swh:1:snp:77163734605b0ec556b01d897b7bb4a7e30d46b6
Tip revision: 754e0b0e35608ed5206d6a67a791563c631cec07 authored by Linus Torvalds on 13 February 2022, 20:13:30 UTC
Linux 5.17-rc4
Linux 5.17-rc4
Tip revision: 754e0b0
pti.c
// SPDX-License-Identifier: GPL-2.0-only
/*
* Copyright(c) 2017 Intel Corporation. All rights reserved.
*
* 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 <linux/cpu.h>
#include <asm/cpufeature.h>
#include <asm/hypervisor.h>
#include <asm/vsyscall.h>
#include <asm/cmdline.h>
#include <asm/pti.h>
#include <asm/tlbflush.h>
#include <asm/desc.h>
#include <asm/sections.h>
#include <asm/set_memory.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);
}
static 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") ||
cpu_mitigations_off()) {
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 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)) {
WARN_ON_ONCE(addr & ~PUD_MASK);
addr = round_up(addr + 1, PUD_SIZE);
continue;
}
pmd = pmd_offset(pud, addr);
if (pmd_none(*pmd)) {
WARN_ON_ONCE(addr & ~PMD_MASK);
addr = round_up(addr + 1, 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 consistency 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 and associated data into the user space visible
* page table.
*/
static void __init pti_clone_user_shared(void)
{
unsigned int cpu;
pti_clone_p4d(CPU_ENTRY_AREA_BASE);
for_each_possible_cpu(cpu) {
/*
* The SYSCALL64 entry code needs one word of scratch space
* in which to spill a register. It lives in the sp2 slot
* of the CPU's TSS.
*
* This is done for all possible CPUs during boot to ensure
* that it's propagated to all mms.
*/
unsigned long va = (unsigned long)&per_cpu(cpu_tss_rw, cpu);
phys_addr_t pa = per_cpu_ptr_to_phys((void *)va);
pte_t *target_pte;
target_pte = pti_user_pagetable_walk_pte(va);
if (WARN_ON(!target_pte))
return;
*target_pte = pfn_pte(pa >> PAGE_SHIFT, PAGE_KERNEL);
}
}
#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 visible 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 text and force it RO.
*/
static void pti_clone_entry_text(void)
{
pti_clone_pgtable((unsigned long) __entry_text_start,
(unsigned long) __entry_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 little 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;
}
/*
* 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)_etext);
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);
}
static 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 (!boot_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)
{
if (!boot_cpu_has(X86_FEATURE_PTI))
return;
/*
* We need to clone everything (again) that maps parts of the
* kernel image.
*/
pti_clone_entry_text();
pti_clone_kernel_text();
debug_checkwx_user();
}