1// SPDX-License-Identifier: GPL-2.0
2#include <linux/mm.h>
3#include <linux/sched.h>
4#include <linux/sched/debug.h>
5#include <linux/init_task.h>
6#include <linux/fs.h>
7
8#include <linux/uaccess.h>
9#include <asm/processor.h>
10#include <asm/desc.h>
11#include <asm/traps.h>
12#include <asm/doublefault.h>
13
14#define ptr_ok(x) ((x) > PAGE_OFFSET && (x) < PAGE_OFFSET + MAXMEM)
15
16#define TSS(x) this_cpu_read(cpu_tss_rw.x86_tss.x)
17
18static void set_df_gdt_entry(unsigned int cpu);
19
20/*
21 * Called by double_fault with CR0.TS and EFLAGS.NT cleared.  The CPU thinks
22 * we're running the doublefault task.  Cannot return.
23 */
24asmlinkage noinstr void __noreturn doublefault_shim(void)
25{
26	unsigned long cr2;
27	struct pt_regs regs;
28
29	BUILD_BUG_ON(sizeof(struct doublefault_stack) != PAGE_SIZE);
30
31	cr2 = native_read_cr2();
32
33	/* Reset back to the normal kernel task. */
34	force_reload_TR();
35	set_df_gdt_entry(smp_processor_id());
36
37	trace_hardirqs_off();
38
39	/*
40	 * Fill in pt_regs.  A downside of doing this in C is that the unwinder
41	 * won't see it (no ENCODE_FRAME_POINTER), so a nested stack dump
42	 * won't successfully unwind to the source of the double fault.
43	 * The main dump from exc_double_fault() is fine, though, since it
44	 * uses these regs directly.
45	 *
46	 * If anyone ever cares, this could be moved to asm.
47	 */
48	regs.ss		= TSS(ss);
49	regs.__ssh	= 0;
50	regs.sp		= TSS(sp);
51	regs.flags	= TSS(flags);
52	regs.cs		= TSS(cs);
53	/* We won't go through the entry asm, so we can leave __csh as 0. */
54	regs.__csh	= 0;
55	regs.ip		= TSS(ip);
56	regs.orig_ax	= 0;
57	regs.gs		= TSS(gs);
58	regs.__gsh	= 0;
59	regs.fs		= TSS(fs);
60	regs.__fsh	= 0;
61	regs.es		= TSS(es);
62	regs.__esh	= 0;
63	regs.ds		= TSS(ds);
64	regs.__dsh	= 0;
65	regs.ax		= TSS(ax);
66	regs.bp		= TSS(bp);
67	regs.di		= TSS(di);
68	regs.si		= TSS(si);
69	regs.dx		= TSS(dx);
70	regs.cx		= TSS(cx);
71	regs.bx		= TSS(bx);
72
73	exc_double_fault(&regs, 0, cr2);
74
75	/*
76	 * x86_32 does not save the original CR3 anywhere on a task switch.
77	 * This means that, even if we wanted to return, we would need to find
78	 * some way to reconstruct CR3.  We could make a credible guess based
79	 * on cpu_tlbstate, but that would be racy and would not account for
80	 * PTI.
81	 */
82	panic("cannot return from double fault\n");
83}
84
85DEFINE_PER_CPU_PAGE_ALIGNED(struct doublefault_stack, doublefault_stack) = {
86	.tss = {
87                /*
88                 * No sp0 or ss0 -- we never run CPL != 0 with this TSS
89                 * active.  sp is filled in later.
90                 */
91		.ldt		= 0,
92	.io_bitmap_base	= IO_BITMAP_OFFSET_INVALID,
93
94		.ip		= (unsigned long) asm_exc_double_fault,
95		.flags		= X86_EFLAGS_FIXED,
96		.es		= __USER_DS,
97		.cs		= __KERNEL_CS,
98		.ss		= __KERNEL_DS,
99		.ds		= __USER_DS,
100		.fs		= __KERNEL_PERCPU,
101		.gs		= 0,
102
103		.__cr3		= __pa_nodebug(swapper_pg_dir),
104	},
105};
106
107static void set_df_gdt_entry(unsigned int cpu)
108{
109	/* Set up doublefault TSS pointer in the GDT */
110	__set_tss_desc(cpu, GDT_ENTRY_DOUBLEFAULT_TSS,
111		       &get_cpu_entry_area(cpu)->doublefault_stack.tss);
112
113}
114
115void doublefault_init_cpu_tss(void)
116{
117	unsigned int cpu = smp_processor_id();
118	struct cpu_entry_area *cea = get_cpu_entry_area(cpu);
119
120	/*
121	 * The linker isn't smart enough to initialize percpu variables that
122	 * point to other places in percpu space.
123	 */
124        this_cpu_write(doublefault_stack.tss.sp,
125                       (unsigned long)&cea->doublefault_stack.stack +
126                       sizeof(doublefault_stack.stack));
127
128	set_df_gdt_entry(cpu);
129}
130