xref: /openbmc/linux/arch/x86/mm/extable.c (revision 798dec33)
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/sev.h>
10 #include <asm/traps.h>
11 #include <asm/kdebug.h>
12 #include <asm/insn-eval.h>
13 #include <asm/sgx.h>
14 
pt_regs_nr(struct pt_regs * regs,int nr)15 static inline unsigned long *pt_regs_nr(struct pt_regs *regs, int nr)
16 {
17 	int reg_offset = pt_regs_offset(regs, nr);
18 	static unsigned long __dummy;
19 
20 	if (WARN_ON_ONCE(reg_offset < 0))
21 		return &__dummy;
22 
23 	return (unsigned long *)((unsigned long)regs + reg_offset);
24 }
25 
26 static inline unsigned long
ex_fixup_addr(const struct exception_table_entry * x)27 ex_fixup_addr(const struct exception_table_entry *x)
28 {
29 	return (unsigned long)&x->fixup + x->fixup;
30 }
31 
ex_handler_default(const struct exception_table_entry * e,struct pt_regs * regs)32 static bool ex_handler_default(const struct exception_table_entry *e,
33 			       struct pt_regs *regs)
34 {
35 	if (e->data & EX_FLAG_CLEAR_AX)
36 		regs->ax = 0;
37 	if (e->data & EX_FLAG_CLEAR_DX)
38 		regs->dx = 0;
39 
40 	regs->ip = ex_fixup_addr(e);
41 	return true;
42 }
43 
44 /*
45  * This is the *very* rare case where we do a "load_unaligned_zeropad()"
46  * and it's a page crosser into a non-existent page.
47  *
48  * This happens when we optimistically load a pathname a word-at-a-time
49  * and the name is less than the full word and the  next page is not
50  * mapped. Typically that only happens for CONFIG_DEBUG_PAGEALLOC.
51  *
52  * NOTE! The faulting address is always a 'mov mem,reg' type instruction
53  * of size 'long', and the exception fixup must always point to right
54  * after the instruction.
55  */
ex_handler_zeropad(const struct exception_table_entry * e,struct pt_regs * regs,unsigned long fault_addr)56 static bool ex_handler_zeropad(const struct exception_table_entry *e,
57 			       struct pt_regs *regs,
58 			       unsigned long fault_addr)
59 {
60 	struct insn insn;
61 	const unsigned long mask = sizeof(long) - 1;
62 	unsigned long offset, addr, next_ip, len;
63 	unsigned long *reg;
64 
65 	next_ip = ex_fixup_addr(e);
66 	len = next_ip - regs->ip;
67 	if (len > MAX_INSN_SIZE)
68 		return false;
69 
70 	if (insn_decode(&insn, (void *) regs->ip, len, INSN_MODE_KERN))
71 		return false;
72 	if (insn.length != len)
73 		return false;
74 
75 	if (insn.opcode.bytes[0] != 0x8b)
76 		return false;
77 	if (insn.opnd_bytes != sizeof(long))
78 		return false;
79 
80 	addr = (unsigned long) insn_get_addr_ref(&insn, regs);
81 	if (addr == ~0ul)
82 		return false;
83 
84 	offset = addr & mask;
85 	addr = addr & ~mask;
86 	if (fault_addr != addr + sizeof(long))
87 		return false;
88 
89 	reg = insn_get_modrm_reg_ptr(&insn, regs);
90 	if (!reg)
91 		return false;
92 
93 	*reg = *(unsigned long *)addr >> (offset * 8);
94 	return ex_handler_default(e, regs);
95 }
96 
ex_handler_fault(const struct exception_table_entry * fixup,struct pt_regs * regs,int trapnr)97 static bool ex_handler_fault(const struct exception_table_entry *fixup,
98 			     struct pt_regs *regs, int trapnr)
99 {
100 	regs->ax = trapnr;
101 	return ex_handler_default(fixup, regs);
102 }
103 
ex_handler_sgx(const struct exception_table_entry * fixup,struct pt_regs * regs,int trapnr)104 static bool ex_handler_sgx(const struct exception_table_entry *fixup,
105 			   struct pt_regs *regs, int trapnr)
106 {
107 	regs->ax = trapnr | SGX_ENCLS_FAULT_FLAG;
108 	return ex_handler_default(fixup, regs);
109 }
110 
111 /*
112  * Handler for when we fail to restore a task's FPU state.  We should never get
113  * here because the FPU state of a task using the FPU (task->thread.fpu.state)
114  * should always be valid.  However, past bugs have allowed userspace to set
115  * reserved bits in the XSAVE area using PTRACE_SETREGSET or sys_rt_sigreturn().
116  * These caused XRSTOR to fail when switching to the task, leaking the FPU
117  * registers of the task previously executing on the CPU.  Mitigate this class
118  * of vulnerability by restoring from the initial state (essentially, zeroing
119  * out all the FPU registers) if we can't restore from the task's FPU state.
120  */
ex_handler_fprestore(const struct exception_table_entry * fixup,struct pt_regs * regs)121 static bool ex_handler_fprestore(const struct exception_table_entry *fixup,
122 				 struct pt_regs *regs)
123 {
124 	regs->ip = ex_fixup_addr(fixup);
125 
126 	WARN_ONCE(1, "Bad FPU state detected at %pB, reinitializing FPU registers.",
127 		  (void *)instruction_pointer(regs));
128 
129 	fpu_reset_from_exception_fixup();
130 	return true;
131 }
132 
133 /*
134  * On x86-64, we end up being imprecise with 'access_ok()', and allow
135  * non-canonical user addresses to make the range comparisons simpler,
136  * and to not have to worry about LAM being enabled.
137  *
138  * In fact, we allow up to one page of "slop" at the sign boundary,
139  * which means that we can do access_ok() by just checking the sign
140  * of the pointer for the common case of having a small access size.
141  */
gp_fault_address_ok(unsigned long fault_address)142 static bool gp_fault_address_ok(unsigned long fault_address)
143 {
144 #ifdef CONFIG_X86_64
145 	/* Is it in the "user space" part of the non-canonical space? */
146 	if (valid_user_address(fault_address))
147 		return true;
148 
149 	/* .. or just above it? */
150 	fault_address -= PAGE_SIZE;
151 	if (valid_user_address(fault_address))
152 		return true;
153 #endif
154 	return false;
155 }
156 
ex_handler_uaccess(const struct exception_table_entry * fixup,struct pt_regs * regs,int trapnr,unsigned long fault_address)157 static bool ex_handler_uaccess(const struct exception_table_entry *fixup,
158 			       struct pt_regs *regs, int trapnr,
159 			       unsigned long fault_address)
160 {
161 	WARN_ONCE(trapnr == X86_TRAP_GP && !gp_fault_address_ok(fault_address),
162 		"General protection fault in user access. Non-canonical address?");
163 	return ex_handler_default(fixup, regs);
164 }
165 
ex_handler_copy(const struct exception_table_entry * fixup,struct pt_regs * regs,int trapnr)166 static bool ex_handler_copy(const struct exception_table_entry *fixup,
167 			    struct pt_regs *regs, int trapnr)
168 {
169 	WARN_ONCE(trapnr == X86_TRAP_GP, "General protection fault in user access. Non-canonical address?");
170 	return ex_handler_fault(fixup, regs, trapnr);
171 }
172 
ex_handler_msr(const struct exception_table_entry * fixup,struct pt_regs * regs,bool wrmsr,bool safe,int reg)173 static bool ex_handler_msr(const struct exception_table_entry *fixup,
174 			   struct pt_regs *regs, bool wrmsr, bool safe, int reg)
175 {
176 	if (__ONCE_LITE_IF(!safe && wrmsr)) {
177 		pr_warn("unchecked MSR access error: WRMSR to 0x%x (tried to write 0x%08x%08x) at rIP: 0x%lx (%pS)\n",
178 			(unsigned int)regs->cx, (unsigned int)regs->dx,
179 			(unsigned int)regs->ax,  regs->ip, (void *)regs->ip);
180 		show_stack_regs(regs);
181 	}
182 
183 	if (__ONCE_LITE_IF(!safe && !wrmsr)) {
184 		pr_warn("unchecked MSR access error: RDMSR from 0x%x at rIP: 0x%lx (%pS)\n",
185 			(unsigned int)regs->cx, regs->ip, (void *)regs->ip);
186 		show_stack_regs(regs);
187 	}
188 
189 	if (!wrmsr) {
190 		/* Pretend that the read succeeded and returned 0. */
191 		regs->ax = 0;
192 		regs->dx = 0;
193 	}
194 
195 	if (safe)
196 		*pt_regs_nr(regs, reg) = -EIO;
197 
198 	return ex_handler_default(fixup, regs);
199 }
200 
ex_handler_clear_fs(const struct exception_table_entry * fixup,struct pt_regs * regs)201 static bool ex_handler_clear_fs(const struct exception_table_entry *fixup,
202 				struct pt_regs *regs)
203 {
204 	if (static_cpu_has(X86_BUG_NULL_SEG))
205 		asm volatile ("mov %0, %%fs" : : "rm" (__USER_DS));
206 	asm volatile ("mov %0, %%fs" : : "rm" (0));
207 	return ex_handler_default(fixup, regs);
208 }
209 
ex_handler_imm_reg(const struct exception_table_entry * fixup,struct pt_regs * regs,int reg,int imm)210 static bool ex_handler_imm_reg(const struct exception_table_entry *fixup,
211 			       struct pt_regs *regs, int reg, int imm)
212 {
213 	*pt_regs_nr(regs, reg) = (long)imm;
214 	return ex_handler_default(fixup, regs);
215 }
216 
ex_handler_ucopy_len(const struct exception_table_entry * fixup,struct pt_regs * regs,int trapnr,unsigned long fault_address,int reg,int imm)217 static bool ex_handler_ucopy_len(const struct exception_table_entry *fixup,
218 				  struct pt_regs *regs, int trapnr,
219 				  unsigned long fault_address,
220 				  int reg, int imm)
221 {
222 	regs->cx = imm * regs->cx + *pt_regs_nr(regs, reg);
223 	return ex_handler_uaccess(fixup, regs, trapnr, fault_address);
224 }
225 
ex_get_fixup_type(unsigned long ip)226 int ex_get_fixup_type(unsigned long ip)
227 {
228 	const struct exception_table_entry *e = search_exception_tables(ip);
229 
230 	return e ? FIELD_GET(EX_DATA_TYPE_MASK, e->data) : EX_TYPE_NONE;
231 }
232 
fixup_exception(struct pt_regs * regs,int trapnr,unsigned long error_code,unsigned long fault_addr)233 int fixup_exception(struct pt_regs *regs, int trapnr, unsigned long error_code,
234 		    unsigned long fault_addr)
235 {
236 	const struct exception_table_entry *e;
237 	int type, reg, imm;
238 
239 #ifdef CONFIG_PNPBIOS
240 	if (unlikely(SEGMENT_IS_PNP_CODE(regs->cs))) {
241 		extern u32 pnp_bios_fault_eip, pnp_bios_fault_esp;
242 		extern u32 pnp_bios_is_utter_crap;
243 		pnp_bios_is_utter_crap = 1;
244 		printk(KERN_CRIT "PNPBIOS fault.. attempting recovery.\n");
245 		__asm__ volatile(
246 			"movl %0, %%esp\n\t"
247 			"jmp *%1\n\t"
248 			: : "g" (pnp_bios_fault_esp), "g" (pnp_bios_fault_eip));
249 		panic("do_trap: can't hit this");
250 	}
251 #endif
252 
253 	e = search_exception_tables(regs->ip);
254 	if (!e)
255 		return 0;
256 
257 	type = FIELD_GET(EX_DATA_TYPE_MASK, e->data);
258 	reg  = FIELD_GET(EX_DATA_REG_MASK,  e->data);
259 	imm  = FIELD_GET(EX_DATA_IMM_MASK,  e->data);
260 
261 	switch (type) {
262 	case EX_TYPE_DEFAULT:
263 	case EX_TYPE_DEFAULT_MCE_SAFE:
264 		return ex_handler_default(e, regs);
265 	case EX_TYPE_FAULT:
266 	case EX_TYPE_FAULT_MCE_SAFE:
267 		return ex_handler_fault(e, regs, trapnr);
268 	case EX_TYPE_UACCESS:
269 		return ex_handler_uaccess(e, regs, trapnr, fault_addr);
270 	case EX_TYPE_COPY:
271 		return ex_handler_copy(e, regs, trapnr);
272 	case EX_TYPE_CLEAR_FS:
273 		return ex_handler_clear_fs(e, regs);
274 	case EX_TYPE_FPU_RESTORE:
275 		return ex_handler_fprestore(e, regs);
276 	case EX_TYPE_BPF:
277 		return ex_handler_bpf(e, regs);
278 	case EX_TYPE_WRMSR:
279 		return ex_handler_msr(e, regs, true, false, reg);
280 	case EX_TYPE_RDMSR:
281 		return ex_handler_msr(e, regs, false, false, reg);
282 	case EX_TYPE_WRMSR_SAFE:
283 		return ex_handler_msr(e, regs, true, true, reg);
284 	case EX_TYPE_RDMSR_SAFE:
285 		return ex_handler_msr(e, regs, false, true, reg);
286 	case EX_TYPE_WRMSR_IN_MCE:
287 		ex_handler_msr_mce(regs, true);
288 		break;
289 	case EX_TYPE_RDMSR_IN_MCE:
290 		ex_handler_msr_mce(regs, false);
291 		break;
292 	case EX_TYPE_POP_REG:
293 		regs->sp += sizeof(long);
294 		fallthrough;
295 	case EX_TYPE_IMM_REG:
296 		return ex_handler_imm_reg(e, regs, reg, imm);
297 	case EX_TYPE_FAULT_SGX:
298 		return ex_handler_sgx(e, regs, trapnr);
299 	case EX_TYPE_UCOPY_LEN:
300 		return ex_handler_ucopy_len(e, regs, trapnr, fault_addr, reg, imm);
301 	case EX_TYPE_ZEROPAD:
302 		return ex_handler_zeropad(e, regs, fault_addr);
303 	}
304 	BUG();
305 }
306 
307 extern unsigned int early_recursion_flag;
308 
309 /* Restricted version used during very early boot */
early_fixup_exception(struct pt_regs * regs,int trapnr)310 void __init early_fixup_exception(struct pt_regs *regs, int trapnr)
311 {
312 	/* Ignore early NMIs. */
313 	if (trapnr == X86_TRAP_NMI)
314 		return;
315 
316 	if (early_recursion_flag > 2)
317 		goto halt_loop;
318 
319 	/*
320 	 * Old CPUs leave the high bits of CS on the stack
321 	 * undefined.  I'm not sure which CPUs do this, but at least
322 	 * the 486 DX works this way.
323 	 * Xen pv domains are not using the default __KERNEL_CS.
324 	 */
325 	if (!xen_pv_domain() && regs->cs != __KERNEL_CS)
326 		goto fail;
327 
328 	/*
329 	 * The full exception fixup machinery is available as soon as
330 	 * the early IDT is loaded.  This means that it is the
331 	 * responsibility of extable users to either function correctly
332 	 * when handlers are invoked early or to simply avoid causing
333 	 * exceptions before they're ready to handle them.
334 	 *
335 	 * This is better than filtering which handlers can be used,
336 	 * because refusing to call a handler here is guaranteed to
337 	 * result in a hard-to-debug panic.
338 	 *
339 	 * Keep in mind that not all vectors actually get here.  Early
340 	 * page faults, for example, are special.
341 	 */
342 	if (fixup_exception(regs, trapnr, regs->orig_ax, 0))
343 		return;
344 
345 	if (trapnr == X86_TRAP_UD) {
346 		if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
347 			/* Skip the ud2. */
348 			regs->ip += LEN_UD2;
349 			return;
350 		}
351 
352 		/*
353 		 * If this was a BUG and report_bug returns or if this
354 		 * was just a normal #UD, we want to continue onward and
355 		 * crash.
356 		 */
357 	}
358 
359 fail:
360 	early_printk("PANIC: early exception 0x%02x IP %lx:%lx error %lx cr2 0x%lx\n",
361 		     (unsigned)trapnr, (unsigned long)regs->cs, regs->ip,
362 		     regs->orig_ax, read_cr2());
363 
364 	show_regs(regs);
365 
366 halt_loop:
367 	while (true)
368 		halt();
369 }
370