summaryrefslogtreecommitdiff
path: root/arch/x86/entry/common.c
blob: 51cc9c7cb9bdc0e1181d08e33c05b5cc1f177830 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
// SPDX-License-Identifier: GPL-2.0-only
/*
 * common.c - C code for kernel entry and exit
 * Copyright (c) 2015 Andrew Lutomirski
 *
 * Based on asm and ptrace code by many authors.  The code here originated
 * in ptrace.c and signal.c.
 */

#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/sched/task_stack.h>
#include <linux/entry-common.h>
#include <linux/mm.h>
#include <linux/smp.h>
#include <linux/errno.h>
#include <linux/ptrace.h>
#include <linux/export.h>
#include <linux/nospec.h>
#include <linux/syscalls.h>
#include <linux/uaccess.h>
#include <linux/init.h>

#ifdef CONFIG_XEN_PV
#include <xen/xen-ops.h>
#include <xen/events.h>
#endif

#include <asm/apic.h>
#include <asm/desc.h>
#include <asm/traps.h>
#include <asm/vdso.h>
#include <asm/cpufeature.h>
#include <asm/fpu/api.h>
#include <asm/nospec-branch.h>
#include <asm/io_bitmap.h>
#include <asm/syscall.h>
#include <asm/irq_stack.h>

#ifdef CONFIG_X86_64

static __always_inline bool do_syscall_x64(struct pt_regs *regs, int nr)
{
	/*
	 * Convert negative numbers to very high and thus out of range
	 * numbers for comparisons.
	 */
	unsigned int unr = nr;

	if (likely(unr < NR_syscalls)) {
		unr = array_index_nospec(unr, NR_syscalls);
		regs->ax = x64_sys_call(regs, unr);
		return true;
	}
	return false;
}

static __always_inline bool do_syscall_x32(struct pt_regs *regs, int nr)
{
	/*
	 * Adjust the starting offset of the table, and convert numbers
	 * < __X32_SYSCALL_BIT to very high and thus out of range
	 * numbers for comparisons.
	 */
	unsigned int xnr = nr - __X32_SYSCALL_BIT;

	if (IS_ENABLED(CONFIG_X86_X32_ABI) && likely(xnr < X32_NR_syscalls)) {
		xnr = array_index_nospec(xnr, X32_NR_syscalls);
		regs->ax = x32_sys_call(regs, xnr);
		return true;
	}
	return false;
}

/* Returns true to return using SYSRET, or false to use IRET */
__visible noinstr bool do_syscall_64(struct pt_regs *regs, int nr)
{
	add_random_kstack_offset();
	nr = syscall_enter_from_user_mode(regs, nr);

	instrumentation_begin();

	if (!do_syscall_x64(regs, nr) && !do_syscall_x32(regs, nr) && nr != -1) {
		/* Invalid system call, but still a system call. */
		regs->ax = __x64_sys_ni_syscall(regs);
	}

	instrumentation_end();
	syscall_exit_to_user_mode(regs);

	/*
	 * Check that the register state is valid for using SYSRET to exit
	 * to userspace.  Otherwise use the slower but fully capable IRET
	 * exit path.
	 */

	/* XEN PV guests always use the IRET path */
	if (cpu_feature_enabled(X86_FEATURE_XENPV))
		return false;

	/* SYSRET requires RCX == RIP and R11 == EFLAGS */
	if (unlikely(regs->cx != regs->ip || regs->r11 != regs->flags))
		return false;

	/* CS and SS must match the values set in MSR_STAR */
	if (unlikely(regs->cs != __USER_CS || regs->ss != __USER_DS))
		return false;

	/*
	 * On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
	 * in kernel space.  This essentially lets the user take over
	 * the kernel, since userspace controls RSP.
	 *
	 * TASK_SIZE_MAX covers all user-accessible addresses other than
	 * the deprecated vsyscall page.
	 */
	if (unlikely(regs->ip >= TASK_SIZE_MAX))
		return false;

	/*
	 * SYSRET cannot restore RF.  It can restore TF, but unlike IRET,
	 * restoring TF results in a trap from userspace immediately after
	 * SYSRET.
	 */
	if (unlikely(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF)))
		return false;

	/* Use SYSRET to exit to userspace */
	return true;
}
#endif

#if defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION)
static __always_inline int syscall_32_enter(struct pt_regs *regs)
{
	if (IS_ENABLED(CONFIG_IA32_EMULATION))
		current_thread_info()->status |= TS_COMPAT;

	return (int)regs->orig_ax;
}

#ifdef CONFIG_IA32_EMULATION
bool __ia32_enabled __ro_after_init = !IS_ENABLED(CONFIG_IA32_EMULATION_DEFAULT_DISABLED);

static int ia32_emulation_override_cmdline(char *arg)
{
	return kstrtobool(arg, &__ia32_enabled);
}
early_param("ia32_emulation", ia32_emulation_override_cmdline);
#endif

/*
 * Invoke a 32-bit syscall.  Called with IRQs on in CONTEXT_KERNEL.
 */
static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs, int nr)
{
	/*
	 * Convert negative numbers to very high and thus out of range
	 * numbers for comparisons.
	 */
	unsigned int unr = nr;

	if (likely(unr < IA32_NR_syscalls)) {
		unr = array_index_nospec(unr, IA32_NR_syscalls);
		regs->ax = ia32_sys_call(regs, unr);
	} else if (nr != -1) {
		regs->ax = __ia32_sys_ni_syscall(regs);
	}
}

#ifdef CONFIG_IA32_EMULATION
static __always_inline bool int80_is_external(void)
{
	const unsigned int offs = (0x80 / 32) * 0x10;
	const u32 bit = BIT(0x80 % 32);

	/* The local APIC on XENPV guests is fake */
	if (cpu_feature_enabled(X86_FEATURE_XENPV))
		return false;

	/*
	 * If vector 0x80 is set in the APIC ISR then this is an external
	 * interrupt. Either from broken hardware or injected by a VMM.
	 *
	 * Note: In guest mode this is only valid for secure guests where
	 * the secure module fully controls the vAPIC exposed to the guest.
	 */
	return apic_read(APIC_ISR + offs) & bit;
}

/**
 * do_int80_emulation - 32-bit legacy syscall C entry from asm
 *
 * This entry point can be used by 32-bit and 64-bit programs to perform
 * 32-bit system calls.  Instances of INT $0x80 can be found inline in
 * various programs and libraries.  It is also used by the vDSO's
 * __kernel_vsyscall fallback for hardware that doesn't support a faster
 * entry method.  Restarted 32-bit system calls also fall back to INT
 * $0x80 regardless of what instruction was originally used to do the
 * system call.
 *
 * This is considered a slow path.  It is not used by most libc
 * implementations on modern hardware except during process startup.
 *
 * The arguments for the INT $0x80 based syscall are on stack in the
 * pt_regs structure:
 *   eax:				system call number
 *   ebx, ecx, edx, esi, edi, ebp:	arg1 - arg 6
 */
__visible noinstr void do_int80_emulation(struct pt_regs *regs)
{
	int nr;

	/* Kernel does not use INT $0x80! */
	if (unlikely(!user_mode(regs))) {
		irqentry_enter(regs);
		instrumentation_begin();
		panic("Unexpected external interrupt 0x80\n");
	}

	/*
	 * Establish kernel context for instrumentation, including for
	 * int80_is_external() below which calls into the APIC driver.
	 * Identical for soft and external interrupts.
	 */
	enter_from_user_mode(regs);

	instrumentation_begin();
	add_random_kstack_offset();

	/* Validate that this is a soft interrupt to the extent possible */
	if (unlikely(int80_is_external()))
		panic("Unexpected external interrupt 0x80\n");

	/*
	 * The low level idtentry code pushed -1 into regs::orig_ax
	 * and regs::ax contains the syscall number.
	 *
	 * User tracing code (ptrace or signal handlers) might assume
	 * that the regs::orig_ax contains a 32-bit number on invoking
	 * a 32-bit syscall.
	 *
	 * Establish the syscall convention by saving the 32bit truncated
	 * syscall number in regs::orig_ax and by invalidating regs::ax.
	 */
	regs->orig_ax = regs->ax & GENMASK(31, 0);
	regs->ax = -ENOSYS;

	nr = syscall_32_enter(regs);

	local_irq_enable();
	nr = syscall_enter_from_user_mode_work(regs, nr);
	do_syscall_32_irqs_on(regs, nr);

	instrumentation_end();
	syscall_exit_to_user_mode(regs);
}

#ifdef CONFIG_X86_FRED
/*
 * A FRED-specific INT80 handler is warranted for the follwing reasons:
 *
 * 1) As INT instructions and hardware interrupts are separate event
 *    types, FRED does not preclude the use of vector 0x80 for external
 *    interrupts. As a result, the FRED setup code does not reserve
 *    vector 0x80 and calling int80_is_external() is not merely
 *    suboptimal but actively incorrect: it could cause a system call
 *    to be incorrectly ignored.
 *
 * 2) It is called only for handling vector 0x80 of event type
 *    EVENT_TYPE_SWINT and will never be called to handle any external
 *    interrupt (event type EVENT_TYPE_EXTINT).
 *
 * 3) FRED has separate entry flows depending on if the event came from
 *    user space or kernel space, and because the kernel does not use
 *    INT insns, the FRED kernel entry handler fred_entry_from_kernel()
 *    falls through to fred_bad_type() if the event type is
 *    EVENT_TYPE_SWINT, i.e., INT insns. So if the kernel is handling
 *    an INT insn, it can only be from a user level.
 *
 * 4) int80_emulation() does a CLEAR_BRANCH_HISTORY. While FRED will
 *    likely take a different approach if it is ever needed: it
 *    probably belongs in either fred_intx()/ fred_other() or
 *    asm_fred_entrypoint_user(), depending on if this ought to be done
 *    for all entries from userspace or only system
 *    calls.
 *
 * 5) INT $0x80 is the fast path for 32-bit system calls under FRED.
 */
DEFINE_FREDENTRY_RAW(int80_emulation)
{
	int nr;

	enter_from_user_mode(regs);

	instrumentation_begin();
	add_random_kstack_offset();

	/*
	 * FRED pushed 0 into regs::orig_ax and regs::ax contains the
	 * syscall number.
	 *
	 * User tracing code (ptrace or signal handlers) might assume
	 * that the regs::orig_ax contains a 32-bit number on invoking
	 * a 32-bit syscall.
	 *
	 * Establish the syscall convention by saving the 32bit truncated
	 * syscall number in regs::orig_ax and by invalidating regs::ax.
	 */
	regs->orig_ax = regs->ax & GENMASK(31, 0);
	regs->ax = -ENOSYS;

	nr = syscall_32_enter(regs);

	local_irq_enable();
	nr = syscall_enter_from_user_mode_work(regs, nr);
	do_syscall_32_irqs_on(regs, nr);

	instrumentation_end();
	syscall_exit_to_user_mode(regs);
}
#endif
#else /* CONFIG_IA32_EMULATION */

/* Handles int $0x80 on a 32bit kernel */
__visible noinstr void do_int80_syscall_32(struct pt_regs *regs)
{
	int nr = syscall_32_enter(regs);

	add_random_kstack_offset();
	/*
	 * Subtlety here: if ptrace pokes something larger than 2^31-1 into
	 * orig_ax, the int return value truncates it. This matches
	 * the semantics of syscall_get_nr().
	 */
	nr = syscall_enter_from_user_mode(regs, nr);
	instrumentation_begin();

	do_syscall_32_irqs_on(regs, nr);

	instrumentation_end();
	syscall_exit_to_user_mode(regs);
}
#endif /* !CONFIG_IA32_EMULATION */

static noinstr bool __do_fast_syscall_32(struct pt_regs *regs)
{
	int nr = syscall_32_enter(regs);
	int res;

	add_random_kstack_offset();
	/*
	 * This cannot use syscall_enter_from_user_mode() as it has to
	 * fetch EBP before invoking any of the syscall entry work
	 * functions.
	 */
	syscall_enter_from_user_mode_prepare(regs);

	instrumentation_begin();
	/* Fetch EBP from where the vDSO stashed it. */
	if (IS_ENABLED(CONFIG_X86_64)) {
		/*
		 * Micro-optimization: the pointer we're following is
		 * explicitly 32 bits, so it can't be out of range.
		 */
		res = __get_user(*(u32 *)&regs->bp,
			 (u32 __user __force *)(unsigned long)(u32)regs->sp);
	} else {
		res = get_user(*(u32 *)&regs->bp,
		       (u32 __user __force *)(unsigned long)(u32)regs->sp);
	}

	if (res) {
		/* User code screwed up. */
		regs->ax = -EFAULT;

		local_irq_disable();
		instrumentation_end();
		irqentry_exit_to_user_mode(regs);
		return false;
	}

	nr = syscall_enter_from_user_mode_work(regs, nr);

	/* Now this is just like a normal syscall. */
	do_syscall_32_irqs_on(regs, nr);

	instrumentation_end();
	syscall_exit_to_user_mode(regs);
	return true;
}

/* Returns true to return using SYSEXIT/SYSRETL, or false to use IRET */
__visible noinstr bool do_fast_syscall_32(struct pt_regs *regs)
{
	/*
	 * Called using the internal vDSO SYSENTER/SYSCALL32 calling
	 * convention.  Adjust regs so it looks like we entered using int80.
	 */
	unsigned long landing_pad = (unsigned long)current->mm->context.vdso +
					vdso_image_32.sym_int80_landing_pad;

	/*
	 * SYSENTER loses EIP, and even SYSCALL32 needs us to skip forward
	 * so that 'regs->ip -= 2' lands back on an int $0x80 instruction.
	 * Fix it up.
	 */
	regs->ip = landing_pad;

	/* Invoke the syscall. If it failed, keep it simple: use IRET. */
	if (!__do_fast_syscall_32(regs))
		return false;

	/*
	 * Check that the register state is valid for using SYSRETL/SYSEXIT
	 * to exit to userspace.  Otherwise use the slower but fully capable
	 * IRET exit path.
	 */

	/* XEN PV guests always use the IRET path */
	if (cpu_feature_enabled(X86_FEATURE_XENPV))
		return false;

	/* EIP must point to the VDSO landing pad */
	if (unlikely(regs->ip != landing_pad))
		return false;

	/* CS and SS must match the values set in MSR_STAR */
	if (unlikely(regs->cs != __USER32_CS || regs->ss != __USER_DS))
		return false;

	/* If the TF, RF, or VM flags are set, use IRET */
	if (unlikely(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF | X86_EFLAGS_VM)))
		return false;

	/* Use SYSRETL/SYSEXIT to exit to userspace */
	return true;
}

/* Returns true to return using SYSEXIT/SYSRETL, or false to use IRET */
__visible noinstr bool do_SYSENTER_32(struct pt_regs *regs)
{
	/* SYSENTER loses RSP, but the vDSO saved it in RBP. */
	regs->sp = regs->bp;

	/* SYSENTER clobbers EFLAGS.IF.  Assume it was set in usermode. */
	regs->flags |= X86_EFLAGS_IF;

	return do_fast_syscall_32(regs);
}
#endif

SYSCALL_DEFINE0(ni_syscall)
{
	return -ENOSYS;
}

#ifdef CONFIG_XEN_PV
#ifndef CONFIG_PREEMPTION
/*
 * Some hypercalls issued by the toolstack can take many 10s of
 * seconds. Allow tasks running hypercalls via the privcmd driver to
 * be voluntarily preempted even if full kernel preemption is
 * disabled.
 *
 * Such preemptible hypercalls are bracketed by
 * xen_preemptible_hcall_begin() and xen_preemptible_hcall_end()
 * calls.
 */
DEFINE_PER_CPU(bool, xen_in_preemptible_hcall);
EXPORT_SYMBOL_GPL(xen_in_preemptible_hcall);

/*
 * In case of scheduling the flag must be cleared and restored after
 * returning from schedule as the task might move to a different CPU.
 */
static __always_inline bool get_and_clear_inhcall(void)
{
	bool inhcall = __this_cpu_read(xen_in_preemptible_hcall);

	__this_cpu_write(xen_in_preemptible_hcall, false);
	return inhcall;
}

static __always_inline void restore_inhcall(bool inhcall)
{
	__this_cpu_write(xen_in_preemptible_hcall, inhcall);
}
#else
static __always_inline bool get_and_clear_inhcall(void) { return false; }
static __always_inline void restore_inhcall(bool inhcall) { }
#endif

static void __xen_pv_evtchn_do_upcall(struct pt_regs *regs)
{
	struct pt_regs *old_regs = set_irq_regs(regs);

	inc_irq_stat(irq_hv_callback_count);

	xen_evtchn_do_upcall();

	set_irq_regs(old_regs);
}

__visible noinstr void xen_pv_evtchn_do_upcall(struct pt_regs *regs)
{
	irqentry_state_t state = irqentry_enter(regs);
	bool inhcall;

	instrumentation_begin();
	run_sysvec_on_irqstack_cond(__xen_pv_evtchn_do_upcall, regs);

	inhcall = get_and_clear_inhcall();
	if (inhcall && !WARN_ON_ONCE(state.exit_rcu)) {
		irqentry_exit_cond_resched();
		instrumentation_end();
		restore_inhcall(inhcall);
	} else {
		instrumentation_end();
		irqentry_exit(regs, state);
	}
}
#endif /* CONFIG_XEN_PV */