summaryrefslogtreecommitdiffstats
path: root/arch/x86/kernel/doublefault_32.c
blob: 759d392cbe9f0ea562a202991f344e06846d52e5 (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
// SPDX-License-Identifier: GPL-2.0
#include <linux/mm.h>
#include <linux/sched.h>
#include <linux/sched/debug.h>
#include <linux/init_task.h>
#include <linux/fs.h>

#include <linux/uaccess.h>
#include <asm/processor.h>
#include <asm/desc.h>
#include <asm/traps.h>

#define ptr_ok(x) ((x) > PAGE_OFFSET && (x) < PAGE_OFFSET + MAXMEM)

#define TSS(x) this_cpu_read(cpu_tss_rw.x86_tss.x)

static void set_df_gdt_entry(unsigned int cpu);

/*
 * Called by double_fault with CR0.TS and EFLAGS.NT cleared.  The CPU thinks
 * we're running the doublefault task.  Cannot return.
 */
asmlinkage noinstr void __noreturn doublefault_shim(void)
{
	unsigned long cr2;
	struct pt_regs regs;

	BUILD_BUG_ON(sizeof(struct doublefault_stack) != PAGE_SIZE);

	cr2 = native_read_cr2();

	/* Reset back to the normal kernel task. */
	force_reload_TR();
	set_df_gdt_entry(smp_processor_id());

	trace_hardirqs_off();

	/*
	 * Fill in pt_regs.  A downside of doing this in C is that the unwinder
	 * won't see it (no ENCODE_FRAME_POINTER), so a nested stack dump
	 * won't successfully unwind to the source of the double fault.
	 * The main dump from exc_double_fault() is fine, though, since it
	 * uses these regs directly.
	 *
	 * If anyone ever cares, this could be moved to asm.
	 */
	regs.ss		= TSS(ss);
	regs.__ssh	= 0;
	regs.sp		= TSS(sp);
	regs.flags	= TSS(flags);
	regs.cs		= TSS(cs);
	/* We won't go through the entry asm, so we can leave __csh as 0. */
	regs.__csh	= 0;
	regs.ip		= TSS(ip);
	regs.orig_ax	= 0;
	regs.gs		= TSS(gs);
	regs.__gsh	= 0;
	regs.fs		= TSS(fs);
	regs.__fsh	= 0;
	regs.es		= TSS(es);
	regs.__esh	= 0;
	regs.ds		= TSS(ds);
	regs.__dsh	= 0;
	regs.ax		= TSS(ax);
	regs.bp		= TSS(bp);
	regs.di		= TSS(di);
	regs.si		= TSS(si);
	regs.dx		= TSS(dx);
	regs.cx		= TSS(cx);
	regs.bx		= TSS(bx);

	exc_double_fault(&regs, 0, cr2);

	/*
	 * x86_32 does not save the original CR3 anywhere on a task switch.
	 * This means that, even if we wanted to return, we would need to find
	 * some way to reconstruct CR3.  We could make a credible guess based
	 * on cpu_tlbstate, but that would be racy and would not account for
	 * PTI.
	 *
	 * Instead, don't bother.  We can return through
	 * rewind_stack_do_exit() instead.
	 */
	panic("cannot return from double fault\n");
}

DEFINE_PER_CPU_PAGE_ALIGNED(struct doublefault_stack, doublefault_stack) = {
	.tss = {
                /*
                 * No sp0 or ss0 -- we never run CPL != 0 with this TSS
                 * active.  sp is filled in later.
                 */
		.ldt		= 0,
	.io_bitmap_base	= IO_BITMAP_OFFSET_INVALID,

		.ip		= (unsigned long) asm_exc_double_fault,
		.flags		= X86_EFLAGS_FIXED,
		.es		= __USER_DS,
		.cs		= __KERNEL_CS,
		.ss		= __KERNEL_DS,
		.ds		= __USER_DS,
		.fs		= __KERNEL_PERCPU,
#ifndef CONFIG_X86_32_LAZY_GS
		.gs		= __KERNEL_STACK_CANARY,
#endif

		.__cr3		= __pa_nodebug(swapper_pg_dir),
	},
};

static void set_df_gdt_entry(unsigned int cpu)
{
	/* Set up doublefault TSS pointer in the GDT */
	__set_tss_desc(cpu, GDT_ENTRY_DOUBLEFAULT_TSS,
		       &get_cpu_entry_area(cpu)->doublefault_stack.tss);

}

void doublefault_init_cpu_tss(void)
{
	unsigned int cpu = smp_processor_id();
	struct cpu_entry_area *cea = get_cpu_entry_area(cpu);

	/*
	 * The linker isn't smart enough to initialize percpu variables that
	 * point to other places in percpu space.
	 */
        this_cpu_write(doublefault_stack.tss.sp,
                       (unsigned long)&cea->doublefault_stack.stack +
                       sizeof(doublefault_stack.stack));

	set_df_gdt_entry(cpu);
}