xref: /linux/arch/x86/mm/extable.c (revision 621cde16e49b3ecf7d59a8106a20aaebfb4a59a9)
1 // SPDX-License-Identifier: GPL-2.0-only
2 #include <linux/extable.h>
3 #include <linux/uaccess.h>
4 #include <linux/sched/debug.h>
5 #include <linux/bitfield.h>
6 #include <xen/xen.h>
7 
8 #include <asm/fpu/api.h>
9 #include <asm/fred.h>
10 #include <asm/sev.h>
11 #include <asm/traps.h>
12 #include <asm/kdebug.h>
13 #include <asm/insn-eval.h>
14 #include <asm/sgx.h>
15 
pt_regs_nr(struct pt_regs * regs,int nr)16 static inline unsigned long *pt_regs_nr(struct pt_regs *regs, int nr)
17 {
18 	int reg_offset = pt_regs_offset(regs, nr);
19 	static unsigned long __dummy;
20 
21 	if (WARN_ON_ONCE(reg_offset < 0))
22 		return &__dummy;
23 
24 	return (unsigned long *)((unsigned long)regs + reg_offset);
25 }
26 
27 static inline unsigned long
ex_fixup_addr(const struct exception_table_entry * x)28 ex_fixup_addr(const struct exception_table_entry *x)
29 {
30 	return (unsigned long)&x->fixup + x->fixup;
31 }
32 
ex_handler_default(const struct exception_table_entry * e,struct pt_regs * regs)33 static bool ex_handler_default(const struct exception_table_entry *e,
34 			       struct pt_regs *regs)
35 {
36 	if (e->data & EX_FLAG_CLEAR_AX)
37 		regs->ax = 0;
38 	if (e->data & EX_FLAG_CLEAR_DX)
39 		regs->dx = 0;
40 
41 	regs->ip = ex_fixup_addr(e);
42 	return true;
43 }
44 
45 /*
46  * This is the *very* rare case where we do a "load_unaligned_zeropad()"
47  * and it's a page crosser into a non-existent page.
48  *
49  * This happens when we optimistically load a pathname a word-at-a-time
50  * and the name is less than the full word and the  next page is not
51  * mapped. Typically that only happens for CONFIG_DEBUG_PAGEALLOC.
52  *
53  * NOTE! The faulting address is always a 'mov mem,reg' type instruction
54  * of size 'long', and the exception fixup must always point to right
55  * after the instruction.
56  */
ex_handler_zeropad(const struct exception_table_entry * e,struct pt_regs * regs,unsigned long fault_addr)57 static bool ex_handler_zeropad(const struct exception_table_entry *e,
58 			       struct pt_regs *regs,
59 			       unsigned long fault_addr)
60 {
61 	struct insn insn;
62 	const unsigned long mask = sizeof(long) - 1;
63 	unsigned long offset, addr, next_ip, len;
64 	unsigned long *reg;
65 
66 	next_ip = ex_fixup_addr(e);
67 	len = next_ip - regs->ip;
68 	if (len > MAX_INSN_SIZE)
69 		return false;
70 
71 	if (insn_decode(&insn, (void *) regs->ip, len, INSN_MODE_KERN))
72 		return false;
73 	if (insn.length != len)
74 		return false;
75 
76 	if (insn.opcode.bytes[0] != 0x8b)
77 		return false;
78 	if (insn.opnd_bytes != sizeof(long))
79 		return false;
80 
81 	addr = (unsigned long) insn_get_addr_ref(&insn, regs);
82 	if (addr == ~0ul)
83 		return false;
84 
85 	offset = addr & mask;
86 	addr = addr & ~mask;
87 	if (fault_addr != addr + sizeof(long))
88 		return false;
89 
90 	reg = insn_get_modrm_reg_ptr(&insn, regs);
91 	if (!reg)
92 		return false;
93 
94 	*reg = *(unsigned long *)addr >> (offset * 8);
95 	return ex_handler_default(e, regs);
96 }
97 
ex_handler_fault(const struct exception_table_entry * fixup,struct pt_regs * regs,int trapnr)98 static bool ex_handler_fault(const struct exception_table_entry *fixup,
99 			     struct pt_regs *regs, int trapnr)
100 {
101 	regs->ax = trapnr;
102 	return ex_handler_default(fixup, regs);
103 }
104 
ex_handler_sgx(const struct exception_table_entry * fixup,struct pt_regs * regs,int trapnr)105 static bool ex_handler_sgx(const struct exception_table_entry *fixup,
106 			   struct pt_regs *regs, int trapnr)
107 {
108 	regs->ax = trapnr | SGX_ENCLS_FAULT_FLAG;
109 	return ex_handler_default(fixup, regs);
110 }
111 
112 /*
113  * Handler for when we fail to restore a task's FPU state.  We should never get
114  * here because the FPU state of a task using the FPU (task->thread.fpu.state)
115  * should always be valid.  However, past bugs have allowed userspace to set
116  * reserved bits in the XSAVE area using PTRACE_SETREGSET or sys_rt_sigreturn().
117  * These caused XRSTOR to fail when switching to the task, leaking the FPU
118  * registers of the task previously executing on the CPU.  Mitigate this class
119  * of vulnerability by restoring from the initial state (essentially, zeroing
120  * out all the FPU registers) if we can't restore from the task's FPU state.
121  */
ex_handler_fprestore(const struct exception_table_entry * fixup,struct pt_regs * regs)122 static bool ex_handler_fprestore(const struct exception_table_entry *fixup,
123 				 struct pt_regs *regs)
124 {
125 	regs->ip = ex_fixup_addr(fixup);
126 
127 	WARN_ONCE(1, "Bad FPU state detected at %pB, reinitializing FPU registers.",
128 		  (void *)instruction_pointer(regs));
129 
130 	fpu_reset_from_exception_fixup();
131 	return true;
132 }
133 
134 /*
135  * On x86-64, we end up being imprecise with 'access_ok()', and allow
136  * non-canonical user addresses to make the range comparisons simpler,
137  * and to not have to worry about LAM being enabled.
138  *
139  * In fact, we allow up to one page of "slop" at the sign boundary,
140  * which means that we can do access_ok() by just checking the sign
141  * of the pointer for the common case of having a small access size.
142  */
gp_fault_address_ok(unsigned long fault_address)143 static bool gp_fault_address_ok(unsigned long fault_address)
144 {
145 #ifdef CONFIG_X86_64
146 	/* Is it in the "user space" part of the non-canonical space? */
147 	if (valid_user_address(fault_address))
148 		return true;
149 
150 	/* .. or just above it? */
151 	fault_address -= PAGE_SIZE;
152 	if (valid_user_address(fault_address))
153 		return true;
154 #endif
155 	return false;
156 }
157 
ex_handler_uaccess(const struct exception_table_entry * fixup,struct pt_regs * regs,int trapnr,unsigned long fault_address)158 static bool ex_handler_uaccess(const struct exception_table_entry *fixup,
159 			       struct pt_regs *regs, int trapnr,
160 			       unsigned long fault_address)
161 {
162 	WARN_ONCE(trapnr == X86_TRAP_GP && !gp_fault_address_ok(fault_address),
163 		"General protection fault in user access. Non-canonical address?");
164 	return ex_handler_default(fixup, regs);
165 }
166 
ex_handler_msr(const struct exception_table_entry * fixup,struct pt_regs * regs,bool wrmsr,bool safe,int reg)167 static bool ex_handler_msr(const struct exception_table_entry *fixup,
168 			   struct pt_regs *regs, bool wrmsr, bool safe, int reg)
169 {
170 	if (__ONCE_LITE_IF(!safe && wrmsr)) {
171 		pr_warn("unchecked MSR access error: WRMSR to 0x%x (tried to write 0x%08x%08x) at rIP: 0x%lx (%pS)\n",
172 			(unsigned int)regs->cx, (unsigned int)regs->dx,
173 			(unsigned int)regs->ax,  regs->ip, (void *)regs->ip);
174 		show_stack_regs(regs);
175 	}
176 
177 	if (__ONCE_LITE_IF(!safe && !wrmsr)) {
178 		pr_warn("unchecked MSR access error: RDMSR from 0x%x at rIP: 0x%lx (%pS)\n",
179 			(unsigned int)regs->cx, regs->ip, (void *)regs->ip);
180 		show_stack_regs(regs);
181 	}
182 
183 	if (!wrmsr) {
184 		/* Pretend that the read succeeded and returned 0. */
185 		regs->ax = 0;
186 		regs->dx = 0;
187 	}
188 
189 	if (safe)
190 		*pt_regs_nr(regs, reg) = -EIO;
191 
192 	return ex_handler_default(fixup, regs);
193 }
194 
ex_handler_clear_fs(const struct exception_table_entry * fixup,struct pt_regs * regs)195 static bool ex_handler_clear_fs(const struct exception_table_entry *fixup,
196 				struct pt_regs *regs)
197 {
198 	if (static_cpu_has(X86_BUG_NULL_SEG))
199 		asm volatile ("mov %0, %%fs" : : "rm" (__USER_DS));
200 	asm volatile ("mov %0, %%fs" : : "rm" (0));
201 	return ex_handler_default(fixup, regs);
202 }
203 
ex_handler_imm_reg(const struct exception_table_entry * fixup,struct pt_regs * regs,int reg,int imm)204 static bool ex_handler_imm_reg(const struct exception_table_entry *fixup,
205 			       struct pt_regs *regs, int reg, int imm)
206 {
207 	*pt_regs_nr(regs, reg) = (long)imm;
208 	return ex_handler_default(fixup, regs);
209 }
210 
ex_handler_ucopy_len(const struct exception_table_entry * fixup,struct pt_regs * regs,int trapnr,unsigned long fault_address,int reg,int imm)211 static bool ex_handler_ucopy_len(const struct exception_table_entry *fixup,
212 				  struct pt_regs *regs, int trapnr,
213 				  unsigned long fault_address,
214 				  int reg, int imm)
215 {
216 	regs->cx = imm * regs->cx + *pt_regs_nr(regs, reg);
217 	return ex_handler_uaccess(fixup, regs, trapnr, fault_address);
218 }
219 
220 #ifdef CONFIG_X86_FRED
ex_handler_eretu(const struct exception_table_entry * fixup,struct pt_regs * regs,unsigned long error_code)221 static bool ex_handler_eretu(const struct exception_table_entry *fixup,
222 			     struct pt_regs *regs, unsigned long error_code)
223 {
224 	struct pt_regs *uregs = (struct pt_regs *)(regs->sp - offsetof(struct pt_regs, orig_ax));
225 	unsigned short ss = uregs->ss;
226 	unsigned short cs = uregs->cs;
227 
228 	/*
229 	 * Move the NMI bit from the invalid stack frame, which caused ERETU
230 	 * to fault, to the fault handler's stack frame, thus to unblock NMI
231 	 * with the fault handler's ERETS instruction ASAP if NMI is blocked.
232 	 */
233 	regs->fred_ss.nmi = uregs->fred_ss.nmi;
234 
235 	/*
236 	 * Sync event information to uregs, i.e., the ERETU return frame, but
237 	 * is it safe to write to the ERETU return frame which is just above
238 	 * current event stack frame?
239 	 *
240 	 * The RSP used by FRED to push a stack frame is not the value in %rsp,
241 	 * it is calculated from %rsp with the following 2 steps:
242 	 * 1) RSP = %rsp - (IA32_FRED_CONFIG & 0x1c0)	// Reserve N*64 bytes
243 	 * 2) RSP = RSP & ~0x3f		// Align to a 64-byte cache line
244 	 * when an event delivery doesn't trigger a stack level change.
245 	 *
246 	 * Here is an example with N*64 (N=1) bytes reserved:
247 	 *
248 	 *  64-byte cache line ==>  ______________
249 	 *                         |___Reserved___|
250 	 *                         |__Event_data__|
251 	 *                         |_____SS_______|
252 	 *                         |_____RSP______|
253 	 *                         |_____FLAGS____|
254 	 *                         |_____CS_______|
255 	 *                         |_____IP_______|
256 	 *  64-byte cache line ==> |__Error_code__| <== ERETU return frame
257 	 *                         |______________|
258 	 *                         |______________|
259 	 *                         |______________|
260 	 *                         |______________|
261 	 *                         |______________|
262 	 *                         |______________|
263 	 *                         |______________|
264 	 *  64-byte cache line ==> |______________| <== RSP after step 1) and 2)
265 	 *                         |___Reserved___|
266 	 *                         |__Event_data__|
267 	 *                         |_____SS_______|
268 	 *                         |_____RSP______|
269 	 *                         |_____FLAGS____|
270 	 *                         |_____CS_______|
271 	 *                         |_____IP_______|
272 	 *  64-byte cache line ==> |__Error_code__| <== ERETS return frame
273 	 *
274 	 * Thus a new FRED stack frame will always be pushed below a previous
275 	 * FRED stack frame ((N*64) bytes may be reserved between), and it is
276 	 * safe to write to a previous FRED stack frame as they never overlap.
277 	 */
278 	fred_info(uregs)->edata = fred_event_data(regs);
279 	uregs->ssx = regs->ssx;
280 	uregs->fred_ss.ss = ss;
281 	/* The NMI bit was moved away above */
282 	uregs->fred_ss.nmi = 0;
283 	uregs->csx = regs->csx;
284 	uregs->fred_cs.sl = 0;
285 	uregs->fred_cs.wfe = 0;
286 	uregs->cs = cs;
287 	uregs->orig_ax = error_code;
288 
289 	return ex_handler_default(fixup, regs);
290 }
291 #endif
292 
ex_get_fixup_type(unsigned long ip)293 int ex_get_fixup_type(unsigned long ip)
294 {
295 	const struct exception_table_entry *e = search_exception_tables(ip);
296 
297 	return e ? FIELD_GET(EX_DATA_TYPE_MASK, e->data) : EX_TYPE_NONE;
298 }
299 
fixup_exception(struct pt_regs * regs,int trapnr,unsigned long error_code,unsigned long fault_addr)300 int fixup_exception(struct pt_regs *regs, int trapnr, unsigned long error_code,
301 		    unsigned long fault_addr)
302 {
303 	const struct exception_table_entry *e;
304 	int type, reg, imm;
305 
306 #ifdef CONFIG_PNPBIOS
307 	if (unlikely(SEGMENT_IS_PNP_CODE(regs->cs))) {
308 		extern u32 pnp_bios_fault_eip, pnp_bios_fault_esp;
309 		extern u32 pnp_bios_is_utter_crap;
310 		pnp_bios_is_utter_crap = 1;
311 		printk(KERN_CRIT "PNPBIOS fault.. attempting recovery.\n");
312 		__asm__ volatile(
313 			"movl %0, %%esp\n\t"
314 			"jmp *%1\n\t"
315 			: : "g" (pnp_bios_fault_esp), "g" (pnp_bios_fault_eip));
316 		panic("do_trap: can't hit this");
317 	}
318 #endif
319 
320 	e = search_exception_tables(regs->ip);
321 	if (!e)
322 		return 0;
323 
324 	type = FIELD_GET(EX_DATA_TYPE_MASK, e->data);
325 	reg  = FIELD_GET(EX_DATA_REG_MASK,  e->data);
326 	imm  = FIELD_GET(EX_DATA_IMM_MASK,  e->data);
327 
328 	switch (type) {
329 	case EX_TYPE_DEFAULT:
330 	case EX_TYPE_DEFAULT_MCE_SAFE:
331 		return ex_handler_default(e, regs);
332 	case EX_TYPE_FAULT:
333 	case EX_TYPE_FAULT_MCE_SAFE:
334 		return ex_handler_fault(e, regs, trapnr);
335 	case EX_TYPE_UACCESS:
336 		return ex_handler_uaccess(e, regs, trapnr, fault_addr);
337 	case EX_TYPE_CLEAR_FS:
338 		return ex_handler_clear_fs(e, regs);
339 	case EX_TYPE_FPU_RESTORE:
340 		return ex_handler_fprestore(e, regs);
341 	case EX_TYPE_BPF:
342 		return ex_handler_bpf(e, regs);
343 	case EX_TYPE_WRMSR:
344 		return ex_handler_msr(e, regs, true, false, reg);
345 	case EX_TYPE_RDMSR:
346 		return ex_handler_msr(e, regs, false, false, reg);
347 	case EX_TYPE_WRMSR_SAFE:
348 		return ex_handler_msr(e, regs, true, true, reg);
349 	case EX_TYPE_RDMSR_SAFE:
350 		return ex_handler_msr(e, regs, false, true, reg);
351 	case EX_TYPE_WRMSR_IN_MCE:
352 		ex_handler_msr_mce(regs, true);
353 		break;
354 	case EX_TYPE_RDMSR_IN_MCE:
355 		ex_handler_msr_mce(regs, false);
356 		break;
357 	case EX_TYPE_POP_REG:
358 		regs->sp += sizeof(long);
359 		fallthrough;
360 	case EX_TYPE_IMM_REG:
361 		return ex_handler_imm_reg(e, regs, reg, imm);
362 	case EX_TYPE_FAULT_SGX:
363 		return ex_handler_sgx(e, regs, trapnr);
364 	case EX_TYPE_UCOPY_LEN:
365 		return ex_handler_ucopy_len(e, regs, trapnr, fault_addr, reg, imm);
366 	case EX_TYPE_ZEROPAD:
367 		return ex_handler_zeropad(e, regs, fault_addr);
368 #ifdef CONFIG_X86_FRED
369 	case EX_TYPE_ERETU:
370 		return ex_handler_eretu(e, regs, error_code);
371 #endif
372 	}
373 	BUG();
374 }
375 
376 extern unsigned int early_recursion_flag;
377 
378 /* Restricted version used during very early boot */
early_fixup_exception(struct pt_regs * regs,int trapnr)379 void __init early_fixup_exception(struct pt_regs *regs, int trapnr)
380 {
381 	/* Ignore early NMIs. */
382 	if (trapnr == X86_TRAP_NMI)
383 		return;
384 
385 	if (early_recursion_flag > 2)
386 		goto halt_loop;
387 
388 	/*
389 	 * Old CPUs leave the high bits of CS on the stack
390 	 * undefined.  I'm not sure which CPUs do this, but at least
391 	 * the 486 DX works this way.
392 	 * Xen pv domains are not using the default __KERNEL_CS.
393 	 */
394 	if (!xen_pv_domain() && regs->cs != __KERNEL_CS)
395 		goto fail;
396 
397 	/*
398 	 * The full exception fixup machinery is available as soon as
399 	 * the early IDT is loaded.  This means that it is the
400 	 * responsibility of extable users to either function correctly
401 	 * when handlers are invoked early or to simply avoid causing
402 	 * exceptions before they're ready to handle them.
403 	 *
404 	 * This is better than filtering which handlers can be used,
405 	 * because refusing to call a handler here is guaranteed to
406 	 * result in a hard-to-debug panic.
407 	 *
408 	 * Keep in mind that not all vectors actually get here.  Early
409 	 * page faults, for example, are special.
410 	 */
411 	if (fixup_exception(regs, trapnr, regs->orig_ax, 0))
412 		return;
413 
414 	if (trapnr == X86_TRAP_UD) {
415 		if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
416 			/* Skip the ud2. */
417 			regs->ip += LEN_UD2;
418 			return;
419 		}
420 
421 		/*
422 		 * If this was a BUG and report_bug returns or if this
423 		 * was just a normal #UD, we want to continue onward and
424 		 * crash.
425 		 */
426 	}
427 
428 fail:
429 	early_printk("PANIC: early exception 0x%02x IP %lx:%lx error %lx cr2 0x%lx\n",
430 		     (unsigned)trapnr, (unsigned long)regs->cs, regs->ip,
431 		     regs->orig_ax, read_cr2());
432 
433 	show_regs(regs);
434 
435 halt_loop:
436 	while (true)
437 		halt();
438 }
439