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/* ----------------------------------------------------------------------- *
*
* Copyright 2014 Intel Corporation; author: H. Peter Anvin
*
* This program is free software; you can redistribute it and/or modify it
* under the terms and conditions of the GNU General Public License,
* version 2, as published by the Free Software Foundation.
*
* This program is distributed in the hope it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
* more details.
*
* ----------------------------------------------------------------------- */
/*
* The IRET instruction, when returning to a 16-bit segment, only
* restores the bottom 16 bits of the user space stack pointer. This
* causes some 16-bit software to break, but it also leaks kernel state
* to user space.
*
* This works around this by creating percpu "ministacks", each of which
* is mapped 2^16 times 64K apart. When we detect that the return SS is
* on the LDT, we copy the IRET frame to the ministack and use the
* relevant alias to return to userspace. The ministacks are mapped
* readonly, so if the IRET fault we promote #GP to #DF which is an IST
* vector and thus has its own stack; we then do the fixup in the #DF
* handler.
*
* This file sets up the ministacks and the related page tables. The
* actual ministack invocation is in entry_64.S.
*/
#include <linux/init.h>
#include <linux/init_task.h>
#include <linux/kernel.h>
#include <linux/percpu.h>
#include <linux/gfp.h>
#include <linux/random.h>
#include <asm/pgtable.h>
#include <asm/pgalloc.h>
#include <asm/setup.h>
#include <asm/espfix.h>
/*
* Note: we only need 6*8 = 48 bytes for the espfix stack, but round
* it up to a cache line to avoid unnecessary sharing.
*/
#define ESPFIX_STACK_SIZE (8*8UL)
#define ESPFIX_STACKS_PER_PAGE (PAGE_SIZE/ESPFIX_STACK_SIZE)
/* There is address space for how many espfix pages? */
#define ESPFIX_PAGE_SPACE (1UL << (PGDIR_SHIFT-PAGE_SHIFT-16))
#define ESPFIX_MAX_CPUS (ESPFIX_STACKS_PER_PAGE * ESPFIX_PAGE_SPACE)
#if CONFIG_NR_CPUS > ESPFIX_MAX_CPUS
# error "Need more than one PGD for the ESPFIX hack"
#endif
#define PGALLOC_GFP (GFP_KERNEL | __GFP_NOTRACK | __GFP_REPEAT | __GFP_ZERO)
/* This contains the *bottom* address of the espfix stack */
DEFINE_PER_CPU_READ_MOSTLY(unsigned long, espfix_stack);
DEFINE_PER_CPU_READ_MOSTLY(unsigned long, espfix_waddr);
/* Initialization mutex - should this be a spinlock? */
static DEFINE_MUTEX(espfix_init_mutex);
/* Page allocation bitmap - each page serves ESPFIX_STACKS_PER_PAGE CPUs */
#define ESPFIX_MAX_PAGES DIV_ROUND_UP(CONFIG_NR_CPUS, ESPFIX_STACKS_PER_PAGE)
static void *espfix_pages[ESPFIX_MAX_PAGES];
static __page_aligned_bss pud_t espfix_pud_page[PTRS_PER_PUD]
__aligned(PAGE_SIZE);
static unsigned int page_random, slot_random;
/*
* This returns the bottom address of the espfix stack for a specific CPU.
* The math allows for a non-power-of-two ESPFIX_STACK_SIZE, in which case
* we have to account for some amount of padding at the end of each page.
*/
static inline unsigned long espfix_base_addr(unsigned int cpu)
{
unsigned long page, slot;
unsigned long addr;
page = (cpu / ESPFIX_STACKS_PER_PAGE) ^ page_random;
slot = (cpu + slot_random) % ESPFIX_STACKS_PER_PAGE;
addr = (page << PAGE_SHIFT) + (slot * ESPFIX_STACK_SIZE);
addr = (addr & 0xffffUL) | ((addr & ~0xffffUL) << 16);
addr += ESPFIX_BASE_ADDR;
return addr;
}
#define PTE_STRIDE (65536/PAGE_SIZE)
#define ESPFIX_PTE_CLONES (PTRS_PER_PTE/PTE_STRIDE)
#define ESPFIX_PMD_CLONES PTRS_PER_PMD
#define ESPFIX_PUD_CLONES (65536/(ESPFIX_PTE_CLONES*ESPFIX_PMD_CLONES))
#define PGTABLE_PROT ((_KERNPG_TABLE & ~_PAGE_RW) | _PAGE_NX)
static void init_espfix_random(void)
{
unsigned long rand;
/*
* This is run before the entropy pools are initialized,
* but this is hopefully better than nothing.
*/
if (!arch_get_random_long(&rand)) {
/* The constant is an arbitrary large prime */
rdtscll(rand);
rand *= 0xc345c6b72fd16123UL;
}
slot_random = rand % ESPFIX_STACKS_PER_PAGE;
page_random = (rand / ESPFIX_STACKS_PER_PAGE)
& (ESPFIX_PAGE_SPACE - 1);
}
void __init init_espfix_bsp(void)
{
pgd_t *pgd_p;
/* Install the espfix pud into the kernel page directory */
pgd_p = &init_level4_pgt[pgd_index(ESPFIX_BASE_ADDR)];
pgd_populate(&init_mm, pgd_p, (pud_t *)espfix_pud_page);
/* Randomize the locations */
init_espfix_random();
/* The rest is the same as for any other processor */
init_espfix_ap();
}
void init_espfix_ap(void)
{
unsigned int cpu, page;
unsigned long addr;
pud_t pud, *pud_p;
pmd_t pmd, *pmd_p;
pte_t pte, *pte_p;
int n;
void *stack_page;
pteval_t ptemask;
/* We only have to do this once... */
if (likely(this_cpu_read(espfix_stack)))
return; /* Already initialized */
cpu = smp_processor_id();
addr = espfix_base_addr(cpu);
page = cpu/ESPFIX_STACKS_PER_PAGE;
/* Did another CPU already set this up? */
stack_page = ACCESS_ONCE(espfix_pages[page]);
if (likely(stack_page))
goto done;
mutex_lock(&espfix_init_mutex);
/* Did we race on the lock? */
stack_page = ACCESS_ONCE(espfix_pages[page]);
if (stack_page)
goto unlock_done;
ptemask = __supported_pte_mask;
pud_p = &espfix_pud_page[pud_index(addr)];
pud = *pud_p;
if (!pud_present(pud)) {
pmd_p = (pmd_t *)__get_free_page(PGALLOC_GFP);
pud = __pud(__pa(pmd_p) | (PGTABLE_PROT & ptemask));
paravirt_alloc_pmd(&init_mm, __pa(pmd_p) >> PAGE_SHIFT);
for (n = 0; n < ESPFIX_PUD_CLONES; n++)
set_pud(&pud_p[n], pud);
}
pmd_p = pmd_offset(&pud, addr);
pmd = *pmd_p;
if (!pmd_present(pmd)) {
pte_p = (pte_t *)__get_free_page(PGALLOC_GFP);
pmd = __pmd(__pa(pte_p) | (PGTABLE_PROT & ptemask));
paravirt_alloc_pte(&init_mm, __pa(pte_p) >> PAGE_SHIFT);
for (n = 0; n < ESPFIX_PMD_CLONES; n++)
set_pmd(&pmd_p[n], pmd);
}
pte_p = pte_offset_kernel(&pmd, addr);
stack_page = (void *)__get_free_page(GFP_KERNEL);
pte = __pte(__pa(stack_page) | (__PAGE_KERNEL_RO & ptemask));
for (n = 0; n < ESPFIX_PTE_CLONES; n++)
set_pte(&pte_p[n*PTE_STRIDE], pte);
/* Job is done for this CPU and any CPU which shares this page */
ACCESS_ONCE(espfix_pages[page]) = stack_page;
unlock_done:
mutex_unlock(&espfix_init_mutex);
done:
this_cpu_write(espfix_stack, addr);
this_cpu_write(espfix_waddr, (unsigned long)stack_page
+ (addr & ~PAGE_MASK));
}
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