[mirror_ubuntu-bionic-kernel.git] / arch / x86 / mm / pti.c

/*
 * Copyright(c) 2017 Intel Corporation. All rights reserved.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of version 2 of the GNU General Public License as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that 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.
 *
 * This code is based in part on work published here:
 *
 *      https://github.com/IAIK/KAISER
 *
 * The original work was written by and and signed off by for the Linux
 * kernel by:
 *
 *   Signed-off-by: Richard Fellner <richard.fellner@student.tugraz.at>
 *   Signed-off-by: Moritz Lipp <moritz.lipp@iaik.tugraz.at>
 *   Signed-off-by: Daniel Gruss <daniel.gruss@iaik.tugraz.at>
 *   Signed-off-by: Michael Schwarz <michael.schwarz@iaik.tugraz.at>
 *
 * Major changes to the original code by: Dave Hansen <dave.hansen@intel.com>
 * Mostly rewritten by Thomas Gleixner <tglx@linutronix.de> and
 *                     Andy Lutomirsky <luto@amacapital.net>
 */
#include <linux/kernel.h>
#include <linux/errno.h>
#include <linux/string.h>
#include <linux/types.h>
#include <linux/bug.h>
#include <linux/init.h>
#include <linux/spinlock.h>
#include <linux/mm.h>
#include <linux/uaccess.h>

#include <asm/cpufeature.h>
#include <asm/hypervisor.h>
#include <asm/vsyscall.h>
#include <asm/cmdline.h>
#include <asm/pti.h>
#include <asm/pgtable.h>
#include <asm/pgalloc.h>
#include <asm/tlbflush.h>
#include <asm/desc.h>
#include <asm/sections.h>

#undef pr_fmt
#define pr_fmt(fmt)     "Kernel/User page tables isolation: " fmt

/* Backporting helper */
#ifndef __GFP_NOTRACK
#define __GFP_NOTRACK   0
#endif

/*
 * Define the page-table levels we clone for user-space on 32
 * and 64 bit.
 */
#ifdef CONFIG_X86_64
#define PTI_LEVEL_KERNEL_IMAGE  PTI_CLONE_PMD
#else
#define PTI_LEVEL_KERNEL_IMAGE  PTI_CLONE_PTE
#endif

static void __init pti_print_if_insecure(const char *reason)
{
        if (boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
                pr_info("%s\n", reason);
}

static void __init pti_print_if_secure(const char *reason)
{
        if (!boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
                pr_info("%s\n", reason);
}

enum pti_mode {
        PTI_AUTO = 0,
        PTI_FORCE_OFF,
        PTI_FORCE_ON
} pti_mode;

void __init pti_check_boottime_disable(void)
{
        char arg[5];
        int ret;

        /* Assume mode is auto unless overridden. */
        pti_mode = PTI_AUTO;

        if (hypervisor_is_type(X86_HYPER_XEN_PV)) {
                pti_mode = PTI_FORCE_OFF;
                pti_print_if_insecure("disabled on XEN PV.");
                return;
        }

        ret = cmdline_find_option(boot_command_line, "pti", arg, sizeof(arg));
        if (ret > 0)  {
                if (ret == 3 && !strncmp(arg, "off", 3)) {
                        pti_mode = PTI_FORCE_OFF;
                        pti_print_if_insecure("disabled on command line.");
                        return;
                }
                if (ret == 2 && !strncmp(arg, "on", 2)) {
                        pti_mode = PTI_FORCE_ON;
                        pti_print_if_secure("force enabled on command line.");
                        goto enable;
                }
                if (ret == 4 && !strncmp(arg, "auto", 4)) {
                        pti_mode = PTI_AUTO;
                        goto autosel;
                }
        }

        if (cmdline_find_option_bool(boot_command_line, "nopti")) {
                pti_mode = PTI_FORCE_OFF;
                pti_print_if_insecure("disabled on command line.");
                return;
        }

autosel:
        if (!boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
                return;
enable:
        setup_force_cpu_cap(X86_FEATURE_PTI);
}

pgd_t __pti_set_user_pgtbl(pgd_t *pgdp, pgd_t pgd)
{
        /*
         * Changes to the high (kernel) portion of the kernelmode page
         * tables are not automatically propagated to the usermode tables.
         *
         * Users should keep in mind that, unlike the kernelmode tables,
         * there is no vmalloc_fault equivalent for the usermode tables.
         * Top-level entries added to init_mm's usermode pgd after boot
         * will not be automatically propagated to other mms.
         */
        if (!pgdp_maps_userspace(pgdp))
                return pgd;

        /*
         * The user page tables get the full PGD, accessible from
         * userspace:
         */
        kernel_to_user_pgdp(pgdp)->pgd = pgd.pgd;

        /*
         * If this is normal user memory, make it NX in the kernel
         * pagetables so that, if we somehow screw up and return to
         * usermode with the kernel CR3 loaded, we'll get a page fault
         * instead of allowing user code to execute with the wrong CR3.
         *
         * As exceptions, we don't set NX if:
         *  - _PAGE_USER is not set.  This could be an executable
         *     EFI runtime mapping or something similar, and the kernel
         *     may execute from it
         *  - we don't have NX support
         *  - we're clearing the PGD (i.e. the new pgd is not present).
         */
        if ((pgd.pgd & (_PAGE_USER|_PAGE_PRESENT)) == (_PAGE_USER|_PAGE_PRESENT) &&
            (__supported_pte_mask & _PAGE_NX))
                pgd.pgd |= _PAGE_NX;

        /* return the copy of the PGD we want the kernel to use: */
        return pgd;
}

/*
 * Walk the user copy of the page tables (optionally) trying to allocate
 * page table pages on the way down.
 *
 * Returns a pointer to a P4D on success, or NULL on failure.
 */
static p4d_t *pti_user_pagetable_walk_p4d(unsigned long address)
{
        pgd_t *pgd = kernel_to_user_pgdp(pgd_offset_k(address));
        gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);

        if (address < PAGE_OFFSET) {
                WARN_ONCE(1, "attempt to walk user address\n");
                return NULL;
        }

        if (pgd_none(*pgd)) {
                unsigned long new_p4d_page = __get_free_page(gfp);
                if (WARN_ON_ONCE(!new_p4d_page))
                        return NULL;

                set_pgd(pgd, __pgd(_KERNPG_TABLE | __pa(new_p4d_page)));
        }
        BUILD_BUG_ON(pgd_large(*pgd) != 0);

        return p4d_offset(pgd, address);
}

/*
 * Walk the user copy of the page tables (optionally) trying to allocate
 * page table pages on the way down.
 *
 * Returns a pointer to a PMD on success, or NULL on failure.
 */
static pmd_t *pti_user_pagetable_walk_pmd(unsigned long address)
{
        gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
        p4d_t *p4d;
        pud_t *pud;

        p4d = pti_user_pagetable_walk_p4d(address);
        if (!p4d)
                return NULL;

        BUILD_BUG_ON(p4d_large(*p4d) != 0);
        if (p4d_none(*p4d)) {
                unsigned long new_pud_page = __get_free_page(gfp);
                if (WARN_ON_ONCE(!new_pud_page))
                        return NULL;

                set_p4d(p4d, __p4d(_KERNPG_TABLE | __pa(new_pud_page)));
        }

        pud = pud_offset(p4d, address);
        /* The user page tables do not use large mappings: */
        if (pud_large(*pud)) {
                WARN_ON(1);
                return NULL;
        }
        if (pud_none(*pud)) {
                unsigned long new_pmd_page = __get_free_page(gfp);
                if (WARN_ON_ONCE(!new_pmd_page))
                        return NULL;

                set_pud(pud, __pud(_KERNPG_TABLE | __pa(new_pmd_page)));
        }

        return pmd_offset(pud, address);
}

/*
 * Walk the shadow copy of the page tables (optionally) trying to allocate
 * page table pages on the way down.  Does not support large pages.
 *
 * Note: this is only used when mapping *new* kernel data into the
 * user/shadow page tables.  It is never used for userspace data.
 *
 * Returns a pointer to a PTE on success, or NULL on failure.
 */
static __init pte_t *pti_user_pagetable_walk_pte(unsigned long address)
{
        gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
        pmd_t *pmd;
        pte_t *pte;

        pmd = pti_user_pagetable_walk_pmd(address);
        if (!pmd)
                return NULL;

        /* We can't do anything sensible if we hit a large mapping. */
        if (pmd_large(*pmd)) {
                WARN_ON(1);
                return NULL;
        }

        if (pmd_none(*pmd)) {
                unsigned long new_pte_page = __get_free_page(gfp);
                if (!new_pte_page)
                        return NULL;

                set_pmd(pmd, __pmd(_KERNPG_TABLE | __pa(new_pte_page)));
        }

        pte = pte_offset_kernel(pmd, address);
        if (pte_flags(*pte) & _PAGE_USER) {
                WARN_ONCE(1, "attempt to walk to user pte\n");
                return NULL;
        }
        return pte;
}

#ifdef CONFIG_X86_VSYSCALL_EMULATION
static void __init pti_setup_vsyscall(void)
{
        pte_t *pte, *target_pte;
        unsigned int level;

        pte = lookup_address(VSYSCALL_ADDR, &level);
        if (!pte || WARN_ON(level != PG_LEVEL_4K) || pte_none(*pte))
                return;

        target_pte = pti_user_pagetable_walk_pte(VSYSCALL_ADDR);
        if (WARN_ON(!target_pte))
                return;

        *target_pte = *pte;
        set_vsyscall_pgtable_user_bits(kernel_to_user_pgdp(swapper_pg_dir));
}
#else
static void __init pti_setup_vsyscall(void) { }
#endif

enum pti_clone_level {
        PTI_CLONE_PMD,
        PTI_CLONE_PTE,
};

static void
pti_clone_pgtable(unsigned long start, unsigned long end,
                  enum pti_clone_level level)
{
        unsigned long addr;

        /*
         * Clone the populated PMDs which cover start to end. These PMD areas
         * can have holes.
         */
        for (addr = start; addr < end;) {
                pte_t *pte, *target_pte;
                pmd_t *pmd, *target_pmd;
                pgd_t *pgd;
                p4d_t *p4d;
                pud_t *pud;

                /* Overflow check */
                if (addr < start)
                        break;

                pgd = pgd_offset_k(addr);
                if (WARN_ON(pgd_none(*pgd)))
                        return;
                p4d = p4d_offset(pgd, addr);
                if (WARN_ON(p4d_none(*p4d)))
                        return;

                pud = pud_offset(p4d, addr);
                if (pud_none(*pud)) {
                        addr += PUD_SIZE;
                        continue;
                }

                pmd = pmd_offset(pud, addr);
                if (pmd_none(*pmd)) {
                        addr += PMD_SIZE;
                        continue;
                }

                if (pmd_large(*pmd) || level == PTI_CLONE_PMD) {
                        target_pmd = pti_user_pagetable_walk_pmd(addr);
                        if (WARN_ON(!target_pmd))
                                return;

                        /*
                         * Only clone present PMDs.  This ensures only setting
                         * _PAGE_GLOBAL on present PMDs.  This should only be
                         * called on well-known addresses anyway, so a non-
                         * present PMD would be a surprise.
                         */
                        if (WARN_ON(!(pmd_flags(*pmd) & _PAGE_PRESENT)))
                                return;

                        /*
                         * Setting 'target_pmd' below creates a mapping in both
                         * the user and kernel page tables.  It is effectively
                         * global, so set it as global in both copies.  Note:
                         * the X86_FEATURE_PGE check is not _required_ because
                         * the CPU ignores _PAGE_GLOBAL when PGE is not
                         * supported.  The check keeps consistentency with
                         * code that only set this bit when supported.
                         */
                        if (boot_cpu_has(X86_FEATURE_PGE))
                                *pmd = pmd_set_flags(*pmd, _PAGE_GLOBAL);

                        /*
                         * Copy the PMD.  That is, the kernelmode and usermode
                         * tables will share the last-level page tables of this
                         * address range
                         */
                        *target_pmd = *pmd;

                        addr += PMD_SIZE;

                } else if (level == PTI_CLONE_PTE) {

                        /* Walk the page-table down to the pte level */
                        pte = pte_offset_kernel(pmd, addr);
                        if (pte_none(*pte)) {
                                addr += PAGE_SIZE;
                                continue;
                        }

                        /* Only clone present PTEs */
                        if (WARN_ON(!(pte_flags(*pte) & _PAGE_PRESENT)))
                                return;

                        /* Allocate PTE in the user page-table */
                        target_pte = pti_user_pagetable_walk_pte(addr);
                        if (WARN_ON(!target_pte))
                                return;

                        /* Set GLOBAL bit in both PTEs */
                        if (boot_cpu_has(X86_FEATURE_PGE))
                                *pte = pte_set_flags(*pte, _PAGE_GLOBAL);

                        /* Clone the PTE */
                        *target_pte = *pte;

                        addr += PAGE_SIZE;

                } else {
                        BUG();
                }
        }
}

#ifdef CONFIG_X86_64
/*
 * Clone a single p4d (i.e. a top-level entry on 4-level systems and a
 * next-level entry on 5-level systems.
 */
static void __init pti_clone_p4d(unsigned long addr)
{
        p4d_t *kernel_p4d, *user_p4d;
        pgd_t *kernel_pgd;

        user_p4d = pti_user_pagetable_walk_p4d(addr);
        if (!user_p4d)
                return;

        kernel_pgd = pgd_offset_k(addr);
        kernel_p4d = p4d_offset(kernel_pgd, addr);
        *user_p4d = *kernel_p4d;
}

/*
 * Clone the CPU_ENTRY_AREA into the user space visible page table.
 */
static void __init pti_clone_user_shared(void)
{
        pti_clone_p4d(CPU_ENTRY_AREA_BASE);
}

#else /* CONFIG_X86_64 */

/*
 * On 32 bit PAE systems with 1GB of Kernel address space there is only
 * one pgd/p4d for the whole kernel. Cloning that would map the whole
 * address space into the user page-tables, making PTI useless. So clone
 * the page-table on the PMD level to prevent that.
 */
static void __init pti_clone_user_shared(void)
{
        unsigned long start, end;

        start = CPU_ENTRY_AREA_BASE;
        end   = start + (PAGE_SIZE * CPU_ENTRY_AREA_PAGES);

        pti_clone_pgtable(start, end, PTI_CLONE_PMD);
}
#endif /* CONFIG_X86_64 */

/*
 * Clone the ESPFIX P4D into the user space visinble page table
 */
static void __init pti_setup_espfix64(void)
{
#ifdef CONFIG_X86_ESPFIX64
        pti_clone_p4d(ESPFIX_BASE_ADDR);
#endif
}

/*
 * Clone the populated PMDs of the entry and irqentry text and force it RO.
 */
static void pti_clone_entry_text(void)
{
        pti_clone_pgtable((unsigned long) __entry_text_start,
                          (unsigned long) __irqentry_text_end,
                          PTI_CLONE_PMD);
}

/*
 * Global pages and PCIDs are both ways to make kernel TLB entries
 * live longer, reduce TLB misses and improve kernel performance.
 * But, leaving all kernel text Global makes it potentially accessible
 * to Meltdown-style attacks which make it trivial to find gadgets or
 * defeat KASLR.
 *
 * Only use global pages when it is really worth it.
 */
static inline bool pti_kernel_image_global_ok(void)
{
        /*
         * Systems with PCIDs get litlle benefit from global
         * kernel text and are not worth the downsides.
         */
        if (cpu_feature_enabled(X86_FEATURE_PCID))
                return false;

        /*
         * Only do global kernel image for pti=auto.  Do the most
         * secure thing (not global) if pti=on specified.
         */
        if (pti_mode != PTI_AUTO)
                return false;

        /*
         * K8 may not tolerate the cleared _PAGE_RW on the userspace
         * global kernel image pages.  Do the safe thing (disable
         * global kernel image).  This is unlikely to ever be
         * noticed because PTI is disabled by default on AMD CPUs.
         */
        if (boot_cpu_has(X86_FEATURE_K8))
                return false;

        /*
         * RANDSTRUCT derives its hardening benefits from the
         * attacker's lack of knowledge about the layout of kernel
         * data structures.  Keep the kernel image non-global in
         * cases where RANDSTRUCT is in use to help keep the layout a
         * secret.
         */
        if (IS_ENABLED(CONFIG_GCC_PLUGIN_RANDSTRUCT))
                return false;

        return true;
}

/*
 * This is the only user for these and it is not arch-generic
 * like the other set_memory.h functions.  Just extern them.
 */
extern int set_memory_nonglobal(unsigned long addr, int numpages);
extern int set_memory_global(unsigned long addr, int numpages);

/*
 * For some configurations, map all of kernel text into the user page
 * tables.  This reduces TLB misses, especially on non-PCID systems.
 */
static void pti_clone_kernel_text(void)
{
        /*
         * rodata is part of the kernel image and is normally
         * readable on the filesystem or on the web.  But, do not
         * clone the areas past rodata, they might contain secrets.
         */
        unsigned long start = PFN_ALIGN(_text);
        unsigned long end_clone  = (unsigned long)__end_rodata_aligned;
        unsigned long end_global = PFN_ALIGN((unsigned long)__stop___ex_table);

        if (!pti_kernel_image_global_ok())
                return;

        pr_debug("mapping partial kernel image into user address space\n");

        /*
         * Note that this will undo _some_ of the work that
         * pti_set_kernel_image_nonglobal() did to clear the
         * global bit.
         */
        pti_clone_pgtable(start, end_clone, PTI_LEVEL_KERNEL_IMAGE);

        /*
         * pti_clone_pgtable() will set the global bit in any PMDs
         * that it clones, but we also need to get any PTEs in
         * the last level for areas that are not huge-page-aligned.
         */

        /* Set the global bit for normal non-__init kernel text: */
        set_memory_global(start, (end_global - start) >> PAGE_SHIFT);
}

void pti_set_kernel_image_nonglobal(void)
{
        /*
         * The identity map is created with PMDs, regardless of the
         * actual length of the kernel.  We need to clear
         * _PAGE_GLOBAL up to a PMD boundary, not just to the end
         * of the image.
         */
        unsigned long start = PFN_ALIGN(_text);
        unsigned long end = ALIGN((unsigned long)_end, PMD_PAGE_SIZE);

        /*
         * This clears _PAGE_GLOBAL from the entire kernel image.
         * pti_clone_kernel_text() map put _PAGE_GLOBAL back for
         * areas that are mapped to userspace.
         */
        set_memory_nonglobal(start, (end - start) >> PAGE_SHIFT);
}

/*
 * Initialize kernel page table isolation
 */
void __init pti_init(void)
{
        if (!static_cpu_has(X86_FEATURE_PTI))
                return;

        pr_info("enabled\n");

#ifdef CONFIG_X86_32
        /*
         * We check for X86_FEATURE_PCID here. But the init-code will
         * clear the feature flag on 32 bit because the feature is not
         * supported on 32 bit anyway. To print the warning we need to
         * check with cpuid directly again.
         */
        if (cpuid_ecx(0x1) & BIT(17)) {
                /* Use printk to work around pr_fmt() */
                printk(KERN_WARNING "\n");
                printk(KERN_WARNING "************************************************************\n");
                printk(KERN_WARNING "** WARNING! WARNING! WARNING! WARNING! WARNING! WARNING!  **\n");
                printk(KERN_WARNING "**                                                        **\n");
                printk(KERN_WARNING "** You are using 32-bit PTI on a 64-bit PCID-capable CPU. **\n");
                printk(KERN_WARNING "** Your performance will increase dramatically if you     **\n");
                printk(KERN_WARNING "** switch to a 64-bit kernel!                             **\n");
                printk(KERN_WARNING "**                                                        **\n");
                printk(KERN_WARNING "** WARNING! WARNING! WARNING! WARNING! WARNING! WARNING!  **\n");
                printk(KERN_WARNING "************************************************************\n");
        }
#endif

        pti_clone_user_shared();

        /* Undo all global bits from the init pagetables in head_64.S: */
        pti_set_kernel_image_nonglobal();
        /* Replace some of the global bits just for shared entry text: */
        pti_clone_entry_text();
        pti_setup_espfix64();
        pti_setup_vsyscall();
}

/*
 * Finalize the kernel mappings in the userspace page-table. Some of the
 * mappings for the kernel image might have changed since pti_init()
 * cloned them. This is because parts of the kernel image have been
 * mapped RO and/or NX.  These changes need to be cloned again to the
 * userspace page-table.
 */
void pti_finalize(void)
{
        /*
         * We need to clone everything (again) that maps parts of the
         * kernel image.
         */
        pti_clone_entry_text();
        pti_clone_kernel_text();

        debug_checkwx_user();
}