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
85const 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 */
115static __printf(3, 4)
116void 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 */
151static 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 */
204static 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 */
272static void force_sig_info_umip_fault(void __user *addr, struct pt_regs *regs)
273{
274 struct task_struct *tsk = current;
275
276 tsk->thread.cr2 = (unsigned long)addr;
277 tsk->thread.error_code = X86_PF_USER | X86_PF_WRITE;
278 tsk->thread.trap_nr = X86_TRAP_PF;
279
280 force_sig_fault(SIGSEGV, SEGV_MAPERR, addr, tsk);
281
282 if (!(show_unhandled_signals && unhandled_signal(tsk, SIGSEGV)))
283 return;
284
285 umip_pr_err(regs, "segfault in emulation. error%x\n",
286 X86_PF_USER | X86_PF_WRITE);
287}
288
289/**
290 * fixup_umip_exception() - Fixup a general protection fault caused by UMIP
291 * @regs: Registers as saved when entering the #GP handler
292 *
293 * The instructions sgdt, sidt, str, smsw, sldt cause a general protection
294 * fault if executed with CPL > 0 (i.e., from user space). If the offending
295 * user-space process is not in long mode, this function fixes the exception
296 * up and provides dummy results for sgdt, sidt and smsw; str and sldt are not
297 * fixed up. Also long mode user-space processes are not fixed up.
298 *
299 * If operands are memory addresses, results are copied to user-space memory as
300 * indicated by the instruction pointed by eIP using the registers indicated in
301 * the instruction operands. If operands are registers, results are copied into
302 * the context that was saved when entering kernel mode.
303 *
304 * Returns:
305 *
306 * True if emulation was successful; false if not.
307 */
308bool fixup_umip_exception(struct pt_regs *regs)
309{
310 int not_copied, nr_copied, reg_offset, dummy_data_size, umip_inst;
311 unsigned long seg_base = 0, *reg_addr;
312 /* 10 bytes is the maximum size of the result of UMIP instructions */
313 unsigned char dummy_data[10] = { 0 };
314 unsigned char buf[MAX_INSN_SIZE];
315 void __user *uaddr;
316 struct insn insn;
317 int seg_defs;
318
319 if (!regs)
320 return false;
321
322 /*
323 * If not in user-space long mode, a custom code segment could be in
324 * use. This is true in protected mode (if the process defined a local
325 * descriptor table), or virtual-8086 mode. In most of the cases
326 * seg_base will be zero as in USER_CS.
327 */
328 if (!user_64bit_mode(regs))
329 seg_base = insn_get_seg_base(regs, INAT_SEG_REG_CS);
330
331 if (seg_base == -1L)
332 return false;
333
334 not_copied = copy_from_user(buf, (void __user *)(seg_base + regs->ip),
335 sizeof(buf));
336 nr_copied = sizeof(buf) - not_copied;
337
338 /*
339 * The copy_from_user above could have failed if user code is protected
340 * by a memory protection key. Give up on emulation in such a case.
341 * Should we issue a page fault?
342 */
343 if (!nr_copied)
344 return false;
345
346 insn_init(&insn, buf, nr_copied, user_64bit_mode(regs));
347
348 /*
349 * Override the default operand and address sizes with what is specified
350 * in the code segment descriptor. The instruction decoder only sets
351 * the address size it to either 4 or 8 address bytes and does nothing
352 * for the operand bytes. This OK for most of the cases, but we could
353 * have special cases where, for instance, a 16-bit code segment
354 * descriptor is used.
355 * If there is an address override prefix, the instruction decoder
356 * correctly updates these values, even for 16-bit defaults.
357 */
358 seg_defs = insn_get_code_seg_params(regs);
359 if (seg_defs == -EINVAL)
360 return false;
361
362 insn.addr_bytes = INSN_CODE_SEG_ADDR_SZ(seg_defs);
363 insn.opnd_bytes = INSN_CODE_SEG_OPND_SZ(seg_defs);
364
365 insn_get_length(&insn);
366 if (nr_copied < insn.length)
367 return false;
368
369 umip_inst = identify_insn(&insn);
370 if (umip_inst < 0)
371 return false;
372
373 umip_pr_warning(regs, "%s instruction cannot be used by applications.\n",
374 umip_insns[umip_inst]);
375
376 /* Do not emulate SLDT, STR or user long mode processes. */
377 if (umip_inst == UMIP_INST_STR || umip_inst == UMIP_INST_SLDT || user_64bit_mode(regs))
378 return false;
379
380 umip_pr_warning(regs, "For now, expensive software emulation returns the result.\n");
381
382 if (emulate_umip_insn(&insn, umip_inst, dummy_data, &dummy_data_size))
383 return false;
384
385 /*
386 * If operand is a register, write result to the copy of the register
387 * value that was pushed to the stack when entering into kernel mode.
388 * Upon exit, the value we write will be restored to the actual hardware
389 * register.
390 */
391 if (X86_MODRM_MOD(insn.modrm.value) == 3) {
392 reg_offset = insn_get_modrm_rm_off(&insn, regs);
393
394 /*
395 * Negative values are usually errors. In memory addressing,
396 * the exception is -EDOM. Since we expect a register operand,
397 * all negative values are errors.
398 */
399 if (reg_offset < 0)
400 return false;
401
402 reg_addr = (unsigned long *)((unsigned long)regs + reg_offset);
403 memcpy(reg_addr, dummy_data, dummy_data_size);
404 } else {
405 uaddr = insn_get_addr_ref(&insn, regs);
406 if ((unsigned long)uaddr == -1L)
407 return false;
408
409 nr_copied = copy_to_user(uaddr, dummy_data, dummy_data_size);
410 if (nr_copied > 0) {
411 /*
412 * If copy fails, send a signal and tell caller that
413 * fault was fixed up.
414 */
415 force_sig_info_umip_fault(uaddr, regs);
416 return true;
417 }
418 }
419
420 /* increase IP to let the program keep going */
421 regs->ip += insn.length;
422 return true;
423}
424