1 // SPDX-License-Identifier: GPL-2.0-only
2 /* ----------------------------------------------------------------------- *
3 *
4 * Copyright 2014 Intel Corporation; author: H. Peter Anvin
5 *
6 * ----------------------------------------------------------------------- */
7
8 /*
9 * The IRET instruction, when returning to a 16-bit segment, only
10 * restores the bottom 16 bits of the user space stack pointer. This
11 * causes some 16-bit software to break, but it also leaks kernel state
12 * to user space.
13 *
14 * This works around this by creating percpu "ministacks", each of which
15 * is mapped 2^16 times 64K apart. When we detect that the return SS is
16 * on the LDT, we copy the IRET frame to the ministack and use the
17 * relevant alias to return to userspace. The ministacks are mapped
18 * readonly, so if the IRET fault we promote #GP to #DF which is an IST
19 * vector and thus has its own stack; we then do the fixup in the #DF
20 * handler.
21 *
22 * This file sets up the ministacks and the related page tables. The
23 * actual ministack invocation is in entry_64.S.
24 */
25
26 #include <linux/init.h>
27 #include <linux/init_task.h>
28 #include <linux/kernel.h>
29 #include <linux/percpu.h>
30 #include <linux/gfp.h>
31 #include <linux/random.h>
32 #include <linux/pgtable.h>
33 #include <asm/pgalloc.h>
34 #include <asm/setup.h>
35 #include <asm/espfix.h>
36
37 /*
38 * Note: we only need 6*8 = 48 bytes for the espfix stack, but round
39 * it up to a cache line to avoid unnecessary sharing.
40 */
41 #define ESPFIX_STACK_SIZE (8*8UL)
42 #define ESPFIX_STACKS_PER_PAGE (PAGE_SIZE/ESPFIX_STACK_SIZE)
43
44 /* There is address space for how many espfix pages? */
45 #define ESPFIX_PAGE_SPACE (1UL << (P4D_SHIFT-PAGE_SHIFT-16))
46
47 #define ESPFIX_MAX_CPUS (ESPFIX_STACKS_PER_PAGE * ESPFIX_PAGE_SPACE)
48 #if CONFIG_NR_CPUS > ESPFIX_MAX_CPUS
49 # error "Need more virtual address space for the ESPFIX hack"
50 #endif
51
52 #define PGALLOC_GFP (GFP_KERNEL | __GFP_ZERO)
53
54 /* This contains the *bottom* address of the espfix stack */
55 DEFINE_PER_CPU_READ_MOSTLY(unsigned long, espfix_stack);
56 DEFINE_PER_CPU_READ_MOSTLY(unsigned long, espfix_waddr);
57
58 /* Initialization mutex - should this be a spinlock? */
59 static DEFINE_MUTEX(espfix_init_mutex);
60
61 /* Page allocation bitmap - each page serves ESPFIX_STACKS_PER_PAGE CPUs */
62 #define ESPFIX_MAX_PAGES DIV_ROUND_UP(CONFIG_NR_CPUS, ESPFIX_STACKS_PER_PAGE)
63 static void *espfix_pages[ESPFIX_MAX_PAGES];
64
65 static __page_aligned_bss pud_t espfix_pud_page[PTRS_PER_PUD]
66 __aligned(PAGE_SIZE);
67
68 static unsigned int page_random, slot_random;
69
70 /*
71 * This returns the bottom address of the espfix stack for a specific CPU.
72 * The math allows for a non-power-of-two ESPFIX_STACK_SIZE, in which case
73 * we have to account for some amount of padding at the end of each page.
74 */
espfix_base_addr(unsigned int cpu)75 static inline unsigned long espfix_base_addr(unsigned int cpu)
76 {
77 unsigned long page, slot;
78 unsigned long addr;
79
80 page = (cpu / ESPFIX_STACKS_PER_PAGE) ^ page_random;
81 slot = (cpu + slot_random) % ESPFIX_STACKS_PER_PAGE;
82 addr = (page << PAGE_SHIFT) + (slot * ESPFIX_STACK_SIZE);
83 addr = (addr & 0xffffUL) | ((addr & ~0xffffUL) << 16);
84 addr += ESPFIX_BASE_ADDR;
85 return addr;
86 }
87
88 #define PTE_STRIDE (65536/PAGE_SIZE)
89 #define ESPFIX_PTE_CLONES (PTRS_PER_PTE/PTE_STRIDE)
90 #define ESPFIX_PMD_CLONES PTRS_PER_PMD
91 #define ESPFIX_PUD_CLONES (65536/(ESPFIX_PTE_CLONES*ESPFIX_PMD_CLONES))
92
93 #define PGTABLE_PROT ((_KERNPG_TABLE & ~_PAGE_RW) | _PAGE_NX)
94
init_espfix_random(void)95 static void init_espfix_random(void)
96 {
97 unsigned long rand = get_random_long();
98
99 slot_random = rand % ESPFIX_STACKS_PER_PAGE;
100 page_random = (rand / ESPFIX_STACKS_PER_PAGE)
101 & (ESPFIX_PAGE_SPACE - 1);
102 }
103
init_espfix_bsp(void)104 void __init init_espfix_bsp(void)
105 {
106 pgd_t *pgd;
107 p4d_t *p4d;
108
109 /* FRED systems always restore the full value of %rsp */
110 if (cpu_feature_enabled(X86_FEATURE_FRED))
111 return;
112
113 /* Install the espfix pud into the kernel page directory */
114 pgd = &init_top_pgt[pgd_index(ESPFIX_BASE_ADDR)];
115 p4d = p4d_alloc(&init_mm, pgd, ESPFIX_BASE_ADDR);
116 p4d_populate(&init_mm, p4d, espfix_pud_page);
117
118 /* Randomize the locations */
119 init_espfix_random();
120
121 /* The rest is the same as for any other processor */
122 init_espfix_ap(0);
123 }
124
init_espfix_ap(int cpu)125 void init_espfix_ap(int cpu)
126 {
127 unsigned int page;
128 unsigned long addr;
129 pud_t pud, *pud_p;
130 pmd_t pmd, *pmd_p;
131 pte_t pte, *pte_p;
132 int n, node;
133 void *stack_page;
134 pteval_t ptemask;
135
136 /* FRED systems always restore the full value of %rsp */
137 if (cpu_feature_enabled(X86_FEATURE_FRED))
138 return;
139
140 /* We only have to do this once... */
141 if (likely(per_cpu(espfix_stack, cpu)))
142 return; /* Already initialized */
143
144 addr = espfix_base_addr(cpu);
145 page = cpu/ESPFIX_STACKS_PER_PAGE;
146
147 /* Did another CPU already set this up? */
148 stack_page = READ_ONCE(espfix_pages[page]);
149 if (likely(stack_page))
150 goto done;
151
152 mutex_lock(&espfix_init_mutex);
153
154 /* Did we race on the lock? */
155 stack_page = READ_ONCE(espfix_pages[page]);
156 if (stack_page)
157 goto unlock_done;
158
159 node = cpu_to_node(cpu);
160 ptemask = __supported_pte_mask;
161
162 pud_p = &espfix_pud_page[pud_index(addr)];
163 pud = *pud_p;
164 if (!pud_present(pud)) {
165 struct page *page = alloc_pages_node(node, PGALLOC_GFP, 0);
166
167 pmd_p = (pmd_t *)page_address(page);
168 pud = __pud(__pa(pmd_p) | (PGTABLE_PROT & ptemask));
169 paravirt_alloc_pmd(&init_mm, __pa(pmd_p) >> PAGE_SHIFT);
170 for (n = 0; n < ESPFIX_PUD_CLONES; n++)
171 set_pud(&pud_p[n], pud);
172 }
173
174 pmd_p = pmd_offset(&pud, addr);
175 pmd = *pmd_p;
176 if (!pmd_present(pmd)) {
177 struct page *page = alloc_pages_node(node, PGALLOC_GFP, 0);
178
179 pte_p = (pte_t *)page_address(page);
180 pmd = __pmd(__pa(pte_p) | (PGTABLE_PROT & ptemask));
181 paravirt_alloc_pte(&init_mm, __pa(pte_p) >> PAGE_SHIFT);
182 for (n = 0; n < ESPFIX_PMD_CLONES; n++)
183 set_pmd(&pmd_p[n], pmd);
184 }
185
186 pte_p = pte_offset_kernel(&pmd, addr);
187 stack_page = page_address(alloc_pages_node(node, GFP_KERNEL, 0));
188 /*
189 * __PAGE_KERNEL_* includes _PAGE_GLOBAL, which we want since
190 * this is mapped to userspace.
191 */
192 pte = __pte(__pa(stack_page) | ((__PAGE_KERNEL_RO | _PAGE_ENC) & ptemask));
193 for (n = 0; n < ESPFIX_PTE_CLONES; n++)
194 set_pte(&pte_p[n*PTE_STRIDE], pte);
195
196 /* Job is done for this CPU and any CPU which shares this page */
197 WRITE_ONCE(espfix_pages[page], stack_page);
198
199 unlock_done:
200 mutex_unlock(&espfix_init_mutex);
201 done:
202 per_cpu(espfix_stack, cpu) = addr;
203 per_cpu(espfix_waddr, cpu) = (unsigned long)stack_page
204 + (addr & ~PAGE_MASK);
205 }
206