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Linux/arch/x86/kernel/ldt.c

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  1 // SPDX-License-Identifier: GPL-2.0
  2 /*
  3  * Copyright (C) 1992 Krishna Balasubramanian and Linus Torvalds
  4  * Copyright (C) 1999 Ingo Molnar <mingo@redhat.com>
  5  * Copyright (C) 2002 Andi Kleen
  6  *
  7  * This handles calls from both 32bit and 64bit mode.
  8  *
  9  * Lock order:
 10  *      contex.ldt_usr_sem
 11  *        mmap_sem
 12  *          context.lock
 13  */
 14 
 15 #include <linux/errno.h>
 16 #include <linux/gfp.h>
 17 #include <linux/sched.h>
 18 #include <linux/string.h>
 19 #include <linux/mm.h>
 20 #include <linux/smp.h>
 21 #include <linux/syscalls.h>
 22 #include <linux/slab.h>
 23 #include <linux/vmalloc.h>
 24 #include <linux/uaccess.h>
 25 
 26 #include <asm/ldt.h>
 27 #include <asm/tlb.h>
 28 #include <asm/desc.h>
 29 #include <asm/mmu_context.h>
 30 #include <asm/syscalls.h>
 31 
 32 static void refresh_ldt_segments(void)
 33 {
 34 #ifdef CONFIG_X86_64
 35         unsigned short sel;
 36 
 37         /*
 38          * Make sure that the cached DS and ES descriptors match the updated
 39          * LDT.
 40          */
 41         savesegment(ds, sel);
 42         if ((sel & SEGMENT_TI_MASK) == SEGMENT_LDT)
 43                 loadsegment(ds, sel);
 44 
 45         savesegment(es, sel);
 46         if ((sel & SEGMENT_TI_MASK) == SEGMENT_LDT)
 47                 loadsegment(es, sel);
 48 #endif
 49 }
 50 
 51 /* context.lock is held by the task which issued the smp function call */
 52 static void flush_ldt(void *__mm)
 53 {
 54         struct mm_struct *mm = __mm;
 55 
 56         if (this_cpu_read(cpu_tlbstate.loaded_mm) != mm)
 57                 return;
 58 
 59         load_mm_ldt(mm);
 60 
 61         refresh_ldt_segments();
 62 }
 63 
 64 /* The caller must call finalize_ldt_struct on the result. LDT starts zeroed. */
 65 static struct ldt_struct *alloc_ldt_struct(unsigned int num_entries)
 66 {
 67         struct ldt_struct *new_ldt;
 68         unsigned int alloc_size;
 69 
 70         if (num_entries > LDT_ENTRIES)
 71                 return NULL;
 72 
 73         new_ldt = kmalloc(sizeof(struct ldt_struct), GFP_KERNEL);
 74         if (!new_ldt)
 75                 return NULL;
 76 
 77         BUILD_BUG_ON(LDT_ENTRY_SIZE != sizeof(struct desc_struct));
 78         alloc_size = num_entries * LDT_ENTRY_SIZE;
 79 
 80         /*
 81          * Xen is very picky: it requires a page-aligned LDT that has no
 82          * trailing nonzero bytes in any page that contains LDT descriptors.
 83          * Keep it simple: zero the whole allocation and never allocate less
 84          * than PAGE_SIZE.
 85          */
 86         if (alloc_size > PAGE_SIZE)
 87                 new_ldt->entries = vzalloc(alloc_size);
 88         else
 89                 new_ldt->entries = (void *)get_zeroed_page(GFP_KERNEL);
 90 
 91         if (!new_ldt->entries) {
 92                 kfree(new_ldt);
 93                 return NULL;
 94         }
 95 
 96         /* The new LDT isn't aliased for PTI yet. */
 97         new_ldt->slot = -1;
 98 
 99         new_ldt->nr_entries = num_entries;
100         return new_ldt;
101 }
102 
103 /*
104  * If PTI is enabled, this maps the LDT into the kernelmode and
105  * usermode tables for the given mm.
106  *
107  * There is no corresponding unmap function.  Even if the LDT is freed, we
108  * leave the PTEs around until the slot is reused or the mm is destroyed.
109  * This is harmless: the LDT is always in ordinary memory, and no one will
110  * access the freed slot.
111  *
112  * If we wanted to unmap freed LDTs, we'd also need to do a flush to make
113  * it useful, and the flush would slow down modify_ldt().
114  */
115 static int
116 map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
117 {
118 #ifdef CONFIG_PAGE_TABLE_ISOLATION
119         bool is_vmalloc, had_top_level_entry;
120         unsigned long va;
121         spinlock_t *ptl;
122         pgd_t *pgd;
123         int i;
124 
125         if (!static_cpu_has(X86_FEATURE_PTI))
126                 return 0;
127 
128         /*
129          * Any given ldt_struct should have map_ldt_struct() called at most
130          * once.
131          */
132         WARN_ON(ldt->slot != -1);
133 
134         /*
135          * Did we already have the top level entry allocated?  We can't
136          * use pgd_none() for this because it doens't do anything on
137          * 4-level page table kernels.
138          */
139         pgd = pgd_offset(mm, LDT_BASE_ADDR);
140         had_top_level_entry = (pgd->pgd != 0);
141 
142         is_vmalloc = is_vmalloc_addr(ldt->entries);
143 
144         for (i = 0; i * PAGE_SIZE < ldt->nr_entries * LDT_ENTRY_SIZE; i++) {
145                 unsigned long offset = i << PAGE_SHIFT;
146                 const void *src = (char *)ldt->entries + offset;
147                 unsigned long pfn;
148                 pgprot_t pte_prot;
149                 pte_t pte, *ptep;
150 
151                 va = (unsigned long)ldt_slot_va(slot) + offset;
152                 pfn = is_vmalloc ? vmalloc_to_pfn(src) :
153                         page_to_pfn(virt_to_page(src));
154                 /*
155                  * Treat the PTI LDT range as a *userspace* range.
156                  * get_locked_pte() will allocate all needed pagetables
157                  * and account for them in this mm.
158                  */
159                 ptep = get_locked_pte(mm, va, &ptl);
160                 if (!ptep)
161                         return -ENOMEM;
162                 /*
163                  * Map it RO so the easy to find address is not a primary
164                  * target via some kernel interface which misses a
165                  * permission check.
166                  */
167                 pte_prot = __pgprot(__PAGE_KERNEL_RO & ~_PAGE_GLOBAL);
168                 /* Filter out unsuppored __PAGE_KERNEL* bits: */
169                 pgprot_val(pte_prot) &= __supported_pte_mask;
170                 pte = pfn_pte(pfn, pte_prot);
171                 set_pte_at(mm, va, ptep, pte);
172                 pte_unmap_unlock(ptep, ptl);
173         }
174 
175         if (mm->context.ldt) {
176                 /*
177                  * We already had an LDT.  The top-level entry should already
178                  * have been allocated and synchronized with the usermode
179                  * tables.
180                  */
181                 WARN_ON(!had_top_level_entry);
182                 if (static_cpu_has(X86_FEATURE_PTI))
183                         WARN_ON(!kernel_to_user_pgdp(pgd)->pgd);
184         } else {
185                 /*
186                  * This is the first time we're mapping an LDT for this process.
187                  * Sync the pgd to the usermode tables.
188                  */
189                 WARN_ON(had_top_level_entry);
190                 if (static_cpu_has(X86_FEATURE_PTI)) {
191                         WARN_ON(kernel_to_user_pgdp(pgd)->pgd);
192                         set_pgd(kernel_to_user_pgdp(pgd), *pgd);
193                 }
194         }
195 
196         va = (unsigned long)ldt_slot_va(slot);
197         flush_tlb_mm_range(mm, va, va + LDT_SLOT_STRIDE, 0);
198 
199         ldt->slot = slot;
200 #endif
201         return 0;
202 }
203 
204 static void free_ldt_pgtables(struct mm_struct *mm)
205 {
206 #ifdef CONFIG_PAGE_TABLE_ISOLATION
207         struct mmu_gather tlb;
208         unsigned long start = LDT_BASE_ADDR;
209         unsigned long end = start + (1UL << PGDIR_SHIFT);
210 
211         if (!static_cpu_has(X86_FEATURE_PTI))
212                 return;
213 
214         tlb_gather_mmu(&tlb, mm, start, end);
215         free_pgd_range(&tlb, start, end, start, end);
216         tlb_finish_mmu(&tlb, start, end);
217 #endif
218 }
219 
220 /* After calling this, the LDT is immutable. */
221 static void finalize_ldt_struct(struct ldt_struct *ldt)
222 {
223         paravirt_alloc_ldt(ldt->entries, ldt->nr_entries);
224 }
225 
226 static void install_ldt(struct mm_struct *mm, struct ldt_struct *ldt)
227 {
228         mutex_lock(&mm->context.lock);
229 
230         /* Synchronizes with READ_ONCE in load_mm_ldt. */
231         smp_store_release(&mm->context.ldt, ldt);
232 
233         /* Activate the LDT for all CPUs using currents mm. */
234         on_each_cpu_mask(mm_cpumask(mm), flush_ldt, mm, true);
235 
236         mutex_unlock(&mm->context.lock);
237 }
238 
239 static void free_ldt_struct(struct ldt_struct *ldt)
240 {
241         if (likely(!ldt))
242                 return;
243 
244         paravirt_free_ldt(ldt->entries, ldt->nr_entries);
245         if (ldt->nr_entries * LDT_ENTRY_SIZE > PAGE_SIZE)
246                 vfree_atomic(ldt->entries);
247         else
248                 free_page((unsigned long)ldt->entries);
249         kfree(ldt);
250 }
251 
252 /*
253  * Called on fork from arch_dup_mmap(). Just copy the current LDT state,
254  * the new task is not running, so nothing can be installed.
255  */
256 int ldt_dup_context(struct mm_struct *old_mm, struct mm_struct *mm)
257 {
258         struct ldt_struct *new_ldt;
259         int retval = 0;
260 
261         if (!old_mm)
262                 return 0;
263 
264         mutex_lock(&old_mm->context.lock);
265         if (!old_mm->context.ldt)
266                 goto out_unlock;
267 
268         new_ldt = alloc_ldt_struct(old_mm->context.ldt->nr_entries);
269         if (!new_ldt) {
270                 retval = -ENOMEM;
271                 goto out_unlock;
272         }
273 
274         memcpy(new_ldt->entries, old_mm->context.ldt->entries,
275                new_ldt->nr_entries * LDT_ENTRY_SIZE);
276         finalize_ldt_struct(new_ldt);
277 
278         retval = map_ldt_struct(mm, new_ldt, 0);
279         if (retval) {
280                 free_ldt_pgtables(mm);
281                 free_ldt_struct(new_ldt);
282                 goto out_unlock;
283         }
284         mm->context.ldt = new_ldt;
285 
286 out_unlock:
287         mutex_unlock(&old_mm->context.lock);
288         return retval;
289 }
290 
291 /*
292  * No need to lock the MM as we are the last user
293  *
294  * 64bit: Don't touch the LDT register - we're already in the next thread.
295  */
296 void destroy_context_ldt(struct mm_struct *mm)
297 {
298         free_ldt_struct(mm->context.ldt);
299         mm->context.ldt = NULL;
300 }
301 
302 void ldt_arch_exit_mmap(struct mm_struct *mm)
303 {
304         free_ldt_pgtables(mm);
305 }
306 
307 static int read_ldt(void __user *ptr, unsigned long bytecount)
308 {
309         struct mm_struct *mm = current->mm;
310         unsigned long entries_size;
311         int retval;
312 
313         down_read(&mm->context.ldt_usr_sem);
314 
315         if (!mm->context.ldt) {
316                 retval = 0;
317                 goto out_unlock;
318         }
319 
320         if (bytecount > LDT_ENTRY_SIZE * LDT_ENTRIES)
321                 bytecount = LDT_ENTRY_SIZE * LDT_ENTRIES;
322 
323         entries_size = mm->context.ldt->nr_entries * LDT_ENTRY_SIZE;
324         if (entries_size > bytecount)
325                 entries_size = bytecount;
326 
327         if (copy_to_user(ptr, mm->context.ldt->entries, entries_size)) {
328                 retval = -EFAULT;
329                 goto out_unlock;
330         }
331 
332         if (entries_size != bytecount) {
333                 /* Zero-fill the rest and pretend we read bytecount bytes. */
334                 if (clear_user(ptr + entries_size, bytecount - entries_size)) {
335                         retval = -EFAULT;
336                         goto out_unlock;
337                 }
338         }
339         retval = bytecount;
340 
341 out_unlock:
342         up_read(&mm->context.ldt_usr_sem);
343         return retval;
344 }
345 
346 static int read_default_ldt(void __user *ptr, unsigned long bytecount)
347 {
348         /* CHECKME: Can we use _one_ random number ? */
349 #ifdef CONFIG_X86_32
350         unsigned long size = 5 * sizeof(struct desc_struct);
351 #else
352         unsigned long size = 128;
353 #endif
354         if (bytecount > size)
355                 bytecount = size;
356         if (clear_user(ptr, bytecount))
357                 return -EFAULT;
358         return bytecount;
359 }
360 
361 static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
362 {
363         struct mm_struct *mm = current->mm;
364         struct ldt_struct *new_ldt, *old_ldt;
365         unsigned int old_nr_entries, new_nr_entries;
366         struct user_desc ldt_info;
367         struct desc_struct ldt;
368         int error;
369 
370         error = -EINVAL;
371         if (bytecount != sizeof(ldt_info))
372                 goto out;
373         error = -EFAULT;
374         if (copy_from_user(&ldt_info, ptr, sizeof(ldt_info)))
375                 goto out;
376 
377         error = -EINVAL;
378         if (ldt_info.entry_number >= LDT_ENTRIES)
379                 goto out;
380         if (ldt_info.contents == 3) {
381                 if (oldmode)
382                         goto out;
383                 if (ldt_info.seg_not_present == 0)
384                         goto out;
385         }
386 
387         if ((oldmode && !ldt_info.base_addr && !ldt_info.limit) ||
388             LDT_empty(&ldt_info)) {
389                 /* The user wants to clear the entry. */
390                 memset(&ldt, 0, sizeof(ldt));
391         } else {
392                 if (!IS_ENABLED(CONFIG_X86_16BIT) && !ldt_info.seg_32bit) {
393                         error = -EINVAL;
394                         goto out;
395                 }
396 
397                 fill_ldt(&ldt, &ldt_info);
398                 if (oldmode)
399                         ldt.avl = 0;
400         }
401 
402         if (down_write_killable(&mm->context.ldt_usr_sem))
403                 return -EINTR;
404 
405         old_ldt       = mm->context.ldt;
406         old_nr_entries = old_ldt ? old_ldt->nr_entries : 0;
407         new_nr_entries = max(ldt_info.entry_number + 1, old_nr_entries);
408 
409         error = -ENOMEM;
410         new_ldt = alloc_ldt_struct(new_nr_entries);
411         if (!new_ldt)
412                 goto out_unlock;
413 
414         if (old_ldt)
415                 memcpy(new_ldt->entries, old_ldt->entries, old_nr_entries * LDT_ENTRY_SIZE);
416 
417         new_ldt->entries[ldt_info.entry_number] = ldt;
418         finalize_ldt_struct(new_ldt);
419 
420         /*
421          * If we are using PTI, map the new LDT into the userspace pagetables.
422          * If there is already an LDT, use the other slot so that other CPUs
423          * will continue to use the old LDT until install_ldt() switches
424          * them over to the new LDT.
425          */
426         error = map_ldt_struct(mm, new_ldt, old_ldt ? !old_ldt->slot : 0);
427         if (error) {
428                 /*
429                  * This only can fail for the first LDT setup. If an LDT is
430                  * already installed then the PTE page is already
431                  * populated. Mop up a half populated page table.
432                  */
433                 if (!WARN_ON_ONCE(old_ldt))
434                         free_ldt_pgtables(mm);
435                 free_ldt_struct(new_ldt);
436                 goto out_unlock;
437         }
438 
439         install_ldt(mm, new_ldt);
440         free_ldt_struct(old_ldt);
441         error = 0;
442 
443 out_unlock:
444         up_write(&mm->context.ldt_usr_sem);
445 out:
446         return error;
447 }
448 
449 SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr ,
450                 unsigned long , bytecount)
451 {
452         int ret = -ENOSYS;
453 
454         switch (func) {
455         case 0:
456                 ret = read_ldt(ptr, bytecount);
457                 break;
458         case 1:
459                 ret = write_ldt(ptr, bytecount, 1);
460                 break;
461         case 2:
462                 ret = read_default_ldt(ptr, bytecount);
463                 break;
464         case 0x11:
465                 ret = write_ldt(ptr, bytecount, 0);
466                 break;
467         }
468         /*
469          * The SYSCALL_DEFINE() macros give us an 'unsigned long'
470          * return type, but tht ABI for sys_modify_ldt() expects
471          * 'int'.  This cast gives us an int-sized value in %rax
472          * for the return code.  The 'unsigned' is necessary so
473          * the compiler does not try to sign-extend the negative
474          * return codes into the high half of the register when
475          * taking the value from int->long.
476          */
477         return (unsigned int)ret;
478 }
479 

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