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 << (P4D_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 virtual address space for the ESPFIX hack"
  58 #endif
  59
  60 #define PGALLOC_GFP (GFP_KERNEL | __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;
 125         p4d_t *p4d;
 126
 127         /* Install the espfix pud into the kernel page directory */
 128         pgd = &init_top_pgt[pgd_index(ESPFIX_BASE_ADDR)];
 129         p4d = p4d_alloc(&init_mm, pgd, ESPFIX_BASE_ADDR);
 130         p4d_populate(&init_mm, p4d, espfix_pud_page);
 131
 132         /* Randomize the locations */
 133         init_espfix_random();
 134
 135         /* The rest is the same as for any other processor */
 136         init_espfix_ap(0);
 137 }
 138
 139 void init_espfix_ap(int cpu)
 140 {
 141         unsigned int page;
 142         unsigned long addr;
 143         pud_t pud, *pud_p;
 144         pmd_t pmd, *pmd_p;
 145         pte_t pte, *pte_p;
 146         int n, node;
 147         void *stack_page;
 148         pteval_t ptemask;
 149
 150         /* We only have to do this once... */
 151         if (likely(per_cpu(espfix_stack, cpu)))
 152                 return;         /* Already initialized */
 153
 154         addr = espfix_base_addr(cpu);
 155         page = cpu/ESPFIX_STACKS_PER_PAGE;
 156
 157         /* Did another CPU already set this up? */
 158         stack_page = READ_ONCE(espfix_pages[page]);
 159         if (likely(stack_page))
 160                 goto done;
 161
 162         mutex_lock(&espfix_init_mutex);
 163
 164         /* Did we race on the lock? */
 165         stack_page = READ_ONCE(espfix_pages[page]);
 166         if (stack_page)
 167                 goto unlock_done;
 168
 169         node = cpu_to_node(cpu);
 170         ptemask = __supported_pte_mask;
 171
 172         pud_p = &espfix_pud_page[pud_index(addr)];
 173         pud = *pud_p;
 174         if (!pud_present(pud)) {
 175                 struct page *page = alloc_pages_node(node, PGALLOC_GFP, 0);
 176
 177                 pmd_p = (pmd_t *)page_address(page);
 178                 pud = __pud(__pa(pmd_p) | (PGTABLE_PROT & ptemask));
 179                 paravirt_alloc_pmd(&init_mm, __pa(pmd_p) >> PAGE_SHIFT);
 180                 for (n = 0; n < ESPFIX_PUD_CLONES; n++)
 181                         set_pud(&pud_p[n], pud);
 182         }
 183
 184         pmd_p = pmd_offset(&pud, addr);
 185         pmd = *pmd_p;
 186         if (!pmd_present(pmd)) {
 187                 struct page *page = alloc_pages_node(node, PGALLOC_GFP, 0);
 188
 189                 pte_p = (pte_t *)page_address(page);
 190                 pmd = __pmd(__pa(pte_p) | (PGTABLE_PROT & ptemask));
 191                 paravirt_alloc_pte(&init_mm, __pa(pte_p) >> PAGE_SHIFT);
 192                 for (n = 0; n < ESPFIX_PMD_CLONES; n++)
 193                         set_pmd(&pmd_p[n], pmd);
 194         }
 195
 196         pte_p = pte_offset_kernel(&pmd, addr);
 197         stack_page = page_address(alloc_pages_node(node, GFP_KERNEL, 0));
 198         /*
 199          * __PAGE_KERNEL_* includes _PAGE_GLOBAL, which we want since
 200          * this is mapped to userspace.
 201          */
 202         pte = __pte(__pa(stack_page) | ((__PAGE_KERNEL_RO | _PAGE_ENC) & ptemask));
 203         for (n = 0; n < ESPFIX_PTE_CLONES; n++)
 204                 set_pte(&pte_p[n*PTE_STRIDE], pte);
 205
 206         /* Job is done for this CPU and any CPU which shares this page */
 207         WRITE_ONCE(espfix_pages[page], stack_page);
 208
 209 unlock_done:
 210         mutex_unlock(&espfix_init_mutex);
 211 done:
 212         per_cpu(espfix_stack, cpu) = addr;
 213         per_cpu(espfix_waddr, cpu) = (unsigned long)stack_page
 214                                       + (addr & ~PAGE_MASK);
 215 }