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 * Emulation is provided for both 32-bit and 64-bit processes.
55 *
56 * Care is taken to appropriately emulate the results when segmentation is
57 * used. That is, rather than relying on USER_DS and USER_CS, the function
58 * insn_get_addr_ref() inspects the segment descriptor pointed by the
59 * registers in pt_regs. This ensures that we correctly obtain the segment
60 * base address and the address and operand sizes even if the user space
61 * application uses a local descriptor table.
62 */
63
64 #define UMIP_DUMMY_GDT_BASE 0xfffffffffffe0000ULL
65 #define UMIP_DUMMY_IDT_BASE 0xffffffffffff0000ULL
66
67 /*
68 * The SGDT and SIDT instructions store the contents of the global descriptor
69 * table and interrupt table registers, respectively. The destination is a
70 * memory operand of X+2 bytes. X bytes are used to store the base address of
71 * the table and 2 bytes are used to store the limit. In 32-bit processes X
72 * has a value of 4, in 64-bit processes X has a value of 8.
73 */
74 #define UMIP_GDT_IDT_BASE_SIZE_64BIT 8
75 #define UMIP_GDT_IDT_BASE_SIZE_32BIT 4
76 #define UMIP_GDT_IDT_LIMIT_SIZE 2
77
78 #define UMIP_INST_SGDT 0 /* 0F 01 /0 */
79 #define UMIP_INST_SIDT 1 /* 0F 01 /1 */
80 #define UMIP_INST_SMSW 2 /* 0F 01 /4 */
81 #define UMIP_INST_SLDT 3 /* 0F 00 /0 */
82 #define UMIP_INST_STR 4 /* 0F 00 /1 */
83
84 const char * const umip_insns[5] = {
85 [UMIP_INST_SGDT] = "SGDT",
86 [UMIP_INST_SIDT] = "SIDT",
87 [UMIP_INST_SMSW] = "SMSW",
88 [UMIP_INST_SLDT] = "SLDT",
89 [UMIP_INST_STR] = "STR",
90 };
91
92 #define umip_pr_err(regs, fmt, ...) \
93 umip_printk(regs, KERN_ERR, fmt, ##__VA_ARGS__)
94 #define umip_pr_warning(regs, fmt, ...) \
95 umip_printk(regs, KERN_WARNING, fmt, ##__VA_ARGS__)
96
97 /**
98 * umip_printk() - Print a rate-limited message
99 * @regs: Register set with the context in which the warning is printed
100 * @log_level: Kernel log level to print the message
101 * @fmt: The text string to print
102 *
103 * Print the text contained in @fmt. The print rate is limited to bursts of 5
104 * messages every two minutes. The purpose of this customized version of
105 * printk() is to print messages when user space processes use any of the
106 * UMIP-protected instructions. Thus, the printed text is prepended with the
107 * task name and process ID number of the current task as well as the
108 * instruction and stack pointers in @regs as seen when entering kernel mode.
109 *
110 * Returns:
111 *
112 * None.
113 */
114 static __printf(3, 4)
115 void umip_printk(const struct pt_regs *regs, const char *log_level,
116 const char *fmt, ...)
117 {
118 /* Bursts of 5 messages every two minutes */
119 static DEFINE_RATELIMIT_STATE(ratelimit, 2 * 60 * HZ, 5);
120 struct task_struct *tsk = current;
121 struct va_format vaf;
122 va_list args;
123
124 if (!__ratelimit(&ratelimit))
125 return;
126
127 va_start(args, fmt);
128 vaf.fmt = fmt;
129 vaf.va = &args;
130 printk("%s" pr_fmt("%s[%d] ip:%lx sp:%lx: %pV"), log_level, tsk->comm,
131 task_pid_nr(tsk), regs->ip, regs->sp, &vaf);
132 va_end(args);
133 }
134
135 /**
136 * identify_insn() - Identify a UMIP-protected instruction
137 * @insn: Instruction structure with opcode and ModRM byte.
138 *
139 * From the opcode and ModRM.reg in @insn identify, if any, a UMIP-protected
140 * instruction that can be emulated.
141 *
142 * Returns:
143 *
144 * On success, a constant identifying a specific UMIP-protected instruction that
145 * can be emulated.
146 *
147 * -EINVAL on error or when not an UMIP-protected instruction that can be
148 * emulated.
149 */
150 static int identify_insn(struct insn *insn)
151 {
152 /* By getting modrm we also get the opcode. */
153 insn_get_modrm(insn);
154
155 if (!insn->modrm.nbytes)
156 return -EINVAL;
157
158 /* All the instructions of interest start with 0x0f. */
159 if (insn->opcode.bytes[0] != 0xf)
160 return -EINVAL;
161
162 if (insn->opcode.bytes[1] == 0x1) {
163 switch (X86_MODRM_REG(insn->modrm.value)) {
164 case 0:
165 return UMIP_INST_SGDT;
166 case 1:
167 return UMIP_INST_SIDT;
168 case 4:
169 return UMIP_INST_SMSW;
170 default:
171 return -EINVAL;
172 }
173 } else if (insn->opcode.bytes[1] == 0x0) {
174 if (X86_MODRM_REG(insn->modrm.value) == 0)
175 return UMIP_INST_SLDT;
176 else if (X86_MODRM_REG(insn->modrm.value) == 1)
177 return UMIP_INST_STR;
178 else
179 return -EINVAL;
180 } else {
181 return -EINVAL;
182 }
183 }
184
185 /**
186 * emulate_umip_insn() - Emulate UMIP instructions and return dummy values
187 * @insn: Instruction structure with operands
188 * @umip_inst: A constant indicating the instruction to emulate
189 * @data: Buffer into which the dummy result is stored
190 * @data_size: Size of the emulated result
191 * @x86_64: true if process is 64-bit, false otherwise
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, bool x86_64)
206 {
207 if (!data || !data_size || !insn)
208 return -EINVAL;
209 /*
210 * These two instructions return the base address and limit of the
211 * global and interrupt descriptor table, respectively. According to the
212 * Intel Software Development manual, the base address can be 24-bit,
213 * 32-bit or 64-bit. Limit is always 16-bit. If the operand size is
214 * 16-bit, the returned value of the base address is supposed to be a
215 * zero-extended 24-byte number. However, it seems that a 32-byte number
216 * is always returned irrespective of the operand size.
217 */
218 if (umip_inst == UMIP_INST_SGDT || umip_inst == UMIP_INST_SIDT) {
219 u64 dummy_base_addr;
220 u16 dummy_limit = 0;
221
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 /*
232 * 64-bit processes use the entire dummy base address.
233 * 32-bit processes use the lower 32 bits of the base address.
234 * dummy_base_addr is always 64 bits, but we memcpy the correct
235 * number of bytes from it to the destination.
236 */
237 if (x86_64)
238 *data_size = UMIP_GDT_IDT_BASE_SIZE_64BIT;
239 else
240 *data_size = UMIP_GDT_IDT_BASE_SIZE_32BIT;
241
242 memcpy(data + 2, &dummy_base_addr, *data_size);
243
244 *data_size += UMIP_GDT_IDT_LIMIT_SIZE;
245 memcpy(data, &dummy_limit, UMIP_GDT_IDT_LIMIT_SIZE);
246
247 } else if (umip_inst == UMIP_INST_SMSW) {
248 unsigned long dummy_value = CR0_STATE;
249
250 /*
251 * Even though the CR0 register has 4 bytes, the number
252 * of bytes to be copied in the result buffer is determined
253 * by whether the operand is a register or a memory location.
254 * If operand is a register, return as many bytes as the operand
255 * size. If operand is memory, return only the two least
256 * siginificant bytes of CR0.
257 */
258 if (X86_MODRM_MOD(insn->modrm.value) == 3)
259 *data_size = insn->opnd_bytes;
260 else
261 *data_size = 2;
262
263 memcpy(data, &dummy_value, *data_size);
264 /* STR and SLDT are not emulated */
265 } else {
266 return -EINVAL;
267 }
268
269 return 0;
270 }
271
272 /**
273 * force_sig_info_umip_fault() - Force a SIGSEGV with SEGV_MAPERR
274 * @addr: Address that caused the signal
275 * @regs: Register set containing the instruction pointer
276 *
277 * Force a SIGSEGV signal with SEGV_MAPERR as the error code. This function is
278 * intended to be used to provide a segmentation fault when the result of the
279 * UMIP emulation could not be copied to the user space memory.
280 *
281 * Returns: none
282 */
283 static void force_sig_info_umip_fault(void __user *addr, struct pt_regs *regs)
284 {
285 struct task_struct *tsk = current;
286
287 tsk->thread.cr2 = (unsigned long)addr;
288 tsk->thread.error_code = X86_PF_USER | X86_PF_WRITE;
289 tsk->thread.trap_nr = X86_TRAP_PF;
290
291 force_sig_fault(SIGSEGV, SEGV_MAPERR, addr);
292
293 if (!(show_unhandled_signals && unhandled_signal(tsk, SIGSEGV)))
294 return;
295
296 umip_pr_err(regs, "segfault in emulation. error%x\n",
297 X86_PF_USER | X86_PF_WRITE);
298 }
299
300 /**
301 * fixup_umip_exception() - Fixup a general protection fault caused by UMIP
302 * @regs: Registers as saved when entering the #GP handler
303 *
304 * The instructions SGDT, SIDT, STR, SMSW and SLDT cause a general protection
305 * fault if executed with CPL > 0 (i.e., from user space). This function fixes
306 * the exception up and provides dummy results for SGDT, SIDT and SMSW; STR
307 * and SLDT are not fixed up.
308 *
309 * If operands are memory addresses, results are copied to user-space memory as
310 * indicated by the instruction pointed by eIP using the registers indicated in
311 * the instruction operands. If operands are registers, results are copied into
312 * the context that was saved when entering kernel mode.
313 *
314 * Returns:
315 *
316 * True if emulation was successful; false if not.
317 */
318 bool fixup_umip_exception(struct pt_regs *regs)
319 {
320 int not_copied, nr_copied, reg_offset, dummy_data_size, umip_inst;
321 unsigned long seg_base = 0, *reg_addr;
322 /* 10 bytes is the maximum size of the result of UMIP instructions */
323 unsigned char dummy_data[10] = { 0 };
324 unsigned char buf[MAX_INSN_SIZE];
325 void __user *uaddr;
326 struct insn insn;
327 int seg_defs;
328
329 if (!regs)
330 return false;
331
332 /*
333 * If not in user-space long mode, a custom code segment could be in
334 * use. This is true in protected mode (if the process defined a local
335 * descriptor table), or virtual-8086 mode. In most of the cases
336 * seg_base will be zero as in USER_CS.
337 */
338 if (!user_64bit_mode(regs))
339 seg_base = insn_get_seg_base(regs, INAT_SEG_REG_CS);
340
341 if (seg_base == -1L)
342 return false;
343
344 not_copied = copy_from_user(buf, (void __user *)(seg_base + regs->ip),
345 sizeof(buf));
346 nr_copied = sizeof(buf) - not_copied;
347
348 /*
349 * The copy_from_user above could have failed if user code is protected
350 * by a memory protection key. Give up on emulation in such a case.
351 * Should we issue a page fault?
352 */
353 if (!nr_copied)
354 return false;
355
356 insn_init(&insn, buf, nr_copied, user_64bit_mode(regs));
357
358 /*
359 * Override the default operand and address sizes with what is specified
360 * in the code segment descriptor. The instruction decoder only sets
361 * the address size it to either 4 or 8 address bytes and does nothing
362 * for the operand bytes. This OK for most of the cases, but we could
363 * have special cases where, for instance, a 16-bit code segment
364 * descriptor is used.
365 * If there is an address override prefix, the instruction decoder
366 * correctly updates these values, even for 16-bit defaults.
367 */
368 seg_defs = insn_get_code_seg_params(regs);
369 if (seg_defs == -EINVAL)
370 return false;
371
372 insn.addr_bytes = INSN_CODE_SEG_ADDR_SZ(seg_defs);
373 insn.opnd_bytes = INSN_CODE_SEG_OPND_SZ(seg_defs);
374
375 insn_get_length(&insn);
376 if (nr_copied < insn.length)
377 return false;
378
379 umip_inst = identify_insn(&insn);
380 if (umip_inst < 0)
381 return false;
382
383 umip_pr_warning(regs, "%s instruction cannot be used by applications.\n",
384 umip_insns[umip_inst]);
385
386 /* Do not emulate (spoof) SLDT or STR. */
387 if (umip_inst == UMIP_INST_STR || umip_inst == UMIP_INST_SLDT)
388 return false;
389
390 umip_pr_warning(regs, "For now, expensive software emulation returns the result.\n");
391
392 if (emulate_umip_insn(&insn, umip_inst, dummy_data, &dummy_data_size,
393 user_64bit_mode(regs)))
394 return false;
395
396 /*
397 * If operand is a register, write result to the copy of the register
398 * value that was pushed to the stack when entering into kernel mode.
399 * Upon exit, the value we write will be restored to the actual hardware
400 * register.
401 */
402 if (X86_MODRM_MOD(insn.modrm.value) == 3) {
403 reg_offset = insn_get_modrm_rm_off(&insn, regs);
404
405 /*
406 * Negative values are usually errors. In memory addressing,
407 * the exception is -EDOM. Since we expect a register operand,
408 * all negative values are errors.
409 */
410 if (reg_offset < 0)
411 return false;
412
413 reg_addr = (unsigned long *)((unsigned long)regs + reg_offset);
414 memcpy(reg_addr, dummy_data, dummy_data_size);
415 } else {
416 uaddr = insn_get_addr_ref(&insn, regs);
417 if ((unsigned long)uaddr == -1L)
418 return false;
419
420 nr_copied = copy_to_user(uaddr, dummy_data, dummy_data_size);
421 if (nr_copied > 0) {
422 /*
423 * If copy fails, send a signal and tell caller that
424 * fault was fixed up.
425 */
426 force_sig_info_umip_fault(uaddr, regs);
427 return true;
428 }
429 }
430
431 /* increase IP to let the program keep going */
432 regs->ip += insn.length;
433 return true;
434 }