2 * Copyright(c) 2017 Intel Corporation. All rights reserved.
4 * This program is free software; you can redistribute it and/or modify
5 * it under the terms of version 2 of the GNU General Public License as
6 * published by the Free Software Foundation.
8 * This program is distributed in the hope that it will be useful, but
9 * WITHOUT ANY WARRANTY; without even the implied warranty of
10 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
11 * General Public License for more details.
13 * This code is based in part on work published here:
15 * https://github.com/IAIK/KAISER
17 * The original work was written by and and signed off by for the Linux
20 * Signed-off-by: Richard Fellner <richard.fellner@student.tugraz.at>
21 * Signed-off-by: Moritz Lipp <moritz.lipp@iaik.tugraz.at>
22 * Signed-off-by: Daniel Gruss <daniel.gruss@iaik.tugraz.at>
23 * Signed-off-by: Michael Schwarz <michael.schwarz@iaik.tugraz.at>
25 * Major changes to the original code by: Dave Hansen <dave.hansen@intel.com>
26 * Mostly rewritten by Thomas Gleixner <tglx@linutronix.de> and
27 * Andy Lutomirsky <luto@amacapital.net>
29 #include <linux/kernel.h>
30 #include <linux/errno.h>
31 #include <linux/string.h>
32 #include <linux/types.h>
33 #include <linux/bug.h>
34 #include <linux/init.h>
35 #include <linux/spinlock.h>
37 #include <linux/uaccess.h>
39 #include <asm/cpufeature.h>
40 #include <asm/hypervisor.h>
41 #include <asm/vsyscall.h>
42 #include <asm/cmdline.h>
44 #include <asm/pgtable.h>
45 #include <asm/pgalloc.h>
46 #include <asm/tlbflush.h>
50 #define pr_fmt(fmt) "Kernel/User page tables isolation: " fmt
52 /* Backporting helper */
54 #define __GFP_NOTRACK 0
57 static void __init pti_print_if_insecure(const char *reason)
59 if (boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
60 pr_info("%s\n", reason);
63 static void __init pti_print_if_secure(const char *reason)
65 if (!boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
66 pr_info("%s\n", reason);
75 void __init pti_check_boottime_disable(void)
80 /* Assume mode is auto unless overridden. */
83 if (hypervisor_is_type(X86_HYPER_XEN_PV)) {
84 pti_mode = PTI_FORCE_OFF;
85 pti_print_if_insecure("disabled on XEN PV.");
89 ret = cmdline_find_option(boot_command_line, "pti", arg, sizeof(arg));
91 if (ret == 3 && !strncmp(arg, "off", 3)) {
92 pti_mode = PTI_FORCE_OFF;
93 pti_print_if_insecure("disabled on command line.");
96 if (ret == 2 && !strncmp(arg, "on", 2)) {
97 pti_mode = PTI_FORCE_ON;
98 pti_print_if_secure("force enabled on command line.");
101 if (ret == 4 && !strncmp(arg, "auto", 4)) {
107 if (cmdline_find_option_bool(boot_command_line, "nopti")) {
108 pti_mode = PTI_FORCE_OFF;
109 pti_print_if_insecure("disabled on command line.");
114 if (!boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
117 setup_force_cpu_cap(X86_FEATURE_PTI);
120 pgd_t __pti_set_user_pgd(pgd_t *pgdp, pgd_t pgd)
123 * Changes to the high (kernel) portion of the kernelmode page
124 * tables are not automatically propagated to the usermode tables.
126 * Users should keep in mind that, unlike the kernelmode tables,
127 * there is no vmalloc_fault equivalent for the usermode tables.
128 * Top-level entries added to init_mm's usermode pgd after boot
129 * will not be automatically propagated to other mms.
131 if (!pgdp_maps_userspace(pgdp))
135 * The user page tables get the full PGD, accessible from
138 kernel_to_user_pgdp(pgdp)->pgd = pgd.pgd;
141 * If this is normal user memory, make it NX in the kernel
142 * pagetables so that, if we somehow screw up and return to
143 * usermode with the kernel CR3 loaded, we'll get a page fault
144 * instead of allowing user code to execute with the wrong CR3.
146 * As exceptions, we don't set NX if:
147 * - _PAGE_USER is not set. This could be an executable
148 * EFI runtime mapping or something similar, and the kernel
149 * may execute from it
150 * - we don't have NX support
151 * - we're clearing the PGD (i.e. the new pgd is not present).
153 if ((pgd.pgd & (_PAGE_USER|_PAGE_PRESENT)) == (_PAGE_USER|_PAGE_PRESENT) &&
154 (__supported_pte_mask & _PAGE_NX))
157 /* return the copy of the PGD we want the kernel to use: */
162 * Walk the user copy of the page tables (optionally) trying to allocate
163 * page table pages on the way down.
165 * Returns a pointer to a P4D on success, or NULL on failure.
167 static p4d_t *pti_user_pagetable_walk_p4d(unsigned long address)
169 pgd_t *pgd = kernel_to_user_pgdp(pgd_offset_k(address));
170 gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
172 if (address < PAGE_OFFSET) {
173 WARN_ONCE(1, "attempt to walk user address\n");
177 if (pgd_none(*pgd)) {
178 unsigned long new_p4d_page = __get_free_page(gfp);
182 set_pgd(pgd, __pgd(_KERNPG_TABLE | __pa(new_p4d_page)));
184 BUILD_BUG_ON(pgd_large(*pgd) != 0);
186 return p4d_offset(pgd, address);
190 * Walk the user copy of the page tables (optionally) trying to allocate
191 * page table pages on the way down.
193 * Returns a pointer to a PMD on success, or NULL on failure.
195 static pmd_t *pti_user_pagetable_walk_pmd(unsigned long address)
197 gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
198 p4d_t *p4d = pti_user_pagetable_walk_p4d(address);
201 BUILD_BUG_ON(p4d_large(*p4d) != 0);
202 if (p4d_none(*p4d)) {
203 unsigned long new_pud_page = __get_free_page(gfp);
207 set_p4d(p4d, __p4d(_KERNPG_TABLE | __pa(new_pud_page)));
210 pud = pud_offset(p4d, address);
211 /* The user page tables do not use large mappings: */
212 if (pud_large(*pud)) {
216 if (pud_none(*pud)) {
217 unsigned long new_pmd_page = __get_free_page(gfp);
221 set_pud(pud, __pud(_KERNPG_TABLE | __pa(new_pmd_page)));
224 return pmd_offset(pud, address);
227 #ifdef CONFIG_X86_VSYSCALL_EMULATION
229 * Walk the shadow copy of the page tables (optionally) trying to allocate
230 * page table pages on the way down. Does not support large pages.
232 * Note: this is only used when mapping *new* kernel data into the
233 * user/shadow page tables. It is never used for userspace data.
235 * Returns a pointer to a PTE on success, or NULL on failure.
237 static __init pte_t *pti_user_pagetable_walk_pte(unsigned long address)
239 gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
240 pmd_t *pmd = pti_user_pagetable_walk_pmd(address);
243 /* We can't do anything sensible if we hit a large mapping. */
244 if (pmd_large(*pmd)) {
249 if (pmd_none(*pmd)) {
250 unsigned long new_pte_page = __get_free_page(gfp);
254 set_pmd(pmd, __pmd(_KERNPG_TABLE | __pa(new_pte_page)));
257 pte = pte_offset_kernel(pmd, address);
258 if (pte_flags(*pte) & _PAGE_USER) {
259 WARN_ONCE(1, "attempt to walk to user pte\n");
265 static void __init pti_setup_vsyscall(void)
267 pte_t *pte, *target_pte;
270 pte = lookup_address(VSYSCALL_ADDR, &level);
271 if (!pte || WARN_ON(level != PG_LEVEL_4K) || pte_none(*pte))
274 target_pte = pti_user_pagetable_walk_pte(VSYSCALL_ADDR);
275 if (WARN_ON(!target_pte))
279 set_vsyscall_pgtable_user_bits(kernel_to_user_pgdp(swapper_pg_dir));
282 static void __init pti_setup_vsyscall(void) { }
286 pti_clone_pmds(unsigned long start, unsigned long end, pmdval_t clear)
291 * Clone the populated PMDs which cover start to end. These PMD areas
294 for (addr = start; addr < end; addr += PMD_SIZE) {
295 pmd_t *pmd, *target_pmd;
300 pgd = pgd_offset_k(addr);
301 if (WARN_ON(pgd_none(*pgd)))
303 p4d = p4d_offset(pgd, addr);
304 if (WARN_ON(p4d_none(*p4d)))
306 pud = pud_offset(p4d, addr);
309 pmd = pmd_offset(pud, addr);
313 target_pmd = pti_user_pagetable_walk_pmd(addr);
314 if (WARN_ON(!target_pmd))
318 * Only clone present PMDs. This ensures only setting
319 * _PAGE_GLOBAL on present PMDs. This should only be
320 * called on well-known addresses anyway, so a non-
321 * present PMD would be a surprise.
323 if (WARN_ON(!(pmd_flags(*pmd) & _PAGE_PRESENT)))
327 * Setting 'target_pmd' below creates a mapping in both
328 * the user and kernel page tables. It is effectively
329 * global, so set it as global in both copies. Note:
330 * the X86_FEATURE_PGE check is not _required_ because
331 * the CPU ignores _PAGE_GLOBAL when PGE is not
332 * supported. The check keeps consistentency with
333 * code that only set this bit when supported.
335 if (boot_cpu_has(X86_FEATURE_PGE))
336 *pmd = pmd_set_flags(*pmd, _PAGE_GLOBAL);
339 * Copy the PMD. That is, the kernelmode and usermode
340 * tables will share the last-level page tables of this
343 *target_pmd = pmd_clear_flags(*pmd, clear);
348 * Clone a single p4d (i.e. a top-level entry on 4-level systems and a
349 * next-level entry on 5-level systems.
351 static void __init pti_clone_p4d(unsigned long addr)
353 p4d_t *kernel_p4d, *user_p4d;
356 user_p4d = pti_user_pagetable_walk_p4d(addr);
357 kernel_pgd = pgd_offset_k(addr);
358 kernel_p4d = p4d_offset(kernel_pgd, addr);
359 *user_p4d = *kernel_p4d;
363 * Clone the CPU_ENTRY_AREA into the user space visible page table.
365 static void __init pti_clone_user_shared(void)
367 pti_clone_p4d(CPU_ENTRY_AREA_BASE);
371 * Clone the ESPFIX P4D into the user space visible page table
373 static void __init pti_setup_espfix64(void)
375 #ifdef CONFIG_X86_ESPFIX64
376 pti_clone_p4d(ESPFIX_BASE_ADDR);
381 * Clone the populated PMDs of the entry and irqentry text and force it RO.
383 static void __init pti_clone_entry_text(void)
385 pti_clone_pmds((unsigned long) __entry_text_start,
386 (unsigned long) __irqentry_text_end,
391 * Global pages and PCIDs are both ways to make kernel TLB entries
392 * live longer, reduce TLB misses and improve kernel performance.
393 * But, leaving all kernel text Global makes it potentially accessible
394 * to Meltdown-style attacks which make it trivial to find gadgets or
397 * Only use global pages when it is really worth it.
399 static inline bool pti_kernel_image_global_ok(void)
402 * Systems with PCIDs get litlle benefit from global
403 * kernel text and are not worth the downsides.
405 if (cpu_feature_enabled(X86_FEATURE_PCID))
409 * Only do global kernel image for pti=auto. Do the most
410 * secure thing (not global) if pti=on specified.
412 if (pti_mode != PTI_AUTO)
416 * K8 may not tolerate the cleared _PAGE_RW on the userspace
417 * global kernel image pages. Do the safe thing (disable
418 * global kernel image). This is unlikely to ever be
419 * noticed because PTI is disabled by default on AMD CPUs.
421 if (boot_cpu_has(X86_FEATURE_K8))
425 * RANDSTRUCT derives its hardening benefits from the
426 * attacker's lack of knowledge about the layout of kernel
427 * data structures. Keep the kernel image non-global in
428 * cases where RANDSTRUCT is in use to help keep the layout a
431 if (IS_ENABLED(CONFIG_GCC_PLUGIN_RANDSTRUCT))
438 * This is the only user for these and it is not arch-generic
439 * like the other set_memory.h functions. Just extern them.
441 extern int set_memory_nonglobal(unsigned long addr, int numpages);
442 extern int set_memory_global(unsigned long addr, int numpages);
445 * For some configurations, map all of kernel text into the user page
446 * tables. This reduces TLB misses, especially on non-PCID systems.
448 void pti_clone_kernel_text(void)
451 * rodata is part of the kernel image and is normally
452 * readable on the filesystem or on the web. But, do not
453 * clone the areas past rodata, they might contain secrets.
455 unsigned long start = PFN_ALIGN(_text);
456 unsigned long end_clone = (unsigned long)__end_rodata_hpage_align;
457 unsigned long end_global = PFN_ALIGN((unsigned long)__stop___ex_table);
459 if (!pti_kernel_image_global_ok())
462 pr_debug("mapping partial kernel image into user address space\n");
465 * Note that this will undo _some_ of the work that
466 * pti_set_kernel_image_nonglobal() did to clear the
469 pti_clone_pmds(start, end_clone, _PAGE_RW);
472 * pti_clone_pmds() will set the global bit in any PMDs
473 * that it clones, but we also need to get any PTEs in
474 * the last level for areas that are not huge-page-aligned.
477 /* Set the global bit for normal non-__init kernel text: */
478 set_memory_global(start, (end_global - start) >> PAGE_SHIFT);
481 void pti_set_kernel_image_nonglobal(void)
484 * The identity map is created with PMDs, regardless of the
485 * actual length of the kernel. We need to clear
486 * _PAGE_GLOBAL up to a PMD boundary, not just to the end
489 unsigned long start = PFN_ALIGN(_text);
490 unsigned long end = ALIGN((unsigned long)_end, PMD_PAGE_SIZE);
493 * This clears _PAGE_GLOBAL from the entire kernel image.
494 * pti_clone_kernel_text() map put _PAGE_GLOBAL back for
495 * areas that are mapped to userspace.
497 set_memory_nonglobal(start, (end - start) >> PAGE_SHIFT);
501 * Initialize kernel page table isolation
503 void __init pti_init(void)
505 if (!static_cpu_has(X86_FEATURE_PTI))
508 pr_info("enabled\n");
510 pti_clone_user_shared();
512 /* Undo all global bits from the init pagetables in head_64.S: */
513 pti_set_kernel_image_nonglobal();
514 /* Replace some of the global bits just for shared entry text: */
515 pti_clone_entry_text();
516 pti_setup_espfix64();
517 pti_setup_vsyscall();