1/* ----------------------------------------------------------------------- *
2 *
3 *   Copyright 2014 Intel Corporation; author: H. Peter Anvin
4 *
5 *   This program is free software; you can redistribute it and/or modify it
6 *   under the terms and conditions of the GNU General Public License,
7 *   version 2, as published by the Free Software Foundation.
8 *
9 *   This program is distributed in the hope it will be useful, but WITHOUT
10 *   ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 *   FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
12 *   more details.
13 *
14 * ----------------------------------------------------------------------- */
15
16/*
17 * The IRET instruction, when returning to a 16-bit segment, only
18 * restores the bottom 16 bits of the user space stack pointer.  This
19 * causes some 16-bit software to break, but it also leaks kernel state
20 * to user space.
21 *
22 * This works around this by creating percpu "ministacks", each of which
23 * is mapped 2^16 times 64K apart.  When we detect that the return SS is
24 * on the LDT, we copy the IRET frame to the ministack and use the
25 * relevant alias to return to userspace.  The ministacks are mapped
26 * readonly, so if the IRET fault we promote #GP to #DF which is an IST
27 * vector and thus has its own stack; we then do the fixup in the #DF
28 * handler.
29 *
30 * This file sets up the ministacks and the related page tables.  The
31 * actual ministack invocation is in entry_64.S.
32 */
33
34#include <linux/init.h>
35#include <linux/init_task.h>
36#include <linux/kernel.h>
37#include <linux/percpu.h>
38#include <linux/gfp.h>
39#include <linux/random.h>
40#include <asm/pgtable.h>
41#include <asm/pgalloc.h>
42#include <asm/setup.h>
43#include <asm/espfix.h>
44
45/*
46 * Note: we only need 6*8 = 48 bytes for the espfix stack, but round
47 * it up to a cache line to avoid unnecessary sharing.
48 */
49#define ESPFIX_STACK_SIZE	(8*8UL)
50#define ESPFIX_STACKS_PER_PAGE	(PAGE_SIZE/ESPFIX_STACK_SIZE)
51
52/* There is address space for how many espfix pages? */
53#define ESPFIX_PAGE_SPACE	(1UL << (PGDIR_SHIFT-PAGE_SHIFT-16))
54
55#define ESPFIX_MAX_CPUS		(ESPFIX_STACKS_PER_PAGE * ESPFIX_PAGE_SPACE)
56#if CONFIG_NR_CPUS > ESPFIX_MAX_CPUS
57# error "Need more than one PGD for the ESPFIX hack"
58#endif
59
60#define PGALLOC_GFP (GFP_KERNEL | __GFP_NOTRACK | __GFP_REPEAT | __GFP_ZERO)
61
62/* This contains the *bottom* address of the espfix stack */
63DEFINE_PER_CPU_READ_MOSTLY(unsigned long, espfix_stack);
64DEFINE_PER_CPU_READ_MOSTLY(unsigned long, espfix_waddr);
65
66/* Initialization mutex - should this be a spinlock? */
67static DEFINE_MUTEX(espfix_init_mutex);
68
69/* Page allocation bitmap - each page serves ESPFIX_STACKS_PER_PAGE CPUs */
70#define ESPFIX_MAX_PAGES  DIV_ROUND_UP(CONFIG_NR_CPUS, ESPFIX_STACKS_PER_PAGE)
71static void *espfix_pages[ESPFIX_MAX_PAGES];
72
73static __page_aligned_bss pud_t espfix_pud_page[PTRS_PER_PUD]
74	__aligned(PAGE_SIZE);
75
76static unsigned int page_random, slot_random;
77
78/*
79 * This returns the bottom address of the espfix stack for a specific CPU.
80 * The math allows for a non-power-of-two ESPFIX_STACK_SIZE, in which case
81 * we have to account for some amount of padding at the end of each page.
82 */
83static inline unsigned long espfix_base_addr(unsigned int cpu)
84{
85	unsigned long page, slot;
86	unsigned long addr;
87
88	page = (cpu / ESPFIX_STACKS_PER_PAGE) ^ page_random;
89	slot = (cpu + slot_random) % ESPFIX_STACKS_PER_PAGE;
90	addr = (page << PAGE_SHIFT) + (slot * ESPFIX_STACK_SIZE);
91	addr = (addr & 0xffffUL) | ((addr & ~0xffffUL) << 16);
92	addr += ESPFIX_BASE_ADDR;
93	return addr;
94}
95
96#define PTE_STRIDE        (65536/PAGE_SIZE)
97#define ESPFIX_PTE_CLONES (PTRS_PER_PTE/PTE_STRIDE)
98#define ESPFIX_PMD_CLONES PTRS_PER_PMD
99#define ESPFIX_PUD_CLONES (65536/(ESPFIX_PTE_CLONES*ESPFIX_PMD_CLONES))
100
101#define PGTABLE_PROT	  ((_KERNPG_TABLE & ~_PAGE_RW) | _PAGE_NX)
102
103static void init_espfix_random(void)
104{
105	unsigned long rand;
106
107	/*
108	 * This is run before the entropy pools are initialized,
109	 * but this is hopefully better than nothing.
110	 */
111	if (!arch_get_random_long(&rand)) {
112		/* The constant is an arbitrary large prime */
113		rand = rdtsc();
114		rand *= 0xc345c6b72fd16123UL;
115	}
116
117	slot_random = rand % ESPFIX_STACKS_PER_PAGE;
118	page_random = (rand / ESPFIX_STACKS_PER_PAGE)
119		& (ESPFIX_PAGE_SPACE - 1);
120}
121
122void __init init_espfix_bsp(void)
123{
124	pgd_t *pgd_p;
125
126	/* Install the espfix pud into the kernel page directory */
127	pgd_p = &init_level4_pgt[pgd_index(ESPFIX_BASE_ADDR)];
128	pgd_populate(&init_mm, pgd_p, (pud_t *)espfix_pud_page);
129
130	/* Randomize the locations */
131	init_espfix_random();
132
133	/* The rest is the same as for any other processor */
134	init_espfix_ap(0);
135}
136
137void init_espfix_ap(int cpu)
138{
139	unsigned int page;
140	unsigned long addr;
141	pud_t pud, *pud_p;
142	pmd_t pmd, *pmd_p;
143	pte_t pte, *pte_p;
144	int n, node;
145	void *stack_page;
146	pteval_t ptemask;
147
148	/* We only have to do this once... */
149	if (likely(per_cpu(espfix_stack, cpu)))
150		return;		/* Already initialized */
151
152	addr = espfix_base_addr(cpu);
153	page = cpu/ESPFIX_STACKS_PER_PAGE;
154
155	/* Did another CPU already set this up? */
156	stack_page = ACCESS_ONCE(espfix_pages[page]);
157	if (likely(stack_page))
158		goto done;
159
160	mutex_lock(&espfix_init_mutex);
161
162	/* Did we race on the lock? */
163	stack_page = ACCESS_ONCE(espfix_pages[page]);
164	if (stack_page)
165		goto unlock_done;
166
167	node = cpu_to_node(cpu);
168	ptemask = __supported_pte_mask;
169
170	pud_p = &espfix_pud_page[pud_index(addr)];
171	pud = *pud_p;
172	if (!pud_present(pud)) {
173		struct page *page = alloc_pages_node(node, PGALLOC_GFP, 0);
174
175		pmd_p = (pmd_t *)page_address(page);
176		pud = __pud(__pa(pmd_p) | (PGTABLE_PROT & ptemask));
177		paravirt_alloc_pmd(&init_mm, __pa(pmd_p) >> PAGE_SHIFT);
178		for (n = 0; n < ESPFIX_PUD_CLONES; n++)
179			set_pud(&pud_p[n], pud);
180	}
181
182	pmd_p = pmd_offset(&pud, addr);
183	pmd = *pmd_p;
184	if (!pmd_present(pmd)) {
185		struct page *page = alloc_pages_node(node, PGALLOC_GFP, 0);
186
187		pte_p = (pte_t *)page_address(page);
188		pmd = __pmd(__pa(pte_p) | (PGTABLE_PROT & ptemask));
189		paravirt_alloc_pte(&init_mm, __pa(pte_p) >> PAGE_SHIFT);
190		for (n = 0; n < ESPFIX_PMD_CLONES; n++)
191			set_pmd(&pmd_p[n], pmd);
192	}
193
194	pte_p = pte_offset_kernel(&pmd, addr);
195	stack_page = page_address(alloc_pages_node(node, GFP_KERNEL, 0));
196	pte = __pte(__pa(stack_page) | (__PAGE_KERNEL_RO & ptemask));
197	for (n = 0; n < ESPFIX_PTE_CLONES; n++)
198		set_pte(&pte_p[n*PTE_STRIDE], pte);
199
200	/* Job is done for this CPU and any CPU which shares this page */
201	ACCESS_ONCE(espfix_pages[page]) = stack_page;
202
203unlock_done:
204	mutex_unlock(&espfix_init_mutex);
205done:
206	per_cpu(espfix_stack, cpu) = addr;
207	per_cpu(espfix_waddr, cpu) = (unsigned long)stack_page
208				      + (addr & ~PAGE_MASK);
209}
210