1 /* 2 * umip.c Emulation for instruction protected by the Intel User-Mode 3 * Instruction 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 * The feature User-Mode Instruction Prevention present in recent Intel 22 * processor prevents a group of instructions (sgdt, sidt, sldt, smsw, and str) 23 * from being executed with CPL > 0. Otherwise, a general protection fault is 24 * issued. 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 * Given that sldt and str are not commonly used in programs that run on WineHQ 49 * or DOSEMU2, they are not emulated. 50 * 51 * The instruction smsw is emulated to return the value that the register CR0 52 * has at boot time as set in the head_32. 53 * 54 * Also, emulation is provided only for 32-bit processes; 64-bit processes 55 * that attempt to use the instructions that UMIP protects will receive the 56 * SIGSEGV signal issued as a consequence of the general protection fault. 57 * 58 * Care is taken to appropriately emulate the results when segmentation is 59 * used. That is, rather than relying on USER_DS and USER_CS, the function 60 * insn_get_addr_ref() inspects the segment descriptor pointed by the 61 * registers in pt_regs. This ensures that we correctly obtain the segment 62 * base address and the address and operand sizes even if the user space 63 * application uses a local descriptor table. 64 */ 65 66 #define UMIP_DUMMY_GDT_BASE 0xfffe0000 67 #define UMIP_DUMMY_IDT_BASE 0xffff0000 68 69 /* 70 * The SGDT and SIDT instructions store the contents of the global descriptor 71 * table and interrupt table registers, respectively. The destination is a 72 * memory operand of X+2 bytes. X bytes are used to store the base address of 73 * the table and 2 bytes are used to store the limit. In 32-bit processes, the 74 * only processes for which emulation is provided, X has a value of 4. 75 */ 76 #define UMIP_GDT_IDT_BASE_SIZE 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 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_warning(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 * 193 * Emulate an instruction protected by UMIP and provide a dummy result. The 194 * result of the emulation is saved in @data. The size of the results depends 195 * on both the instruction and type of operand (register vs memory address). 196 * The size of the result is updated in @data_size. Caller is responsible 197 * of providing a @data buffer of at least UMIP_GDT_IDT_BASE_SIZE + 198 * UMIP_GDT_IDT_LIMIT_SIZE bytes. 199 * 200 * Returns: 201 * 202 * 0 on success, -EINVAL on error while emulating. 203 */ 204 static int emulate_umip_insn(struct insn *insn, int umip_inst, 205 unsigned char *data, int *data_size) 206 { 207 unsigned long dummy_base_addr, dummy_value; 208 unsigned short dummy_limit = 0; 209 210 if (!data || !data_size || !insn) 211 return -EINVAL; 212 /* 213 * These two instructions return the base address and limit of the 214 * global and interrupt descriptor table, respectively. According to the 215 * Intel Software Development manual, the base address can be 24-bit, 216 * 32-bit or 64-bit. Limit is always 16-bit. If the operand size is 217 * 16-bit, the returned value of the base address is supposed to be a 218 * zero-extended 24-byte number. However, it seems that a 32-byte number 219 * is always returned irrespective of the operand size. 220 */ 221 if (umip_inst == UMIP_INST_SGDT || umip_inst == UMIP_INST_SIDT) { 222 /* SGDT and SIDT do not use registers operands. */ 223 if (X86_MODRM_MOD(insn->modrm.value) == 3) 224 return -EINVAL; 225 226 if (umip_inst == UMIP_INST_SGDT) 227 dummy_base_addr = UMIP_DUMMY_GDT_BASE; 228 else 229 dummy_base_addr = UMIP_DUMMY_IDT_BASE; 230 231 *data_size = UMIP_GDT_IDT_LIMIT_SIZE + UMIP_GDT_IDT_BASE_SIZE; 232 233 memcpy(data + 2, &dummy_base_addr, UMIP_GDT_IDT_BASE_SIZE); 234 memcpy(data, &dummy_limit, UMIP_GDT_IDT_LIMIT_SIZE); 235 236 } else if (umip_inst == UMIP_INST_SMSW) { 237 dummy_value = CR0_STATE; 238 239 /* 240 * Even though the CR0 register has 4 bytes, the number 241 * of bytes to be copied in the result buffer is determined 242 * by whether the operand is a register or a memory location. 243 * If operand is a register, return as many bytes as the operand 244 * size. If operand is memory, return only the two least 245 * siginificant bytes of CR0. 246 */ 247 if (X86_MODRM_MOD(insn->modrm.value) == 3) 248 *data_size = insn->opnd_bytes; 249 else 250 *data_size = 2; 251 252 memcpy(data, &dummy_value, *data_size); 253 /* STR and SLDT are not emulated */ 254 } else { 255 return -EINVAL; 256 } 257 258 return 0; 259 } 260 261 /** 262 * force_sig_info_umip_fault() - Force a SIGSEGV with SEGV_MAPERR 263 * @addr: Address that caused the signal 264 * @regs: Register set containing the instruction pointer 265 * 266 * Force a SIGSEGV signal with SEGV_MAPERR as the error code. This function is 267 * intended to be used to provide a segmentation fault when the result of the 268 * UMIP emulation could not be copied to the user space memory. 269 * 270 * Returns: none 271 */ 272 static void force_sig_info_umip_fault(void __user *addr, struct pt_regs *regs) 273 { 274 siginfo_t info; 275 struct task_struct *tsk = current; 276 277 tsk->thread.cr2 = (unsigned long)addr; 278 tsk->thread.error_code = X86_PF_USER | X86_PF_WRITE; 279 tsk->thread.trap_nr = X86_TRAP_PF; 280 281 info.si_signo = SIGSEGV; 282 info.si_errno = 0; 283 info.si_code = SEGV_MAPERR; 284 info.si_addr = addr; 285 force_sig_info(SIGSEGV, &info, tsk); 286 287 if (!(show_unhandled_signals && unhandled_signal(tsk, SIGSEGV))) 288 return; 289 290 umip_pr_err(regs, "segfault in emulation. error%x\n", 291 X86_PF_USER | X86_PF_WRITE); 292 } 293 294 /** 295 * fixup_umip_exception() - Fixup a general protection fault caused by UMIP 296 * @regs: Registers as saved when entering the #GP handler 297 * 298 * The instructions sgdt, sidt, str, smsw, sldt cause a general protection 299 * fault if executed with CPL > 0 (i.e., from user space). If the offending 300 * user-space process is not in long mode, this function fixes the exception 301 * up and provides dummy results for sgdt, sidt and smsw; str and sldt are not 302 * fixed up. Also long mode user-space processes are not fixed up. 303 * 304 * If operands are memory addresses, results are copied to user-space memory as 305 * indicated by the instruction pointed by eIP using the registers indicated in 306 * the instruction operands. If operands are registers, results are copied into 307 * the context that was saved when entering kernel mode. 308 * 309 * Returns: 310 * 311 * True if emulation was successful; false if not. 312 */ 313 bool fixup_umip_exception(struct pt_regs *regs) 314 { 315 int not_copied, nr_copied, reg_offset, dummy_data_size, umip_inst; 316 unsigned long seg_base = 0, *reg_addr; 317 /* 10 bytes is the maximum size of the result of UMIP instructions */ 318 unsigned char dummy_data[10] = { 0 }; 319 unsigned char buf[MAX_INSN_SIZE]; 320 void __user *uaddr; 321 struct insn insn; 322 int seg_defs; 323 324 if (!regs) 325 return false; 326 327 /* 328 * If not in user-space long mode, a custom code segment could be in 329 * use. This is true in protected mode (if the process defined a local 330 * descriptor table), or virtual-8086 mode. In most of the cases 331 * seg_base will be zero as in USER_CS. 332 */ 333 if (!user_64bit_mode(regs)) 334 seg_base = insn_get_seg_base(regs, INAT_SEG_REG_CS); 335 336 if (seg_base == -1L) 337 return false; 338 339 not_copied = copy_from_user(buf, (void __user *)(seg_base + regs->ip), 340 sizeof(buf)); 341 nr_copied = sizeof(buf) - not_copied; 342 343 /* 344 * The copy_from_user above could have failed if user code is protected 345 * by a memory protection key. Give up on emulation in such a case. 346 * Should we issue a page fault? 347 */ 348 if (!nr_copied) 349 return false; 350 351 insn_init(&insn, buf, nr_copied, user_64bit_mode(regs)); 352 353 /* 354 * Override the default operand and address sizes with what is specified 355 * in the code segment descriptor. The instruction decoder only sets 356 * the address size it to either 4 or 8 address bytes and does nothing 357 * for the operand bytes. This OK for most of the cases, but we could 358 * have special cases where, for instance, a 16-bit code segment 359 * descriptor is used. 360 * If there is an address override prefix, the instruction decoder 361 * correctly updates these values, even for 16-bit defaults. 362 */ 363 seg_defs = insn_get_code_seg_params(regs); 364 if (seg_defs == -EINVAL) 365 return false; 366 367 insn.addr_bytes = INSN_CODE_SEG_ADDR_SZ(seg_defs); 368 insn.opnd_bytes = INSN_CODE_SEG_OPND_SZ(seg_defs); 369 370 insn_get_length(&insn); 371 if (nr_copied < insn.length) 372 return false; 373 374 umip_inst = identify_insn(&insn); 375 if (umip_inst < 0) 376 return false; 377 378 umip_pr_warning(regs, "%s instruction cannot be used by applications.\n", 379 umip_insns[umip_inst]); 380 381 /* Do not emulate SLDT, STR or user long mode processes. */ 382 if (umip_inst == UMIP_INST_STR || umip_inst == UMIP_INST_SLDT || user_64bit_mode(regs)) 383 return false; 384 385 umip_pr_warning(regs, "For now, expensive software emulation returns the result.\n"); 386 387 if (emulate_umip_insn(&insn, umip_inst, dummy_data, &dummy_data_size)) 388 return false; 389 390 /* 391 * If operand is a register, write result to the copy of the register 392 * value that was pushed to the stack when entering into kernel mode. 393 * Upon exit, the value we write will be restored to the actual hardware 394 * register. 395 */ 396 if (X86_MODRM_MOD(insn.modrm.value) == 3) { 397 reg_offset = insn_get_modrm_rm_off(&insn, regs); 398 399 /* 400 * Negative values are usually errors. In memory addressing, 401 * the exception is -EDOM. Since we expect a register operand, 402 * all negative values are errors. 403 */ 404 if (reg_offset < 0) 405 return false; 406 407 reg_addr = (unsigned long *)((unsigned long)regs + reg_offset); 408 memcpy(reg_addr, dummy_data, dummy_data_size); 409 } else { 410 uaddr = insn_get_addr_ref(&insn, regs); 411 if ((unsigned long)uaddr == -1L) 412 return false; 413 414 nr_copied = copy_to_user(uaddr, dummy_data, dummy_data_size); 415 if (nr_copied > 0) { 416 /* 417 * If copy fails, send a signal and tell caller that 418 * fault was fixed up. 419 */ 420 force_sig_info_umip_fault(uaddr, regs); 421 return true; 422 } 423 } 424 425 /* increase IP to let the program keep going */ 426 regs->ip += insn.length; 427 return true; 428 } 429