static __printf(3, 4) 116 void umip_printk(const struct pt_regs *regs, const char *log_level, 117 const char *fmt, ...) 118 { 119 /* Bursts of 5 messages every two minutes */ 120 static DEFINE_RATELIMIT_STATE(ratelimit, 2 * 60 * HZ, 5); 121 struct task_struct *tsk = current; 122 struct va_format vaf; 123 va_list args; 124 125 if (!__ratelimit(&ratelimit)) 126 return; 127 128 va_start(args, fmt); 129 vaf.fmt = fmt; 130 vaf.va = &args; 131 printk("%s" pr_fmt("%s[%d] ip:%lx sp:%lx: %pV"), log_level, tsk->comm, 132 task_pid_nr(tsk), regs->ip, regs->sp, &vaf); 133 va_end(args); 134 } 135 136 /** 137 * identify_insn() - Identify a UMIP-protected instruction 138 * @insn: Instruction structure with opcode and ModRM byte. 139 * 140 * From the opcode and ModRM.reg in @insn identify, if any, a UMIP-protected 141 * instruction that can be emulated. 142 * 143 * Returns: 144 * 145 * On success, a constant identifying a specific UMIP-protected instruction that 146 * can be emulated. 147 * 148 * -EINVAL on error or when not an UMIP-protected instruction that can be 149 * emulated. 150 */ 151 static int identify_insn(struct insn *insn) 152 { 153 /* By getting modrm we also get the opcode. */ 154 insn_get_modrm(insn); 155 156 if (!insn->modrm.nbytes) 157 return -EINVAL; 158 159 /* All the instructions of interest start with 0x0f. */ 160 if (insn->opcode.bytes[0] != 0xf) 161 return -EINVAL; 162 163 if (insn->opcode.bytes[1] == 0x1) { 164 switch (X86_MODRM_REG(insn->modrm.value)) { 165 case 0: 166 return UMIP_INST_SGDT; 167 case 1: 168 return UMIP_INST_SIDT; 169 case 4: 170 return UMIP_INST_SMSW; 171 default: 172 return -EINVAL; 173 } 174 } else if (insn->opcode.bytes[1] == 0x0) { 175 if (X86_MODRM_REG(insn->modrm.value) == 0) 176 return UMIP_INST_SLDT; 177 else if (X86_MODRM_REG(insn->modrm.value) == 1) 178 return UMIP_INST_STR; 179 else 180 return -EINVAL; 181 } else { 182 return -EINVAL; 183 } 184 } 185 186 /** 187 * emulate_umip_insn() - Emulate UMIP instructions and return dummy values 188 * @insn: Instruction structure with operands 189 * @umip_inst: A constant indicating the instruction to emulate 190 * @data: Buffer into which the dummy result is stored 191 * @data_size: Size of the emulated result 192 * @x86_64: true if process is 64-bit, false otherwise 193 * 194 * Emulate an instruction protected by UMIP and provide a dummy result. The 195 * result of the emulation is saved in @data. The size of the results depends 196 * on both the instruction and type of operand (register vs memory address). 197 * The size of the result is updated in @data_size. Caller is responsible 198 * of providing a @data buffer of at least UMIP_GDT_IDT_BASE_SIZE + 199 * UMIP_GDT_IDT_LIMIT_SIZE bytes. 200 * 201 * Returns: 202 * 203 * 0 on success, -EINVAL on error while emulating. 204 */ 205 static int emulate_umip_insn(struct insn *insn, int umip_inst, 206 unsigned char *data, int *data_size, bool x86_64) 207 { 208 if (!data || !data_size || !insn) 209 return -EINVAL; 210 /* 211 * These two instructions return the base address and limit of the 212 * global and interrupt descriptor table, respectively. According to the 213 * Intel Software Development manual, the base address can be 24-bit, 214 * 32-bit or 64-bit. Limit is always 16-bit. If the operand size is 215 * 16-bit, the returned value of the base address is supposed to be a 216 * zero-extended 24-byte number. However, it seems that a 32-byte number 217 * is always returned irrespective of the operand size. 218 */ 219 if (umip_inst == UMIP_INST_SGDT || umip_inst == UMIP_INST_SIDT) { 220 u64 dummy_base_addr; 221 u16 dummy_limit = 0; 222 223 /* SGDT and SIDT do not use registers operands. */ 224 if (X86_MODRM_MOD(insn->modrm.value) == 3) 225 return -EINVAL; 226 227 if (umip_inst == UMIP_INST_SGDT) 228 dummy_base_addr = UMIP_DUMMY_GDT_BASE; 229 else 230 dummy_base_addr = UMIP_DUMMY_IDT_BASE; 231 232 /* 233 * 64-bit processes use the entire dummy base address. 234 * 32-bit processes use the lower 32 bits of the base address. 235 * dummy_base_addr is always 64 bits, but we memcpy the correct 236 * number of bytes from it to the destination. 237 */ 238 if (x86_64) 239 *data_size = UMIP_GDT_IDT_BASE_SIZE_64BIT; 240 else 241 *data_size = UMIP_GDT_IDT_BASE_SIZE_32BIT; 242 243 memcpy(data + 2, &dummy_base_addr, *data_size); 244 245 *data_size += UMIP_GDT_IDT_LIMIT_SIZE; 246 memcpy(data, &dummy_limit, UMIP_GDT_IDT_LIMIT_SIZE); 247 248 } else if (umip_inst == UMIP_INST_SMSW || umip_inst == UMIP_INST_SLDT || 249 umip_inst == UMIP_INST_STR) { 250 unsigned long dummy_value; 251 252 if (umip_inst == UMIP_INST_SMSW) { 253 dummy_value = CR0_STATE; 254 } else if (umip_inst == UMIP_INST_STR) { 255 dummy_value = GDT_ENTRY_TSS * 8; 256 } else if (umip_inst == UMIP_INST_SLDT) { 257 #ifdef CONFIG_MODIFY_LDT_SYSCALL 258 down_read(¤t->mm->context.ldt_usr_sem); 259 if (current->mm->context.ldt) 260 dummy_value = GDT_ENTRY_LDT * 8; 261 else 262 dummy_value = 0; 263 up_read(¤t->mm->context.ldt_usr_sem); 264 #else 265 dummy_value = 0; 266 #endif 267 } 268 269 /* 270 * For these 3 instructions, the number 271 * of bytes to be copied in the result buffer is determined 272 * by whether the operand is a register or a memory location. 273 * If operand is a register, return as many bytes as the operand 274 * size. If operand is memory, return only the two least 275 * significant bytes. 276 */ 277 if (X86_MODRM_MOD(insn->modrm.value) == 3) 278 *data_size = insn->opnd_bytes; 279 else 280 *data_size = 2; 281 282 memcpy(data, &dummy_value, *data_size); 283 } else { 284 return -EINVAL; 285 } 286 287 return 0; 288 } 289 290 /** 291 * force_sig_info_umip_fault() - Force a SIGSEGV with SEGV_MAPERR 292 * @addr: Address that caused the signal 293 * @regs: Register set containing the instruction pointer 294 * 295 * Force a SIGSEGV signal with SEGV_MAPERR as the error code. This function is 296 * intended to be used to provide a segmentation fault when the result of the 297 * UMIP emulation could not be copied to the user space memory. 298 * 299 * Returns: none 300 */ 301 static void force_sig_info_umip_fault(void __user *addr, struct pt_regs *regs) 302 { 303 struct task_struct *tsk = current; 304 305 tsk->thread.cr2 = (unsigned long)addr; 306 tsk->thread.error_code = X86_PF_USER | X86_PF_WRITE; 307 tsk->thread.trap_nr = X86_TRAP_PF; 308 309 force_sig_fault(SIGSEGV, SEGV_MAPERR, addr); 310 311 if (!(show_unhandled_signals && unhandled_signal(tsk, SIGSEGV))) 312 return; 313 314 umip_pr_err(regs, "segfault in emulation. error%x\n", 315 X86_PF_USER | X86_PF_WRITE); 316 } 317 318 /** 319 * fixup_umip_exception() - Fixup a general protection fault caused by UMIP 320 * @regs: Registers as saved when entering the #GP handler 321 * 322 * The instructions SGDT, SIDT, STR, SMSW and SLDT cause a general protection 323 * fault if executed with CPL > 0 (i.e., from user space). This function fixes 324 * the exception up and provides dummy results for SGDT, SIDT and SMSW; STR 325 * and SLDT are not fixed up. 326 * 327 * If operands are memory addresses, results are copied to user-space memory as 328 * indicated by the instruction pointed by eIP using the registers indicated in 329 * the instruction operands. If operands are registers, results are copied into 330 * the context that was saved when entering kernel mode. 331 * 332 * Returns: 333 * 334 * True if emulation was successful; false if not. 335 */ 336 bool fixup_umip_exception(struct pt_regs *regs) 337 { 338 int nr_copied, reg_offset, dummy_data_size, umip_inst; 339 /* 10 bytes is the maximum size of the result of UMIP instructions */ 340 unsigned char dummy_data[10] = { 0 }; 341 unsigned char buf[MAX_INSN_SIZE]; 342 unsigned long *reg_addr; 343 void __user *uaddr; 344 struct insn insn; 345 346 if (!regs) 347 return false; 348 349 nr_copied = insn_fetch_from_user(regs, buf); 350 351 /* 352 * The insn_fetch_from_user above could have failed if user code 353 * is protected by a memory protection key. Give up on emulation 354 * in such a case. Should we issue a page fault? 355 */ 356 if (!nr_copied) 357 return false; 358 359 if (!insn_decode_from_regs(&insn, regs, buf, nr_copied)) 360 return false; 361 362 umip_inst = identify_insn(&insn); 363 if (umip_inst < 0) 364 return false; 365 366 umip_pr_warn(regs, "%s instruction cannot be used by applications.\n", 367 umip_insns[umip_inst]); 368 369 umip_pr_warn(regs, "For now, expensive software emulation returns the result.\n"); 370 371 if (emulate_umip_insn(&insn, umip_inst, dummy_data, &dummy_data_size, 372 user_64bit_mode(regs))) 373 return false; 374 375 /* 376 * If operand is a register, write result to the copy of the register 377 * value that was pushed to the stack when entering into kernel mode. 378 * Upon exit, the value we write will be restored to the actual hardware 379 * register. 380 */ 381 if (X86_MODRM_MOD(insn.modrm.value) == 3) { 382 reg_offset = insn_get_modrm_rm_off(&insn, regs); 383 384 /* 385 * Negative values are usually errors. In memory addressing, 386 * the exception is -EDOM. Since we expect a register operand, 387 * all negative values are errors. 388 */ 389 if (reg_offset < 0) 390 return false; 391 392 reg_addr = (unsigned long *)((unsigned long)regs + reg_offset); 393 memcpy(reg_addr, dummy_data, dummy_data_size); 394 } else { 395 uaddr = insn_get_addr_ref(&insn, regs); 396 if ((unsigned long)uaddr == -1L) 397 return false; 398 399 nr_copied = copy_to_user(uaddr, dummy_data, dummy_data_size); 400 if (nr_copied > 0) { 401 /* 402 * If copy fails, send a signal and tell caller that 403 * fault was fixed up. 404 */ 405 force_sig_info_umip_fault(uaddr, regs); 406 return true; 407 } 408 } 409 410 /* increase IP to let the program keep going */ 411 regs->ip += insn.length; 412 return true; 413 } 414