arch/x86/kernel/espfix_64.c

   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
 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();
 135 }
 136
 137 void init_espfix_ap(void)
 138 {
 139         unsigned int cpu, 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;
 145         void *stack_page;
 146         pteval_t ptemask;
 147
 148         /* We only have to do this once... */
 149         if (likely(this_cpu_read(espfix_stack)))
 150                 return;         /* Already initialized */
 151
 152         cpu = smp_processor_id();
 153         addr = espfix_base_addr(cpu);
 154         page = cpu/ESPFIX_STACKS_PER_PAGE;
 155
 156         /* Did another CPU already set this up? */
 157         stack_page = ACCESS_ONCE(espfix_pages[page]);
 158         if (likely(stack_page))
 159                 goto done;
 160
 161         mutex_lock(&espfix_init_mutex);
 162
 163         /* Did we race on the lock? */
 164         stack_page = ACCESS_ONCE(espfix_pages[page]);
 165         if (stack_page)
 166                 goto unlock_done;
 167
 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                 pmd_p = (pmd_t *)__get_free_page(PGALLOC_GFP);
 174                 pud = __pud(__pa(pmd_p) | (PGTABLE_PROT & ptemask));
 175                 paravirt_alloc_pmd(&init_mm, __pa(pmd_p) >> PAGE_SHIFT);
 176                 for (n = 0; n < ESPFIX_PUD_CLONES; n++)
 177                         set_pud(&pud_p[n], pud);
 178         }
 179
 180         pmd_p = pmd_offset(&pud, addr);
 181         pmd = *pmd_p;
 182         if (!pmd_present(pmd)) {
 183                 pte_p = (pte_t *)__get_free_page(PGALLOC_GFP);
 184                 pmd = __pmd(__pa(pte_p) | (PGTABLE_PROT & ptemask));
 185                 paravirt_alloc_pte(&init_mm, __pa(pte_p) >> PAGE_SHIFT);
 186                 for (n = 0; n < ESPFIX_PMD_CLONES; n++)
 187                         set_pmd(&pmd_p[n], pmd);
 188         }
 189
 190         pte_p = pte_offset_kernel(&pmd, addr);
 191         stack_page = (void *)__get_free_page(GFP_KERNEL);
 192         pte = __pte(__pa(stack_page) | (__PAGE_KERNEL_RO & 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         ACCESS_ONCE(espfix_pages[page]) = stack_page;
 198
 199 unlock_done:
 200         mutex_unlock(&espfix_init_mutex);
 201 done:
 202         this_cpu_write(espfix_stack, addr);
 203         this_cpu_write(espfix_waddr, (unsigned long)stack_page
 204                        + (addr & ~PAGE_MASK));
 205 }