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Linux/arch/x86/mm/pti.c

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  1 /*
  2  * Copyright(c) 2017 Intel Corporation. All rights reserved.
  3  *
  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.
  7  *
  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.
 12  *
 13  * This code is based in part on work published here:
 14  *
 15  *      https://github.com/IAIK/KAISER
 16  *
 17  * The original work was written by and and signed off by for the Linux
 18  * kernel by:
 19  *
 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>
 24  *
 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>
 28  */
 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>
 36 #include <linux/mm.h>
 37 #include <linux/uaccess.h>
 38 
 39 #include <asm/cpufeature.h>
 40 #include <asm/hypervisor.h>
 41 #include <asm/vsyscall.h>
 42 #include <asm/cmdline.h>
 43 #include <asm/pti.h>
 44 #include <asm/pgtable.h>
 45 #include <asm/pgalloc.h>
 46 #include <asm/tlbflush.h>
 47 #include <asm/desc.h>
 48 
 49 #undef pr_fmt
 50 #define pr_fmt(fmt)     "Kernel/User page tables isolation: " fmt
 51 
 52 /* Backporting helper */
 53 #ifndef __GFP_NOTRACK
 54 #define __GFP_NOTRACK   0
 55 #endif
 56 
 57 static void __init pti_print_if_insecure(const char *reason)
 58 {
 59         if (boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
 60                 pr_info("%s\n", reason);
 61 }
 62 
 63 static void __init pti_print_if_secure(const char *reason)
 64 {
 65         if (!boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
 66                 pr_info("%s\n", reason);
 67 }
 68 
 69 void __init pti_check_boottime_disable(void)
 70 {
 71         char arg[5];
 72         int ret;
 73 
 74         if (hypervisor_is_type(X86_HYPER_XEN_PV)) {
 75                 pti_print_if_insecure("disabled on XEN PV.");
 76                 return;
 77         }
 78 
 79         ret = cmdline_find_option(boot_command_line, "pti", arg, sizeof(arg));
 80         if (ret > 0)  {
 81                 if (ret == 3 && !strncmp(arg, "off", 3)) {
 82                         pti_print_if_insecure("disabled on command line.");
 83                         return;
 84                 }
 85                 if (ret == 2 && !strncmp(arg, "on", 2)) {
 86                         pti_print_if_secure("force enabled on command line.");
 87                         goto enable;
 88                 }
 89                 if (ret == 4 && !strncmp(arg, "auto", 4))
 90                         goto autosel;
 91         }
 92 
 93         if (cmdline_find_option_bool(boot_command_line, "nopti")) {
 94                 pti_print_if_insecure("disabled on command line.");
 95                 return;
 96         }
 97 
 98 autosel:
 99         if (!boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
100                 return;
101 enable:
102         setup_force_cpu_cap(X86_FEATURE_PTI);
103 }
104 
105 pgd_t __pti_set_user_pgd(pgd_t *pgdp, pgd_t pgd)
106 {
107         /*
108          * Changes to the high (kernel) portion of the kernelmode page
109          * tables are not automatically propagated to the usermode tables.
110          *
111          * Users should keep in mind that, unlike the kernelmode tables,
112          * there is no vmalloc_fault equivalent for the usermode tables.
113          * Top-level entries added to init_mm's usermode pgd after boot
114          * will not be automatically propagated to other mms.
115          */
116         if (!pgdp_maps_userspace(pgdp))
117                 return pgd;
118 
119         /*
120          * The user page tables get the full PGD, accessible from
121          * userspace:
122          */
123         kernel_to_user_pgdp(pgdp)->pgd = pgd.pgd;
124 
125         /*
126          * If this is normal user memory, make it NX in the kernel
127          * pagetables so that, if we somehow screw up and return to
128          * usermode with the kernel CR3 loaded, we'll get a page fault
129          * instead of allowing user code to execute with the wrong CR3.
130          *
131          * As exceptions, we don't set NX if:
132          *  - _PAGE_USER is not set.  This could be an executable
133          *     EFI runtime mapping or something similar, and the kernel
134          *     may execute from it
135          *  - we don't have NX support
136          *  - we're clearing the PGD (i.e. the new pgd is not present).
137          */
138         if ((pgd.pgd & (_PAGE_USER|_PAGE_PRESENT)) == (_PAGE_USER|_PAGE_PRESENT) &&
139             (__supported_pte_mask & _PAGE_NX))
140                 pgd.pgd |= _PAGE_NX;
141 
142         /* return the copy of the PGD we want the kernel to use: */
143         return pgd;
144 }
145 
146 /*
147  * Walk the user copy of the page tables (optionally) trying to allocate
148  * page table pages on the way down.
149  *
150  * Returns a pointer to a P4D on success, or NULL on failure.
151  */
152 static __init p4d_t *pti_user_pagetable_walk_p4d(unsigned long address)
153 {
154         pgd_t *pgd = kernel_to_user_pgdp(pgd_offset_k(address));
155         gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
156 
157         if (address < PAGE_OFFSET) {
158                 WARN_ONCE(1, "attempt to walk user address\n");
159                 return NULL;
160         }
161 
162         if (pgd_none(*pgd)) {
163                 unsigned long new_p4d_page = __get_free_page(gfp);
164                 if (!new_p4d_page)
165                         return NULL;
166 
167                 set_pgd(pgd, __pgd(_KERNPG_TABLE | __pa(new_p4d_page)));
168         }
169         BUILD_BUG_ON(pgd_large(*pgd) != 0);
170 
171         return p4d_offset(pgd, address);
172 }
173 
174 /*
175  * Walk the user copy of the page tables (optionally) trying to allocate
176  * page table pages on the way down.
177  *
178  * Returns a pointer to a PMD on success, or NULL on failure.
179  */
180 static __init pmd_t *pti_user_pagetable_walk_pmd(unsigned long address)
181 {
182         gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
183         p4d_t *p4d = pti_user_pagetable_walk_p4d(address);
184         pud_t *pud;
185 
186         BUILD_BUG_ON(p4d_large(*p4d) != 0);
187         if (p4d_none(*p4d)) {
188                 unsigned long new_pud_page = __get_free_page(gfp);
189                 if (!new_pud_page)
190                         return NULL;
191 
192                 set_p4d(p4d, __p4d(_KERNPG_TABLE | __pa(new_pud_page)));
193         }
194 
195         pud = pud_offset(p4d, address);
196         /* The user page tables do not use large mappings: */
197         if (pud_large(*pud)) {
198                 WARN_ON(1);
199                 return NULL;
200         }
201         if (pud_none(*pud)) {
202                 unsigned long new_pmd_page = __get_free_page(gfp);
203                 if (!new_pmd_page)
204                         return NULL;
205 
206                 set_pud(pud, __pud(_KERNPG_TABLE | __pa(new_pmd_page)));
207         }
208 
209         return pmd_offset(pud, address);
210 }
211 
212 #ifdef CONFIG_X86_VSYSCALL_EMULATION
213 /*
214  * Walk the shadow copy of the page tables (optionally) trying to allocate
215  * page table pages on the way down.  Does not support large pages.
216  *
217  * Note: this is only used when mapping *new* kernel data into the
218  * user/shadow page tables.  It is never used for userspace data.
219  *
220  * Returns a pointer to a PTE on success, or NULL on failure.
221  */
222 static __init pte_t *pti_user_pagetable_walk_pte(unsigned long address)
223 {
224         gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
225         pmd_t *pmd = pti_user_pagetable_walk_pmd(address);
226         pte_t *pte;
227 
228         /* We can't do anything sensible if we hit a large mapping. */
229         if (pmd_large(*pmd)) {
230                 WARN_ON(1);
231                 return NULL;
232         }
233 
234         if (pmd_none(*pmd)) {
235                 unsigned long new_pte_page = __get_free_page(gfp);
236                 if (!new_pte_page)
237                         return NULL;
238 
239                 set_pmd(pmd, __pmd(_KERNPG_TABLE | __pa(new_pte_page)));
240         }
241 
242         pte = pte_offset_kernel(pmd, address);
243         if (pte_flags(*pte) & _PAGE_USER) {
244                 WARN_ONCE(1, "attempt to walk to user pte\n");
245                 return NULL;
246         }
247         return pte;
248 }
249 
250 static void __init pti_setup_vsyscall(void)
251 {
252         pte_t *pte, *target_pte;
253         unsigned int level;
254 
255         pte = lookup_address(VSYSCALL_ADDR, &level);
256         if (!pte || WARN_ON(level != PG_LEVEL_4K) || pte_none(*pte))
257                 return;
258 
259         target_pte = pti_user_pagetable_walk_pte(VSYSCALL_ADDR);
260         if (WARN_ON(!target_pte))
261                 return;
262 
263         *target_pte = *pte;
264         set_vsyscall_pgtable_user_bits(kernel_to_user_pgdp(swapper_pg_dir));
265 }
266 #else
267 static void __init pti_setup_vsyscall(void) { }
268 #endif
269 
270 static void __init
271 pti_clone_pmds(unsigned long start, unsigned long end, pmdval_t clear)
272 {
273         unsigned long addr;
274 
275         /*
276          * Clone the populated PMDs which cover start to end. These PMD areas
277          * can have holes.
278          */
279         for (addr = start; addr < end; addr += PMD_SIZE) {
280                 pmd_t *pmd, *target_pmd;
281                 pgd_t *pgd;
282                 p4d_t *p4d;
283                 pud_t *pud;
284 
285                 pgd = pgd_offset_k(addr);
286                 if (WARN_ON(pgd_none(*pgd)))
287                         return;
288                 p4d = p4d_offset(pgd, addr);
289                 if (WARN_ON(p4d_none(*p4d)))
290                         return;
291                 pud = pud_offset(p4d, addr);
292                 if (pud_none(*pud))
293                         continue;
294                 pmd = pmd_offset(pud, addr);
295                 if (pmd_none(*pmd))
296                         continue;
297 
298                 target_pmd = pti_user_pagetable_walk_pmd(addr);
299                 if (WARN_ON(!target_pmd))
300                         return;
301 
302                 /*
303                  * Copy the PMD.  That is, the kernelmode and usermode
304                  * tables will share the last-level page tables of this
305                  * address range
306                  */
307                 *target_pmd = pmd_clear_flags(*pmd, clear);
308         }
309 }
310 
311 /*
312  * Clone a single p4d (i.e. a top-level entry on 4-level systems and a
313  * next-level entry on 5-level systems.
314  */
315 static void __init pti_clone_p4d(unsigned long addr)
316 {
317         p4d_t *kernel_p4d, *user_p4d;
318         pgd_t *kernel_pgd;
319 
320         user_p4d = pti_user_pagetable_walk_p4d(addr);
321         kernel_pgd = pgd_offset_k(addr);
322         kernel_p4d = p4d_offset(kernel_pgd, addr);
323         *user_p4d = *kernel_p4d;
324 }
325 
326 /*
327  * Clone the CPU_ENTRY_AREA into the user space visible page table.
328  */
329 static void __init pti_clone_user_shared(void)
330 {
331         pti_clone_p4d(CPU_ENTRY_AREA_BASE);
332 }
333 
334 /*
335  * Clone the ESPFIX P4D into the user space visible page table
336  */
337 static void __init pti_setup_espfix64(void)
338 {
339 #ifdef CONFIG_X86_ESPFIX64
340         pti_clone_p4d(ESPFIX_BASE_ADDR);
341 #endif
342 }
343 
344 /*
345  * Clone the populated PMDs of the entry and irqentry text and force it RO.
346  */
347 static void __init pti_clone_entry_text(void)
348 {
349         pti_clone_pmds((unsigned long) __entry_text_start,
350                         (unsigned long) __irqentry_text_end,
351                        _PAGE_RW | _PAGE_GLOBAL);
352 }
353 
354 /*
355  * Initialize kernel page table isolation
356  */
357 void __init pti_init(void)
358 {
359         if (!static_cpu_has(X86_FEATURE_PTI))
360                 return;
361 
362         pr_info("enabled\n");
363 
364         pti_clone_user_shared();
365         pti_clone_entry_text();
366         pti_setup_espfix64();
367         pti_setup_vsyscall();
368 }
369 

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