/* ----------------------------------------------------------------------- *
 *
 *   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 << (P4D_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 virtual address space for the ESPFIX hack"
#endif

#define PGALLOC_GFP (GFP_KERNEL | __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 */
                rand = rdtsc();
                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;
        p4d_t *p4d;

        /* Install the espfix pud into the kernel page directory */
        pgd = &init_top_pgt[pgd_index(ESPFIX_BASE_ADDR)];
        p4d = p4d_alloc(&init_mm, pgd, ESPFIX_BASE_ADDR);
        p4d_populate(&init_mm, p4d, espfix_pud_page);

        /* Randomize the locations */
        init_espfix_random();

        /* The rest is the same as for any other processor */
        init_espfix_ap(0);
}

void init_espfix_ap(int cpu)
{
        unsigned int page;
        unsigned long addr;
        pud_t pud, *pud_p;
        pmd_t pmd, *pmd_p;
        pte_t pte, *pte_p;
        int n, node;
        void *stack_page;
        pteval_t ptemask;

        /* We only have to do this once... */
        if (likely(per_cpu(espfix_stack, cpu)))
                return;         /* Already initialized */

        addr = espfix_base_addr(cpu);
        page = cpu/ESPFIX_STACKS_PER_PAGE;

        /* Did another CPU already set this up? */
        stack_page = READ_ONCE(espfix_pages[page]);
        if (likely(stack_page))
                goto done;

        mutex_lock(&espfix_init_mutex);

        /* Did we race on the lock? */
        stack_page = READ_ONCE(espfix_pages[page]);
        if (stack_page)
                goto unlock_done;

        node = cpu_to_node(cpu);
        ptemask = __supported_pte_mask;

        pud_p = &espfix_pud_page[pud_index(addr)];
        pud = *pud_p;
        if (!pud_present(pud)) {
                struct page *page = alloc_pages_node(node, PGALLOC_GFP, 0);

                pmd_p = (pmd_t *)page_address(page);
                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)) {
                struct page *page = alloc_pages_node(node, PGALLOC_GFP, 0);

                pte_p = (pte_t *)page_address(page);
                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 = page_address(alloc_pages_node(node, GFP_KERNEL, 0));
        /*
         * __PAGE_KERNEL_* includes _PAGE_GLOBAL, which we want since
         * this is mapped to userspace.
         */
        pte = __pte(__pa(stack_page) | ((__PAGE_KERNEL_RO | _PAGE_ENC) & 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 */
        WRITE_ONCE(espfix_pages[page], stack_page);

unlock_done:
        mutex_unlock(&espfix_init_mutex);
done:
        per_cpu(espfix_stack, cpu) = addr;
        per_cpu(espfix_waddr, cpu) = (unsigned long)stack_page
                                      + (addr & ~PAGE_MASK);
}