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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
85static 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_debug(regs, fmt, ...) \
96 umip_printk(regs, KERN_DEBUG, 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 * @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 */
205static 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 */
301static 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 */
336bool 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 /*
350 * Give up on emulation if fetching the instruction failed. Should a
351 * page fault or a #GP be issued?
352 */
353 nr_copied = insn_fetch_from_user(regs, buf);
354 if (nr_copied <= 0)
355 return false;
356
357 if (!insn_decode_from_regs(&insn, regs, buf, nr_copied))
358 return false;
359
360 umip_inst = identify_insn(&insn);
361 if (umip_inst < 0)
362 return false;
363
364 umip_pr_debug(regs, "%s instruction cannot be used by applications.\n",
365 umip_insns[umip_inst]);
366
367 umip_pr_debug(regs, "For now, expensive software emulation returns the result.\n");
368
369 if (emulate_umip_insn(&insn, umip_inst, dummy_data, &dummy_data_size,
370 user_64bit_mode(regs)))
371 return false;
372
373 /*
374 * If operand is a register, write result to the copy of the register
375 * value that was pushed to the stack when entering into kernel mode.
376 * Upon exit, the value we write will be restored to the actual hardware
377 * register.
378 */
379 if (X86_MODRM_MOD(insn.modrm.value) == 3) {
380 reg_offset = insn_get_modrm_rm_off(&insn, regs);
381
382 /*
383 * Negative values are usually errors. In memory addressing,
384 * the exception is -EDOM. Since we expect a register operand,
385 * all negative values are errors.
386 */
387 if (reg_offset < 0)
388 return false;
389
390 reg_addr = (unsigned long *)((unsigned long)regs + reg_offset);
391 memcpy(reg_addr, dummy_data, dummy_data_size);
392 } else {
393 uaddr = insn_get_addr_ref(&insn, regs);
394 if ((unsigned long)uaddr == -1L)
395 return false;
396
397 nr_copied = copy_to_user(uaddr, dummy_data, dummy_data_size);
398 if (nr_copied > 0) {
399 /*
400 * If copy fails, send a signal and tell caller that
401 * fault was fixed up.
402 */
403 force_sig_info_umip_fault(uaddr, regs);
404 return true;
405 }
406 }
407
408 /* increase IP to let the program keep going */
409 regs->ip += insn.length;
410 return true;
411}