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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, NONE } vsyscall_mode =
 46 #ifdef CONFIG_LEGACY_VSYSCALL_NONE
 47         NONE;
 48 #else
 49         EMULATE;
 50 #endif
 51 
 52 static int __init vsyscall_setup(char *str)
 53 {
 54         if (str) {
 55                 if (!strcmp("emulate", str))
 56                         vsyscall_mode = EMULATE;
 57                 else if (!strcmp("none", str))
 58                         vsyscall_mode = NONE;
 59                 else
 60                         return -EINVAL;
 61 
 62                 return 0;
 63         }
 64 
 65         return -EINVAL;
 66 }
 67 early_param("vsyscall", vsyscall_setup);
 68 
 69 static void warn_bad_vsyscall(const char *level, struct pt_regs *regs,
 70                               const char *message)
 71 {
 72         if (!show_unhandled_signals)
 73                 return;
 74 
 75         printk_ratelimited("%s%s[%d] %s ip:%lx cs:%lx sp:%lx ax:%lx si:%lx di:%lx\n",
 76                            level, current->comm, task_pid_nr(current),
 77                            message, regs->ip, regs->cs,
 78                            regs->sp, regs->ax, regs->si, regs->di);
 79 }
 80 
 81 static int addr_to_vsyscall_nr(unsigned long addr)
 82 {
 83         int nr;
 84 
 85         if ((addr & ~0xC00UL) != VSYSCALL_ADDR)
 86                 return -EINVAL;
 87 
 88         nr = (addr & 0xC00UL) >> 10;
 89         if (nr >= 3)
 90                 return -EINVAL;
 91 
 92         return nr;
 93 }
 94 
 95 static bool write_ok_or_segv(unsigned long ptr, size_t size)
 96 {
 97         /*
 98          * XXX: if access_ok, get_user, and put_user handled
 99          * sig_on_uaccess_err, this could go away.
100          */
101 
102         if (!access_ok((void __user *)ptr, size)) {
103                 struct thread_struct *thread = &current->thread;
104 
105                 thread->error_code      = X86_PF_USER | X86_PF_WRITE;
106                 thread->cr2             = ptr;
107                 thread->trap_nr         = X86_TRAP_PF;
108 
109                 force_sig_fault(SIGSEGV, SEGV_MAPERR, (void __user *)ptr, current);
110                 return false;
111         } else {
112                 return true;
113         }
114 }
115 
116 bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
117 {
118         struct task_struct *tsk;
119         unsigned long caller;
120         int vsyscall_nr, syscall_nr, tmp;
121         int prev_sig_on_uaccess_err;
122         long ret;
123         unsigned long orig_dx;
124 
125         /*
126          * No point in checking CS -- the only way to get here is a user mode
127          * trap to a high address, which means that we're in 64-bit user code.
128          */
129 
130         WARN_ON_ONCE(address != regs->ip);
131 
132         if (vsyscall_mode == NONE) {
133                 warn_bad_vsyscall(KERN_INFO, regs,
134                                   "vsyscall attempted with vsyscall=none");
135                 return false;
136         }
137 
138         vsyscall_nr = addr_to_vsyscall_nr(address);
139 
140         trace_emulate_vsyscall(vsyscall_nr);
141 
142         if (vsyscall_nr < 0) {
143                 warn_bad_vsyscall(KERN_WARNING, regs,
144                                   "misaligned vsyscall (exploit attempt or buggy program) -- look up the vsyscall kernel parameter if you need a workaround");
145                 goto sigsegv;
146         }
147 
148         if (get_user(caller, (unsigned long __user *)regs->sp) != 0) {
149                 warn_bad_vsyscall(KERN_WARNING, regs,
150                                   "vsyscall with bad stack (exploit attempt?)");
151                 goto sigsegv;
152         }
153 
154         tsk = current;
155 
156         /*
157          * Check for access_ok violations and find the syscall nr.
158          *
159          * NULL is a valid user pointer (in the access_ok sense) on 32-bit and
160          * 64-bit, so we don't need to special-case it here.  For all the
161          * vsyscalls, NULL means "don't write anything" not "write it at
162          * address 0".
163          */
164         switch (vsyscall_nr) {
165         case 0:
166                 if (!write_ok_or_segv(regs->di, sizeof(struct timeval)) ||
167                     !write_ok_or_segv(regs->si, sizeof(struct timezone))) {
168                         ret = -EFAULT;
169                         goto check_fault;
170                 }
171 
172                 syscall_nr = __NR_gettimeofday;
173                 break;
174 
175         case 1:
176                 if (!write_ok_or_segv(regs->di, sizeof(time_t))) {
177                         ret = -EFAULT;
178                         goto check_fault;
179                 }
180 
181                 syscall_nr = __NR_time;
182                 break;
183 
184         case 2:
185                 if (!write_ok_or_segv(regs->di, sizeof(unsigned)) ||
186                     !write_ok_or_segv(regs->si, sizeof(unsigned))) {
187                         ret = -EFAULT;
188                         goto check_fault;
189                 }
190 
191                 syscall_nr = __NR_getcpu;
192                 break;
193         }
194 
195         /*
196          * Handle seccomp.  regs->ip must be the original value.
197          * See seccomp_send_sigsys and Documentation/userspace-api/seccomp_filter.rst.
198          *
199          * We could optimize the seccomp disabled case, but performance
200          * here doesn't matter.
201          */
202         regs->orig_ax = syscall_nr;
203         regs->ax = -ENOSYS;
204         tmp = secure_computing(NULL);
205         if ((!tmp && regs->orig_ax != syscall_nr) || regs->ip != address) {
206                 warn_bad_vsyscall(KERN_DEBUG, regs,
207                                   "seccomp tried to change syscall nr or ip");
208                 do_exit(SIGSYS);
209         }
210         regs->orig_ax = -1;
211         if (tmp)
212                 goto do_ret;  /* skip requested */
213 
214         /*
215          * With a real vsyscall, page faults cause SIGSEGV.  We want to
216          * preserve that behavior to make writing exploits harder.
217          */
218         prev_sig_on_uaccess_err = current->thread.sig_on_uaccess_err;
219         current->thread.sig_on_uaccess_err = 1;
220 
221         ret = -EFAULT;
222         switch (vsyscall_nr) {
223         case 0:
224                 /* this decodes regs->di and regs->si on its own */
225                 ret = __x64_sys_gettimeofday(regs);
226                 break;
227 
228         case 1:
229                 /* this decodes regs->di on its own */
230                 ret = __x64_sys_time(regs);
231                 break;
232 
233         case 2:
234                 /* while we could clobber regs->dx, we didn't in the past... */
235                 orig_dx = regs->dx;
236                 regs->dx = 0;
237                 /* this decodes regs->di, regs->si and regs->dx on its own */
238                 ret = __x64_sys_getcpu(regs);
239                 regs->dx = orig_dx;
240                 break;
241         }
242 
243         current->thread.sig_on_uaccess_err = prev_sig_on_uaccess_err;
244 
245 check_fault:
246         if (ret == -EFAULT) {
247                 /* Bad news -- userspace fed a bad pointer to a vsyscall. */
248                 warn_bad_vsyscall(KERN_INFO, regs,
249                                   "vsyscall fault (exploit attempt?)");
250 
251                 /*
252                  * If we failed to generate a signal for any reason,
253                  * generate one here.  (This should be impossible.)
254                  */
255                 if (WARN_ON_ONCE(!sigismember(&tsk->pending.signal, SIGBUS) &&
256                                  !sigismember(&tsk->pending.signal, SIGSEGV)))
257                         goto sigsegv;
258 
259                 return true;  /* Don't emulate the ret. */
260         }
261 
262         regs->ax = ret;
263 
264 do_ret:
265         /* Emulate a ret instruction. */
266         regs->ip = caller;
267         regs->sp += 8;
268         return true;
269 
270 sigsegv:
271         force_sig(SIGSEGV, current);
272         return true;
273 }
274 
275 /*
276  * A pseudo VMA to allow ptrace access for the vsyscall page.  This only
277  * covers the 64bit vsyscall page now. 32bit has a real VMA now and does
278  * not need special handling anymore:
279  */
280 static const char *gate_vma_name(struct vm_area_struct *vma)
281 {
282         return "[vsyscall]";
283 }
284 static const struct vm_operations_struct gate_vma_ops = {
285         .name = gate_vma_name,
286 };
287 static struct vm_area_struct gate_vma = {
288         .vm_start       = VSYSCALL_ADDR,
289         .vm_end         = VSYSCALL_ADDR + PAGE_SIZE,
290         .vm_page_prot   = PAGE_READONLY_EXEC,
291         .vm_flags       = VM_READ | VM_EXEC,
292         .vm_ops         = &gate_vma_ops,
293 };
294 
295 struct vm_area_struct *get_gate_vma(struct mm_struct *mm)
296 {
297 #ifdef CONFIG_COMPAT
298         if (!mm || mm->context.ia32_compat)
299                 return NULL;
300 #endif
301         if (vsyscall_mode == NONE)
302                 return NULL;
303         return &gate_vma;
304 }
305 
306 int in_gate_area(struct mm_struct *mm, unsigned long addr)
307 {
308         struct vm_area_struct *vma = get_gate_vma(mm);
309 
310         if (!vma)
311                 return 0;
312 
313         return (addr >= vma->vm_start) && (addr < vma->vm_end);
314 }
315 
316 /*
317  * Use this when you have no reliable mm, typically from interrupt
318  * context. It is less reliable than using a task's mm and may give
319  * false positives.
320  */
321 int in_gate_area_no_mm(unsigned long addr)
322 {
323         return vsyscall_mode != NONE && (addr & PAGE_MASK) == VSYSCALL_ADDR;
324 }
325 
326 /*
327  * The VSYSCALL page is the only user-accessible page in the kernel address
328  * range.  Normally, the kernel page tables can have _PAGE_USER clear, but
329  * the tables covering VSYSCALL_ADDR need _PAGE_USER set if vsyscalls
330  * are enabled.
331  *
332  * Some day we may create a "minimal" vsyscall mode in which we emulate
333  * vsyscalls but leave the page not present.  If so, we skip calling
334  * this.
335  */
336 void __init set_vsyscall_pgtable_user_bits(pgd_t *root)
337 {
338         pgd_t *pgd;
339         p4d_t *p4d;
340         pud_t *pud;
341         pmd_t *pmd;
342 
343         pgd = pgd_offset_pgd(root, VSYSCALL_ADDR);
344         set_pgd(pgd, __pgd(pgd_val(*pgd) | _PAGE_USER));
345         p4d = p4d_offset(pgd, VSYSCALL_ADDR);
346 #if CONFIG_PGTABLE_LEVELS >= 5
347         set_p4d(p4d, __p4d(p4d_val(*p4d) | _PAGE_USER));
348 #endif
349         pud = pud_offset(p4d, VSYSCALL_ADDR);
350         set_pud(pud, __pud(pud_val(*pud) | _PAGE_USER));
351         pmd = pmd_offset(pud, VSYSCALL_ADDR);
352         set_pmd(pmd, __pmd(pmd_val(*pmd) | _PAGE_USER));
353 }
354 
355 void __init map_vsyscall(void)
356 {
357         extern char __vsyscall_page;
358         unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);
359 
360         if (vsyscall_mode != NONE) {
361                 __set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
362                              PAGE_KERNEL_VVAR);
363                 set_vsyscall_pgtable_user_bits(swapper_pg_dir);
364         }
365 
366         BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
367                      (unsigned long)VSYSCALL_ADDR);
368 }
369 

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