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_ZERO) 61 62 /* This contains the *bottom* address of the espfix stack */ 63 DEFINE_PER_CPU_READ_MOSTLY(unsigned long, espfix_stack); 64 DEFINE_PER_CPU_READ_MOSTLY(unsigned long, espfix_waddr); 65 66 /* Initialization mutex - should this be a spinlock? */ 67 static 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) 71 static void *espfix_pages[ESPFIX_MAX_PAGES]; 72 73 static __page_aligned_bss pud_t espfix_pud_page[PTRS_PER_PUD] 74 __aligned(PAGE_SIZE); 75 76 static 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 */ 83 static 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) 102 103 static 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 } 121 122 void __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 } 136 137 void 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; 202 203 unlock_done: 204 mutex_unlock(&espfix_init_mutex); 205 done: 206 per_cpu(espfix_stack, cpu) = addr; 207 per_cpu(espfix_waddr, cpu) = (unsigned long)stack_page 208 + (addr & ~PAGE_MASK); 209 } 210