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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                 pte_t pte, *ptep;
149 
150                 va = (unsigned long)ldt_slot_va(slot) + offset;
151                 pfn = is_vmalloc ? vmalloc_to_pfn(src) :
152                         page_to_pfn(virt_to_page(src));
153                 /*
154                  * Treat the PTI LDT range as a *userspace* range.
155                  * get_locked_pte() will allocate all needed pagetables
156                  * and account for them in this mm.
157                  */
158                 ptep = get_locked_pte(mm, va, &ptl);
159                 if (!ptep)
160                         return -ENOMEM;
161                 /*
162                  * Map it RO so the easy to find address is not a primary
163                  * target via some kernel interface which misses a
164                  * permission check.
165                  */
166                 pte = pfn_pte(pfn, __pgprot(__PAGE_KERNEL_RO & ~_PAGE_GLOBAL));
167                 set_pte_at(mm, va, ptep, pte);
168                 pte_unmap_unlock(ptep, ptl);
169         }
170 
171         if (mm->context.ldt) {
172                 /*
173                  * We already had an LDT.  The top-level entry should already
174                  * have been allocated and synchronized with the usermode
175                  * tables.
176                  */
177                 WARN_ON(!had_top_level_entry);
178                 if (static_cpu_has(X86_FEATURE_PTI))
179                         WARN_ON(!kernel_to_user_pgdp(pgd)->pgd);
180         } else {
181                 /*
182                  * This is the first time we're mapping an LDT for this process.
183                  * Sync the pgd to the usermode tables.
184                  */
185                 WARN_ON(had_top_level_entry);
186                 if (static_cpu_has(X86_FEATURE_PTI)) {
187                         WARN_ON(kernel_to_user_pgdp(pgd)->pgd);
188                         set_pgd(kernel_to_user_pgdp(pgd), *pgd);
189                 }
190         }
191 
192         va = (unsigned long)ldt_slot_va(slot);
193         flush_tlb_mm_range(mm, va, va + LDT_SLOT_STRIDE, 0);
194 
195         ldt->slot = slot;
196 #endif
197         return 0;
198 }
199 
200 static void free_ldt_pgtables(struct mm_struct *mm)
201 {
202 #ifdef CONFIG_PAGE_TABLE_ISOLATION
203         struct mmu_gather tlb;
204         unsigned long start = LDT_BASE_ADDR;
205         unsigned long end = start + (1UL << PGDIR_SHIFT);
206 
207         if (!static_cpu_has(X86_FEATURE_PTI))
208                 return;
209 
210         tlb_gather_mmu(&tlb, mm, start, end);
211         free_pgd_range(&tlb, start, end, start, end);
212         tlb_finish_mmu(&tlb, start, end);
213 #endif
214 }
215 
216 /* After calling this, the LDT is immutable. */
217 static void finalize_ldt_struct(struct ldt_struct *ldt)
218 {
219         paravirt_alloc_ldt(ldt->entries, ldt->nr_entries);
220 }
221 
222 static void install_ldt(struct mm_struct *mm, struct ldt_struct *ldt)
223 {
224         mutex_lock(&mm->context.lock);
225 
226         /* Synchronizes with READ_ONCE in load_mm_ldt. */
227         smp_store_release(&mm->context.ldt, ldt);
228 
229         /* Activate the LDT for all CPUs using currents mm. */
230         on_each_cpu_mask(mm_cpumask(mm), flush_ldt, mm, true);
231 
232         mutex_unlock(&mm->context.lock);
233 }
234 
235 static void free_ldt_struct(struct ldt_struct *ldt)
236 {
237         if (likely(!ldt))
238                 return;
239 
240         paravirt_free_ldt(ldt->entries, ldt->nr_entries);
241         if (ldt->nr_entries * LDT_ENTRY_SIZE > PAGE_SIZE)
242                 vfree_atomic(ldt->entries);
243         else
244                 free_page((unsigned long)ldt->entries);
245         kfree(ldt);
246 }
247 
248 /*
249  * Called on fork from arch_dup_mmap(). Just copy the current LDT state,
250  * the new task is not running, so nothing can be installed.
251  */
252 int ldt_dup_context(struct mm_struct *old_mm, struct mm_struct *mm)
253 {
254         struct ldt_struct *new_ldt;
255         int retval = 0;
256 
257         if (!old_mm)
258                 return 0;
259 
260         mutex_lock(&old_mm->context.lock);
261         if (!old_mm->context.ldt)
262                 goto out_unlock;
263 
264         new_ldt = alloc_ldt_struct(old_mm->context.ldt->nr_entries);
265         if (!new_ldt) {
266                 retval = -ENOMEM;
267                 goto out_unlock;
268         }
269 
270         memcpy(new_ldt->entries, old_mm->context.ldt->entries,
271                new_ldt->nr_entries * LDT_ENTRY_SIZE);
272         finalize_ldt_struct(new_ldt);
273 
274         retval = map_ldt_struct(mm, new_ldt, 0);
275         if (retval) {
276                 free_ldt_pgtables(mm);
277                 free_ldt_struct(new_ldt);
278                 goto out_unlock;
279         }
280         mm->context.ldt = new_ldt;
281 
282 out_unlock:
283         mutex_unlock(&old_mm->context.lock);
284         return retval;
285 }
286 
287 /*
288  * No need to lock the MM as we are the last user
289  *
290  * 64bit: Don't touch the LDT register - we're already in the next thread.
291  */
292 void destroy_context_ldt(struct mm_struct *mm)
293 {
294         free_ldt_struct(mm->context.ldt);
295         mm->context.ldt = NULL;
296 }
297 
298 void ldt_arch_exit_mmap(struct mm_struct *mm)
299 {
300         free_ldt_pgtables(mm);
301 }
302 
303 static int read_ldt(void __user *ptr, unsigned long bytecount)
304 {
305         struct mm_struct *mm = current->mm;
306         unsigned long entries_size;
307         int retval;
308 
309         down_read(&mm->context.ldt_usr_sem);
310 
311         if (!mm->context.ldt) {
312                 retval = 0;
313                 goto out_unlock;
314         }
315 
316         if (bytecount > LDT_ENTRY_SIZE * LDT_ENTRIES)
317                 bytecount = LDT_ENTRY_SIZE * LDT_ENTRIES;
318 
319         entries_size = mm->context.ldt->nr_entries * LDT_ENTRY_SIZE;
320         if (entries_size > bytecount)
321                 entries_size = bytecount;
322 
323         if (copy_to_user(ptr, mm->context.ldt->entries, entries_size)) {
324                 retval = -EFAULT;
325                 goto out_unlock;
326         }
327 
328         if (entries_size != bytecount) {
329                 /* Zero-fill the rest and pretend we read bytecount bytes. */
330                 if (clear_user(ptr + entries_size, bytecount - entries_size)) {
331                         retval = -EFAULT;
332                         goto out_unlock;
333                 }
334         }
335         retval = bytecount;
336 
337 out_unlock:
338         up_read(&mm->context.ldt_usr_sem);
339         return retval;
340 }
341 
342 static int read_default_ldt(void __user *ptr, unsigned long bytecount)
343 {
344         /* CHECKME: Can we use _one_ random number ? */
345 #ifdef CONFIG_X86_32
346         unsigned long size = 5 * sizeof(struct desc_struct);
347 #else
348         unsigned long size = 128;
349 #endif
350         if (bytecount > size)
351                 bytecount = size;
352         if (clear_user(ptr, bytecount))
353                 return -EFAULT;
354         return bytecount;
355 }
356 
357 static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
358 {
359         struct mm_struct *mm = current->mm;
360         struct ldt_struct *new_ldt, *old_ldt;
361         unsigned int old_nr_entries, new_nr_entries;
362         struct user_desc ldt_info;
363         struct desc_struct ldt;
364         int error;
365 
366         error = -EINVAL;
367         if (bytecount != sizeof(ldt_info))
368                 goto out;
369         error = -EFAULT;
370         if (copy_from_user(&ldt_info, ptr, sizeof(ldt_info)))
371                 goto out;
372 
373         error = -EINVAL;
374         if (ldt_info.entry_number >= LDT_ENTRIES)
375                 goto out;
376         if (ldt_info.contents == 3) {
377                 if (oldmode)
378                         goto out;
379                 if (ldt_info.seg_not_present == 0)
380                         goto out;
381         }
382 
383         if ((oldmode && !ldt_info.base_addr && !ldt_info.limit) ||
384             LDT_empty(&ldt_info)) {
385                 /* The user wants to clear the entry. */
386                 memset(&ldt, 0, sizeof(ldt));
387         } else {
388                 if (!IS_ENABLED(CONFIG_X86_16BIT) && !ldt_info.seg_32bit) {
389                         error = -EINVAL;
390                         goto out;
391                 }
392 
393                 fill_ldt(&ldt, &ldt_info);
394                 if (oldmode)
395                         ldt.avl = 0;
396         }
397 
398         if (down_write_killable(&mm->context.ldt_usr_sem))
399                 return -EINTR;
400 
401         old_ldt       = mm->context.ldt;
402         old_nr_entries = old_ldt ? old_ldt->nr_entries : 0;
403         new_nr_entries = max(ldt_info.entry_number + 1, old_nr_entries);
404 
405         error = -ENOMEM;
406         new_ldt = alloc_ldt_struct(new_nr_entries);
407         if (!new_ldt)
408                 goto out_unlock;
409 
410         if (old_ldt)
411                 memcpy(new_ldt->entries, old_ldt->entries, old_nr_entries * LDT_ENTRY_SIZE);
412 
413         new_ldt->entries[ldt_info.entry_number] = ldt;
414         finalize_ldt_struct(new_ldt);
415 
416         /*
417          * If we are using PTI, map the new LDT into the userspace pagetables.
418          * If there is already an LDT, use the other slot so that other CPUs
419          * will continue to use the old LDT until install_ldt() switches
420          * them over to the new LDT.
421          */
422         error = map_ldt_struct(mm, new_ldt, old_ldt ? !old_ldt->slot : 0);
423         if (error) {
424                 /*
425                  * This only can fail for the first LDT setup. If an LDT is
426                  * already installed then the PTE page is already
427                  * populated. Mop up a half populated page table.
428                  */
429                 if (!WARN_ON_ONCE(old_ldt))
430                         free_ldt_pgtables(mm);
431                 free_ldt_struct(new_ldt);
432                 goto out_unlock;
433         }
434 
435         install_ldt(mm, new_ldt);
436         free_ldt_struct(old_ldt);
437         error = 0;
438 
439 out_unlock:
440         up_write(&mm->context.ldt_usr_sem);
441 out:
442         return error;
443 }
444 
445 SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr ,
446                 unsigned long , bytecount)
447 {
448         int ret = -ENOSYS;
449 
450         switch (func) {
451         case 0:
452                 ret = read_ldt(ptr, bytecount);
453                 break;
454         case 1:
455                 ret = write_ldt(ptr, bytecount, 1);
456                 break;
457         case 2:
458                 ret = read_default_ldt(ptr, bytecount);
459                 break;
460         case 0x11:
461                 ret = write_ldt(ptr, bytecount, 0);
462                 break;
463         }
464         /*
465          * The SYSCALL_DEFINE() macros give us an 'unsigned long'
466          * return type, but tht ABI for sys_modify_ldt() expects
467          * 'int'.  This cast gives us an int-sized value in %rax
468          * for the return code.  The 'unsigned' is necessary so
469          * the compiler does not try to sign-extend the negative
470          * return codes into the high half of the register when
471          * taking the value from int->long.
472          */
473         return (unsigned int)ret;
474 }
475 

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