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Linux/arch/x86/entry/vsyscall/vsyscall_64.c

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  1 // SPDX-License-Identifier: GPL-2.0
  2 /*
  3  * Copyright (c) 2012-2014 Andy Lutomirski <luto@amacapital.net>
  4  *
  5  * Based on the original implementation which is:
  6  *  Copyright (C) 2001 Andrea Arcangeli <andrea@suse.de> SuSE
  7  *  Copyright 2003 Andi Kleen, SuSE Labs.
  8  *
  9  *  Parts of the original code have been moved to arch/x86/vdso/vma.c
 10  *
 11  * This file implements vsyscall emulation.  vsyscalls are a legacy ABI:
 12  * Userspace can request certain kernel services by calling fixed
 13  * addresses.  This concept is problematic:
 14  *
 15  * - It interferes with ASLR.
 16  * - It's awkward to write code that lives in kernel addresses but is
 17  *   callable by userspace at fixed addresses.
 18  * - The whole concept is impossible for 32-bit compat userspace.
 19  * - UML cannot easily virtualize a vsyscall.
 20  *
 21  * As of mid-2014, I believe that there is no new userspace code that
 22  * will use a vsyscall if the vDSO is present.  I hope that there will
 23  * soon be no new userspace code that will ever use a vsyscall.
 24  *
 25  * The code in this file emulates vsyscalls when notified of a page
 26  * fault to a vsyscall address.
 27  */
 28 
 29 #include <linux/kernel.h>
 30 #include <linux/timer.h>
 31 #include <linux/sched/signal.h>
 32 #include <linux/mm_types.h>
 33 #include <linux/syscalls.h>
 34 #include <linux/ratelimit.h>
 35 
 36 #include <asm/vsyscall.h>
 37 #include <asm/unistd.h>
 38 #include <asm/fixmap.h>
 39 #include <asm/traps.h>
 40 #include <asm/paravirt.h>
 41 
 42 #define CREATE_TRACE_POINTS
 43 #include "vsyscall_trace.h"
 44 
 45 static enum { EMULATE, XONLY, NONE } vsyscall_mode __ro_after_init =
 46 #ifdef CONFIG_LEGACY_VSYSCALL_NONE
 47         NONE;
 48 #elif defined(CONFIG_LEGACY_VSYSCALL_XONLY)
 49         XONLY;
 50 #else
 51         EMULATE;
 52 #endif
 53 
 54 static int __init vsyscall_setup(char *str)
 55 {
 56         if (str) {
 57                 if (!strcmp("emulate", str))
 58                         vsyscall_mode = EMULATE;
 59                 else if (!strcmp("xonly", str))
 60                         vsyscall_mode = XONLY;
 61                 else if (!strcmp("none", str))
 62                         vsyscall_mode = NONE;
 63                 else
 64                         return -EINVAL;
 65 
 66                 return 0;
 67         }
 68 
 69         return -EINVAL;
 70 }
 71 early_param("vsyscall", vsyscall_setup);
 72 
 73 static void warn_bad_vsyscall(const char *level, struct pt_regs *regs,
 74                               const char *message)
 75 {
 76         if (!show_unhandled_signals)
 77                 return;
 78 
 79         printk_ratelimited("%s%s[%d] %s ip:%lx cs:%lx sp:%lx ax:%lx si:%lx di:%lx\n",
 80                            level, current->comm, task_pid_nr(current),
 81                            message, regs->ip, regs->cs,
 82                            regs->sp, regs->ax, regs->si, regs->di);
 83 }
 84 
 85 static int addr_to_vsyscall_nr(unsigned long addr)
 86 {
 87         int nr;
 88 
 89         if ((addr & ~0xC00UL) != VSYSCALL_ADDR)
 90                 return -EINVAL;
 91 
 92         nr = (addr & 0xC00UL) >> 10;
 93         if (nr >= 3)
 94                 return -EINVAL;
 95 
 96         return nr;
 97 }
 98 
 99 static bool write_ok_or_segv(unsigned long ptr, size_t size)
100 {
101         /*
102          * XXX: if access_ok, get_user, and put_user handled
103          * sig_on_uaccess_err, this could go away.
104          */
105 
106         if (!access_ok((void __user *)ptr, size)) {
107                 struct thread_struct *thread = &current->thread;
108 
109                 thread->error_code      = X86_PF_USER | X86_PF_WRITE;
110                 thread->cr2             = ptr;
111                 thread->trap_nr         = X86_TRAP_PF;
112 
113                 force_sig_fault(SIGSEGV, SEGV_MAPERR, (void __user *)ptr);
114                 return false;
115         } else {
116                 return true;
117         }
118 }
119 
120 bool emulate_vsyscall(unsigned long error_code,
121                       struct pt_regs *regs, unsigned long address)
122 {
123         struct task_struct *tsk;
124         unsigned long caller;
125         int vsyscall_nr, syscall_nr, tmp;
126         int prev_sig_on_uaccess_err;
127         long ret;
128         unsigned long orig_dx;
129 
130         /* Write faults or kernel-privilege faults never get fixed up. */
131         if ((error_code & (X86_PF_WRITE | X86_PF_USER)) != X86_PF_USER)
132                 return false;
133 
134         if (!(error_code & X86_PF_INSTR)) {
135                 /* Failed vsyscall read */
136                 if (vsyscall_mode == EMULATE)
137                         return false;
138 
139                 /*
140                  * User code tried and failed to read the vsyscall page.
141                  */
142                 warn_bad_vsyscall(KERN_INFO, regs, "vsyscall read attempt denied -- look up the vsyscall kernel parameter if you need a workaround");
143                 return false;
144         }
145 
146         /*
147          * No point in checking CS -- the only way to get here is a user mode
148          * trap to a high address, which means that we're in 64-bit user code.
149          */
150 
151         WARN_ON_ONCE(address != regs->ip);
152 
153         if (vsyscall_mode == NONE) {
154                 warn_bad_vsyscall(KERN_INFO, regs,
155                                   "vsyscall attempted with vsyscall=none");
156                 return false;
157         }
158 
159         vsyscall_nr = addr_to_vsyscall_nr(address);
160 
161         trace_emulate_vsyscall(vsyscall_nr);
162 
163         if (vsyscall_nr < 0) {
164                 warn_bad_vsyscall(KERN_WARNING, regs,
165                                   "misaligned vsyscall (exploit attempt or buggy program) -- look up the vsyscall kernel parameter if you need a workaround");
166                 goto sigsegv;
167         }
168 
169         if (get_user(caller, (unsigned long __user *)regs->sp) != 0) {
170                 warn_bad_vsyscall(KERN_WARNING, regs,
171                                   "vsyscall with bad stack (exploit attempt?)");
172                 goto sigsegv;
173         }
174 
175         tsk = current;
176 
177         /*
178          * Check for access_ok violations and find the syscall nr.
179          *
180          * NULL is a valid user pointer (in the access_ok sense) on 32-bit and
181          * 64-bit, so we don't need to special-case it here.  For all the
182          * vsyscalls, NULL means "don't write anything" not "write it at
183          * address 0".
184          */
185         switch (vsyscall_nr) {
186         case 0:
187                 if (!write_ok_or_segv(regs->di, sizeof(struct __kernel_old_timeval)) ||
188                     !write_ok_or_segv(regs->si, sizeof(struct timezone))) {
189                         ret = -EFAULT;
190                         goto check_fault;
191                 }
192 
193                 syscall_nr = __NR_gettimeofday;
194                 break;
195 
196         case 1:
197                 if (!write_ok_or_segv(regs->di, sizeof(__kernel_old_time_t))) {
198                         ret = -EFAULT;
199                         goto check_fault;
200                 }
201 
202                 syscall_nr = __NR_time;
203                 break;
204 
205         case 2:
206                 if (!write_ok_or_segv(regs->di, sizeof(unsigned)) ||
207                     !write_ok_or_segv(regs->si, sizeof(unsigned))) {
208                         ret = -EFAULT;
209                         goto check_fault;
210                 }
211 
212                 syscall_nr = __NR_getcpu;
213                 break;
214         }
215 
216         /*
217          * Handle seccomp.  regs->ip must be the original value.
218          * See seccomp_send_sigsys and Documentation/userspace-api/seccomp_filter.rst.
219          *
220          * We could optimize the seccomp disabled case, but performance
221          * here doesn't matter.
222          */
223         regs->orig_ax = syscall_nr;
224         regs->ax = -ENOSYS;
225         tmp = secure_computing();
226         if ((!tmp && regs->orig_ax != syscall_nr) || regs->ip != address) {
227                 warn_bad_vsyscall(KERN_DEBUG, regs,
228                                   "seccomp tried to change syscall nr or ip");
229                 do_exit(SIGSYS);
230         }
231         regs->orig_ax = -1;
232         if (tmp)
233                 goto do_ret;  /* skip requested */
234 
235         /*
236          * With a real vsyscall, page faults cause SIGSEGV.  We want to
237          * preserve that behavior to make writing exploits harder.
238          */
239         prev_sig_on_uaccess_err = current->thread.sig_on_uaccess_err;
240         current->thread.sig_on_uaccess_err = 1;
241 
242         ret = -EFAULT;
243         switch (vsyscall_nr) {
244         case 0:
245                 /* this decodes regs->di and regs->si on its own */
246                 ret = __x64_sys_gettimeofday(regs);
247                 break;
248 
249         case 1:
250                 /* this decodes regs->di on its own */
251                 ret = __x64_sys_time(regs);
252                 break;
253 
254         case 2:
255                 /* while we could clobber regs->dx, we didn't in the past... */
256                 orig_dx = regs->dx;
257                 regs->dx = 0;
258                 /* this decodes regs->di, regs->si and regs->dx on its own */
259                 ret = __x64_sys_getcpu(regs);
260                 regs->dx = orig_dx;
261                 break;
262         }
263 
264         current->thread.sig_on_uaccess_err = prev_sig_on_uaccess_err;
265 
266 check_fault:
267         if (ret == -EFAULT) {
268                 /* Bad news -- userspace fed a bad pointer to a vsyscall. */
269                 warn_bad_vsyscall(KERN_INFO, regs,
270                                   "vsyscall fault (exploit attempt?)");
271 
272                 /*
273                  * If we failed to generate a signal for any reason,
274                  * generate one here.  (This should be impossible.)
275                  */
276                 if (WARN_ON_ONCE(!sigismember(&tsk->pending.signal, SIGBUS) &&
277                                  !sigismember(&tsk->pending.signal, SIGSEGV)))
278                         goto sigsegv;
279 
280                 return true;  /* Don't emulate the ret. */
281         }
282 
283         regs->ax = ret;
284 
285 do_ret:
286         /* Emulate a ret instruction. */
287         regs->ip = caller;
288         regs->sp += 8;
289         return true;
290 
291 sigsegv:
292         force_sig(SIGSEGV);
293         return true;
294 }
295 
296 /*
297  * A pseudo VMA to allow ptrace access for the vsyscall page.  This only
298  * covers the 64bit vsyscall page now. 32bit has a real VMA now and does
299  * not need special handling anymore:
300  */
301 static const char *gate_vma_name(struct vm_area_struct *vma)
302 {
303         return "[vsyscall]";
304 }
305 static const struct vm_operations_struct gate_vma_ops = {
306         .name = gate_vma_name,
307 };
308 static struct vm_area_struct gate_vma __ro_after_init = {
309         .vm_start       = VSYSCALL_ADDR,
310         .vm_end         = VSYSCALL_ADDR + PAGE_SIZE,
311         .vm_page_prot   = PAGE_READONLY_EXEC,
312         .vm_flags       = VM_READ | VM_EXEC,
313         .vm_ops         = &gate_vma_ops,
314 };
315 
316 struct vm_area_struct *get_gate_vma(struct mm_struct *mm)
317 {
318 #ifdef CONFIG_COMPAT
319         if (!mm || mm->context.ia32_compat)
320                 return NULL;
321 #endif
322         if (vsyscall_mode == NONE)
323                 return NULL;
324         return &gate_vma;
325 }
326 
327 int in_gate_area(struct mm_struct *mm, unsigned long addr)
328 {
329         struct vm_area_struct *vma = get_gate_vma(mm);
330 
331         if (!vma)
332                 return 0;
333 
334         return (addr >= vma->vm_start) && (addr < vma->vm_end);
335 }
336 
337 /*
338  * Use this when you have no reliable mm, typically from interrupt
339  * context. It is less reliable than using a task's mm and may give
340  * false positives.
341  */
342 int in_gate_area_no_mm(unsigned long addr)
343 {
344         return vsyscall_mode != NONE && (addr & PAGE_MASK) == VSYSCALL_ADDR;
345 }
346 
347 /*
348  * The VSYSCALL page is the only user-accessible page in the kernel address
349  * range.  Normally, the kernel page tables can have _PAGE_USER clear, but
350  * the tables covering VSYSCALL_ADDR need _PAGE_USER set if vsyscalls
351  * are enabled.
352  *
353  * Some day we may create a "minimal" vsyscall mode in which we emulate
354  * vsyscalls but leave the page not present.  If so, we skip calling
355  * this.
356  */
357 void __init set_vsyscall_pgtable_user_bits(pgd_t *root)
358 {
359         pgd_t *pgd;
360         p4d_t *p4d;
361         pud_t *pud;
362         pmd_t *pmd;
363 
364         pgd = pgd_offset_pgd(root, VSYSCALL_ADDR);
365         set_pgd(pgd, __pgd(pgd_val(*pgd) | _PAGE_USER));
366         p4d = p4d_offset(pgd, VSYSCALL_ADDR);
367 #if CONFIG_PGTABLE_LEVELS >= 5
368         set_p4d(p4d, __p4d(p4d_val(*p4d) | _PAGE_USER));
369 #endif
370         pud = pud_offset(p4d, VSYSCALL_ADDR);
371         set_pud(pud, __pud(pud_val(*pud) | _PAGE_USER));
372         pmd = pmd_offset(pud, VSYSCALL_ADDR);
373         set_pmd(pmd, __pmd(pmd_val(*pmd) | _PAGE_USER));
374 }
375 
376 void __init map_vsyscall(void)
377 {
378         extern char __vsyscall_page;
379         unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);
380 
381         /*
382          * For full emulation, the page needs to exist for real.  In
383          * execute-only mode, there is no PTE at all backing the vsyscall
384          * page.
385          */
386         if (vsyscall_mode == EMULATE) {
387                 __set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
388                              PAGE_KERNEL_VVAR);
389                 set_vsyscall_pgtable_user_bits(swapper_pg_dir);
390         }
391 
392         if (vsyscall_mode == XONLY)
393                 gate_vma.vm_flags = VM_EXEC;
394 
395         BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
396                      (unsigned long)VSYSCALL_ADDR);
397 }
398 

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