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 */ 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 rdtscll(rand); 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 pteval_t ptemask; 126 127 ptemask = __supported_pte_mask; 128 129 /* Install the espfix pud into the kernel page directory */ 130 pgd_p = &init_level4_pgt[pgd_index(ESPFIX_BASE_ADDR)]; 131 pgd_populate(&init_mm, pgd_p, (pud_t *)espfix_pud_page); 132 133 /* Randomize the locations */ 134 init_espfix_random(); 135 136 /* The rest is the same as for any other processor */ 137 init_espfix_ap(); 138 } 139 140 void init_espfix_ap(void) 141 { 142 unsigned int cpu, page; 143 unsigned long addr; 144 pud_t pud, *pud_p; 145 pmd_t pmd, *pmd_p; 146 pte_t pte, *pte_p; 147 int n; 148 void *stack_page; 149 pteval_t ptemask; 150 151 /* We only have to do this once... */ 152 if (likely(this_cpu_read(espfix_stack))) 153 return; /* Already initialized */ 154 155 cpu = smp_processor_id(); 156 addr = espfix_base_addr(cpu); 157 page = cpu/ESPFIX_STACKS_PER_PAGE; 158 159 /* Did another CPU already set this up? */ 160 stack_page = ACCESS_ONCE(espfix_pages[page]); 161 if (likely(stack_page)) 162 goto done; 163 164 mutex_lock(&espfix_init_mutex); 165 166 /* Did we race on the lock? */ 167 stack_page = ACCESS_ONCE(espfix_pages[page]); 168 if (stack_page) 169 goto unlock_done; 170 171 ptemask = __supported_pte_mask; 172 173 pud_p = &espfix_pud_page[pud_index(addr)]; 174 pud = *pud_p; 175 if (!pud_present(pud)) { 176 pmd_p = (pmd_t *)__get_free_page(PGALLOC_GFP); 177 pud = __pud(__pa(pmd_p) | (PGTABLE_PROT & ptemask)); 178 paravirt_alloc_pmd(&init_mm, __pa(pmd_p) >> PAGE_SHIFT); 179 for (n = 0; n < ESPFIX_PUD_CLONES; n++) 180 set_pud(&pud_p[n], pud); 181 } 182 183 pmd_p = pmd_offset(&pud, addr); 184 pmd = *pmd_p; 185 if (!pmd_present(pmd)) { 186 pte_p = (pte_t *)__get_free_page(PGALLOC_GFP); 187 pmd = __pmd(__pa(pte_p) | (PGTABLE_PROT & ptemask)); 188 paravirt_alloc_pte(&init_mm, __pa(pte_p) >> PAGE_SHIFT); 189 for (n = 0; n < ESPFIX_PMD_CLONES; n++) 190 set_pmd(&pmd_p[n], pmd); 191 } 192 193 pte_p = pte_offset_kernel(&pmd, addr); 194 stack_page = (void *)__get_free_page(GFP_KERNEL); 195 pte = __pte(__pa(stack_page) | (__PAGE_KERNEL_RO & ptemask)); 196 for (n = 0; n < ESPFIX_PTE_CLONES; n++) 197 set_pte(&pte_p[n*PTE_STRIDE], pte); 198 199 /* Job is done for this CPU and any CPU which shares this page */ 200 ACCESS_ONCE(espfix_pages[page]) = stack_page; 201 202 unlock_done: 203 mutex_unlock(&espfix_init_mutex); 204 done: 205 this_cpu_write(espfix_stack, addr); 206 this_cpu_write(espfix_waddr, (unsigned long)stack_page 207 + (addr & ~PAGE_MASK)); 208 } 209