1 // SPDX-License-Identifier: GPL-2.0-only
3 * Copyright(c) 2017 Intel Corporation. All rights reserved.
5 * This code is based in part on work published here:
7 * https://github.com/IAIK/KAISER
9 * The original work was written by and and signed off by for the Linux
12 * Signed-off-by: Richard Fellner <richard.fellner@student.tugraz.at>
13 * Signed-off-by: Moritz Lipp <moritz.lipp@iaik.tugraz.at>
14 * Signed-off-by: Daniel Gruss <daniel.gruss@iaik.tugraz.at>
15 * Signed-off-by: Michael Schwarz <michael.schwarz@iaik.tugraz.at>
17 * Major changes to the original code by: Dave Hansen <dave.hansen@intel.com>
18 * Mostly rewritten by Thomas Gleixner <tglx@linutronix.de> and
19 * Andy Lutomirsky <luto@amacapital.net>
21 #include <linux/kernel.h>
22 #include <linux/errno.h>
23 #include <linux/string.h>
24 #include <linux/types.h>
25 #include <linux/bug.h>
26 #include <linux/init.h>
27 #include <linux/spinlock.h>
29 #include <linux/uaccess.h>
30 #include <linux/cpu.h>
32 #include <asm/cpufeature.h>
33 #include <asm/hypervisor.h>
34 #include <asm/vsyscall.h>
35 #include <asm/cmdline.h>
37 #include <asm/tlbflush.h>
39 #include <asm/sections.h>
40 #include <asm/set_memory.h>
43 #define pr_fmt(fmt) "Kernel/User page tables isolation: " fmt
45 /* Backporting helper */
47 #define __GFP_NOTRACK 0
51 * Define the page-table levels we clone for user-space on 32
55 #define PTI_LEVEL_KERNEL_IMAGE PTI_CLONE_PMD
57 #define PTI_LEVEL_KERNEL_IMAGE PTI_CLONE_PTE
60 static void __init pti_print_if_insecure(const char *reason)
62 if (boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
63 pr_info("%s\n", reason);
66 static void __init pti_print_if_secure(const char *reason)
68 if (!boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
69 pr_info("%s\n", reason);
72 static enum pti_mode {
78 void __init pti_check_boottime_disable(void)
83 /* Assume mode is auto unless overridden. */
86 if (hypervisor_is_type(X86_HYPER_XEN_PV)) {
87 pti_mode = PTI_FORCE_OFF;
88 pti_print_if_insecure("disabled on XEN PV.");
92 ret = cmdline_find_option(boot_command_line, "pti", arg, sizeof(arg));
94 if (ret == 3 && !strncmp(arg, "off", 3)) {
95 pti_mode = PTI_FORCE_OFF;
96 pti_print_if_insecure("disabled on command line.");
99 if (ret == 2 && !strncmp(arg, "on", 2)) {
100 pti_mode = PTI_FORCE_ON;
101 pti_print_if_secure("force enabled on command line.");
104 if (ret == 4 && !strncmp(arg, "auto", 4)) {
110 if (cmdline_find_option_bool(boot_command_line, "nopti") ||
111 cpu_mitigations_off()) {
112 pti_mode = PTI_FORCE_OFF;
113 pti_print_if_insecure("disabled on command line.");
118 if (!boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
121 setup_force_cpu_cap(X86_FEATURE_PTI);
124 pgd_t __pti_set_user_pgtbl(pgd_t *pgdp, pgd_t pgd)
127 * Changes to the high (kernel) portion of the kernelmode page
128 * tables are not automatically propagated to the usermode tables.
130 * Users should keep in mind that, unlike the kernelmode tables,
131 * there is no vmalloc_fault equivalent for the usermode tables.
132 * Top-level entries added to init_mm's usermode pgd after boot
133 * will not be automatically propagated to other mms.
135 if (!pgdp_maps_userspace(pgdp))
139 * The user page tables get the full PGD, accessible from
142 kernel_to_user_pgdp(pgdp)->pgd = pgd.pgd;
145 * If this is normal user memory, make it NX in the kernel
146 * pagetables so that, if we somehow screw up and return to
147 * usermode with the kernel CR3 loaded, we'll get a page fault
148 * instead of allowing user code to execute with the wrong CR3.
150 * As exceptions, we don't set NX if:
151 * - _PAGE_USER is not set. This could be an executable
152 * EFI runtime mapping or something similar, and the kernel
153 * may execute from it
154 * - we don't have NX support
155 * - we're clearing the PGD (i.e. the new pgd is not present).
157 if ((pgd.pgd & (_PAGE_USER|_PAGE_PRESENT)) == (_PAGE_USER|_PAGE_PRESENT) &&
158 (__supported_pte_mask & _PAGE_NX))
161 /* return the copy of the PGD we want the kernel to use: */
166 * Walk the user copy of the page tables (optionally) trying to allocate
167 * page table pages on the way down.
169 * Returns a pointer to a P4D on success, or NULL on failure.
171 static p4d_t *pti_user_pagetable_walk_p4d(unsigned long address)
173 pgd_t *pgd = kernel_to_user_pgdp(pgd_offset_k(address));
174 gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
176 if (address < PAGE_OFFSET) {
177 WARN_ONCE(1, "attempt to walk user address\n");
181 if (pgd_none(*pgd)) {
182 unsigned long new_p4d_page = __get_free_page(gfp);
183 if (WARN_ON_ONCE(!new_p4d_page))
186 set_pgd(pgd, __pgd(_KERNPG_TABLE | __pa(new_p4d_page)));
188 BUILD_BUG_ON(pgd_large(*pgd) != 0);
190 return p4d_offset(pgd, address);
194 * Walk the user copy of the page tables (optionally) trying to allocate
195 * page table pages on the way down.
197 * Returns a pointer to a PMD on success, or NULL on failure.
199 static pmd_t *pti_user_pagetable_walk_pmd(unsigned long address)
201 gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
205 p4d = pti_user_pagetable_walk_p4d(address);
209 BUILD_BUG_ON(p4d_large(*p4d) != 0);
210 if (p4d_none(*p4d)) {
211 unsigned long new_pud_page = __get_free_page(gfp);
212 if (WARN_ON_ONCE(!new_pud_page))
215 set_p4d(p4d, __p4d(_KERNPG_TABLE | __pa(new_pud_page)));
218 pud = pud_offset(p4d, address);
219 /* The user page tables do not use large mappings: */
220 if (pud_large(*pud)) {
224 if (pud_none(*pud)) {
225 unsigned long new_pmd_page = __get_free_page(gfp);
226 if (WARN_ON_ONCE(!new_pmd_page))
229 set_pud(pud, __pud(_KERNPG_TABLE | __pa(new_pmd_page)));
232 return pmd_offset(pud, address);
236 * Walk the shadow copy of the page tables (optionally) trying to allocate
237 * page table pages on the way down. Does not support large pages.
239 * Note: this is only used when mapping *new* kernel data into the
240 * user/shadow page tables. It is never used for userspace data.
242 * Returns a pointer to a PTE on success, or NULL on failure.
244 static pte_t *pti_user_pagetable_walk_pte(unsigned long address)
246 gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
250 pmd = pti_user_pagetable_walk_pmd(address);
254 /* We can't do anything sensible if we hit a large mapping. */
255 if (pmd_large(*pmd)) {
260 if (pmd_none(*pmd)) {
261 unsigned long new_pte_page = __get_free_page(gfp);
265 set_pmd(pmd, __pmd(_KERNPG_TABLE | __pa(new_pte_page)));
268 pte = pte_offset_kernel(pmd, address);
269 if (pte_flags(*pte) & _PAGE_USER) {
270 WARN_ONCE(1, "attempt to walk to user pte\n");
276 #ifdef CONFIG_X86_VSYSCALL_EMULATION
277 static void __init pti_setup_vsyscall(void)
279 pte_t *pte, *target_pte;
282 pte = lookup_address(VSYSCALL_ADDR, &level);
283 if (!pte || WARN_ON(level != PG_LEVEL_4K) || pte_none(*pte))
286 target_pte = pti_user_pagetable_walk_pte(VSYSCALL_ADDR);
287 if (WARN_ON(!target_pte))
291 set_vsyscall_pgtable_user_bits(kernel_to_user_pgdp(swapper_pg_dir));
294 static void __init pti_setup_vsyscall(void) { }
297 enum pti_clone_level {
303 pti_clone_pgtable(unsigned long start, unsigned long end,
304 enum pti_clone_level level)
309 * Clone the populated PMDs which cover start to end. These PMD areas
312 for (addr = start; addr < end;) {
313 pte_t *pte, *target_pte;
314 pmd_t *pmd, *target_pmd;
323 pgd = pgd_offset_k(addr);
324 if (WARN_ON(pgd_none(*pgd)))
326 p4d = p4d_offset(pgd, addr);
327 if (WARN_ON(p4d_none(*p4d)))
330 pud = pud_offset(p4d, addr);
331 if (pud_none(*pud)) {
332 WARN_ON_ONCE(addr & ~PUD_MASK);
333 addr = round_up(addr + 1, PUD_SIZE);
337 pmd = pmd_offset(pud, addr);
338 if (pmd_none(*pmd)) {
339 WARN_ON_ONCE(addr & ~PMD_MASK);
340 addr = round_up(addr + 1, PMD_SIZE);
344 if (pmd_large(*pmd) || level == PTI_CLONE_PMD) {
345 target_pmd = pti_user_pagetable_walk_pmd(addr);
346 if (WARN_ON(!target_pmd))
350 * Only clone present PMDs. This ensures only setting
351 * _PAGE_GLOBAL on present PMDs. This should only be
352 * called on well-known addresses anyway, so a non-
353 * present PMD would be a surprise.
355 if (WARN_ON(!(pmd_flags(*pmd) & _PAGE_PRESENT)))
359 * Setting 'target_pmd' below creates a mapping in both
360 * the user and kernel page tables. It is effectively
361 * global, so set it as global in both copies. Note:
362 * the X86_FEATURE_PGE check is not _required_ because
363 * the CPU ignores _PAGE_GLOBAL when PGE is not
364 * supported. The check keeps consistentency with
365 * code that only set this bit when supported.
367 if (boot_cpu_has(X86_FEATURE_PGE))
368 *pmd = pmd_set_flags(*pmd, _PAGE_GLOBAL);
371 * Copy the PMD. That is, the kernelmode and usermode
372 * tables will share the last-level page tables of this
379 } else if (level == PTI_CLONE_PTE) {
381 /* Walk the page-table down to the pte level */
382 pte = pte_offset_kernel(pmd, addr);
383 if (pte_none(*pte)) {
388 /* Only clone present PTEs */
389 if (WARN_ON(!(pte_flags(*pte) & _PAGE_PRESENT)))
392 /* Allocate PTE in the user page-table */
393 target_pte = pti_user_pagetable_walk_pte(addr);
394 if (WARN_ON(!target_pte))
397 /* Set GLOBAL bit in both PTEs */
398 if (boot_cpu_has(X86_FEATURE_PGE))
399 *pte = pte_set_flags(*pte, _PAGE_GLOBAL);
414 * Clone a single p4d (i.e. a top-level entry on 4-level systems and a
415 * next-level entry on 5-level systems.
417 static void __init pti_clone_p4d(unsigned long addr)
419 p4d_t *kernel_p4d, *user_p4d;
422 user_p4d = pti_user_pagetable_walk_p4d(addr);
426 kernel_pgd = pgd_offset_k(addr);
427 kernel_p4d = p4d_offset(kernel_pgd, addr);
428 *user_p4d = *kernel_p4d;
432 * Clone the CPU_ENTRY_AREA and associated data into the user space visible
435 static void __init pti_clone_user_shared(void)
439 pti_clone_p4d(CPU_ENTRY_AREA_BASE);
441 for_each_possible_cpu(cpu) {
443 * The SYSCALL64 entry code needs to be able to find the
444 * thread stack and needs one word of scratch space in which
445 * to spill a register. All of this lives in the TSS, in
446 * the sp1 and sp2 slots.
448 * This is done for all possible CPUs during boot to ensure
449 * that it's propagated to all mms.
452 unsigned long va = (unsigned long)&per_cpu(cpu_tss_rw, cpu);
453 phys_addr_t pa = per_cpu_ptr_to_phys((void *)va);
456 target_pte = pti_user_pagetable_walk_pte(va);
457 if (WARN_ON(!target_pte))
460 *target_pte = pfn_pte(pa >> PAGE_SHIFT, PAGE_KERNEL);
464 #else /* CONFIG_X86_64 */
467 * On 32 bit PAE systems with 1GB of Kernel address space there is only
468 * one pgd/p4d for the whole kernel. Cloning that would map the whole
469 * address space into the user page-tables, making PTI useless. So clone
470 * the page-table on the PMD level to prevent that.
472 static void __init pti_clone_user_shared(void)
474 unsigned long start, end;
476 start = CPU_ENTRY_AREA_BASE;
477 end = start + (PAGE_SIZE * CPU_ENTRY_AREA_PAGES);
479 pti_clone_pgtable(start, end, PTI_CLONE_PMD);
481 #endif /* CONFIG_X86_64 */
484 * Clone the ESPFIX P4D into the user space visible page table
486 static void __init pti_setup_espfix64(void)
488 #ifdef CONFIG_X86_ESPFIX64
489 pti_clone_p4d(ESPFIX_BASE_ADDR);
494 * Clone the populated PMDs of the entry text and force it RO.
496 static void pti_clone_entry_text(void)
498 pti_clone_pgtable((unsigned long) __entry_text_start,
499 (unsigned long) __entry_text_end,
504 * Global pages and PCIDs are both ways to make kernel TLB entries
505 * live longer, reduce TLB misses and improve kernel performance.
506 * But, leaving all kernel text Global makes it potentially accessible
507 * to Meltdown-style attacks which make it trivial to find gadgets or
510 * Only use global pages when it is really worth it.
512 static inline bool pti_kernel_image_global_ok(void)
515 * Systems with PCIDs get litlle benefit from global
516 * kernel text and are not worth the downsides.
518 if (cpu_feature_enabled(X86_FEATURE_PCID))
522 * Only do global kernel image for pti=auto. Do the most
523 * secure thing (not global) if pti=on specified.
525 if (pti_mode != PTI_AUTO)
529 * K8 may not tolerate the cleared _PAGE_RW on the userspace
530 * global kernel image pages. Do the safe thing (disable
531 * global kernel image). This is unlikely to ever be
532 * noticed because PTI is disabled by default on AMD CPUs.
534 if (boot_cpu_has(X86_FEATURE_K8))
538 * RANDSTRUCT derives its hardening benefits from the
539 * attacker's lack of knowledge about the layout of kernel
540 * data structures. Keep the kernel image non-global in
541 * cases where RANDSTRUCT is in use to help keep the layout a
544 if (IS_ENABLED(CONFIG_GCC_PLUGIN_RANDSTRUCT))
551 * For some configurations, map all of kernel text into the user page
552 * tables. This reduces TLB misses, especially on non-PCID systems.
554 static void pti_clone_kernel_text(void)
557 * rodata is part of the kernel image and is normally
558 * readable on the filesystem or on the web. But, do not
559 * clone the areas past rodata, they might contain secrets.
561 unsigned long start = PFN_ALIGN(_text);
562 unsigned long end_clone = (unsigned long)__end_rodata_aligned;
563 unsigned long end_global = PFN_ALIGN((unsigned long)_etext);
565 if (!pti_kernel_image_global_ok())
568 pr_debug("mapping partial kernel image into user address space\n");
571 * Note that this will undo _some_ of the work that
572 * pti_set_kernel_image_nonglobal() did to clear the
575 pti_clone_pgtable(start, end_clone, PTI_LEVEL_KERNEL_IMAGE);
578 * pti_clone_pgtable() will set the global bit in any PMDs
579 * that it clones, but we also need to get any PTEs in
580 * the last level for areas that are not huge-page-aligned.
583 /* Set the global bit for normal non-__init kernel text: */
584 set_memory_global(start, (end_global - start) >> PAGE_SHIFT);
587 static void pti_set_kernel_image_nonglobal(void)
590 * The identity map is created with PMDs, regardless of the
591 * actual length of the kernel. We need to clear
592 * _PAGE_GLOBAL up to a PMD boundary, not just to the end
595 unsigned long start = PFN_ALIGN(_text);
596 unsigned long end = ALIGN((unsigned long)_end, PMD_PAGE_SIZE);
599 * This clears _PAGE_GLOBAL from the entire kernel image.
600 * pti_clone_kernel_text() map put _PAGE_GLOBAL back for
601 * areas that are mapped to userspace.
603 set_memory_nonglobal(start, (end - start) >> PAGE_SHIFT);
607 * Initialize kernel page table isolation
609 void __init pti_init(void)
611 if (!boot_cpu_has(X86_FEATURE_PTI))
614 pr_info("enabled\n");
618 * We check for X86_FEATURE_PCID here. But the init-code will
619 * clear the feature flag on 32 bit because the feature is not
620 * supported on 32 bit anyway. To print the warning we need to
621 * check with cpuid directly again.
623 if (cpuid_ecx(0x1) & BIT(17)) {
624 /* Use printk to work around pr_fmt() */
625 printk(KERN_WARNING "\n");
626 printk(KERN_WARNING "************************************************************\n");
627 printk(KERN_WARNING "** WARNING! WARNING! WARNING! WARNING! WARNING! WARNING! **\n");
628 printk(KERN_WARNING "** **\n");
629 printk(KERN_WARNING "** You are using 32-bit PTI on a 64-bit PCID-capable CPU. **\n");
630 printk(KERN_WARNING "** Your performance will increase dramatically if you **\n");
631 printk(KERN_WARNING "** switch to a 64-bit kernel! **\n");
632 printk(KERN_WARNING "** **\n");
633 printk(KERN_WARNING "** WARNING! WARNING! WARNING! WARNING! WARNING! WARNING! **\n");
634 printk(KERN_WARNING "************************************************************\n");
638 pti_clone_user_shared();
640 /* Undo all global bits from the init pagetables in head_64.S: */
641 pti_set_kernel_image_nonglobal();
642 /* Replace some of the global bits just for shared entry text: */
643 pti_clone_entry_text();
644 pti_setup_espfix64();
645 pti_setup_vsyscall();
649 * Finalize the kernel mappings in the userspace page-table. Some of the
650 * mappings for the kernel image might have changed since pti_init()
651 * cloned them. This is because parts of the kernel image have been
652 * mapped RO and/or NX. These changes need to be cloned again to the
653 * userspace page-table.
655 void pti_finalize(void)
657 if (!boot_cpu_has(X86_FEATURE_PTI))
660 * We need to clone everything (again) that maps parts of the
663 pti_clone_entry_text();
664 pti_clone_kernel_text();
666 debug_checkwx_user();