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

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  1 /*
  2  * Copyright (c) 2012-2014 Andy Lutomirski <luto@amacapital.net>
  3  *
  4  * Based on the original implementation which is:
  5  *  Copyright (C) 2001 Andrea Arcangeli <andrea@suse.de> SuSE
  6  *  Copyright 2003 Andi Kleen, SuSE Labs.
  7  *
  8  *  Parts of the original code have been moved to arch/x86/vdso/vma.c
  9  *
 10  * This file implements vsyscall emulation.  vsyscalls are a legacy ABI:
 11  * Userspace can request certain kernel services by calling fixed
 12  * addresses.  This concept is problematic:
 13  *
 14  * - It interferes with ASLR.
 15  * - It's awkward to write code that lives in kernel addresses but is
 16  *   callable by userspace at fixed addresses.
 17  * - The whole concept is impossible for 32-bit compat userspace.
 18  * - UML cannot easily virtualize a vsyscall.
 19  *
 20  * As of mid-2014, I believe that there is no new userspace code that
 21  * will use a vsyscall if the vDSO is present.  I hope that there will
 22  * soon be no new userspace code that will ever use a vsyscall.
 23  *
 24  * The code in this file emulates vsyscalls when notified of a page
 25  * fault to a vsyscall address.
 26  */
 27 
 28 #include <linux/kernel.h>
 29 #include <linux/timer.h>
 30 #include <linux/syscalls.h>
 31 #include <linux/ratelimit.h>
 32 
 33 #include <asm/vsyscall.h>
 34 #include <asm/unistd.h>
 35 #include <asm/fixmap.h>
 36 #include <asm/traps.h>
 37 
 38 #define CREATE_TRACE_POINTS
 39 #include "vsyscall_trace.h"
 40 
 41 static enum { EMULATE, NATIVE, NONE } vsyscall_mode =
 42 #if defined(CONFIG_LEGACY_VSYSCALL_NATIVE)
 43         NATIVE;
 44 #elif defined(CONFIG_LEGACY_VSYSCALL_NONE)
 45         NONE;
 46 #else
 47         EMULATE;
 48 #endif
 49 
 50 static int __init vsyscall_setup(char *str)
 51 {
 52         if (str) {
 53                 if (!strcmp("emulate", str))
 54                         vsyscall_mode = EMULATE;
 55                 else if (!strcmp("native", str))
 56                         vsyscall_mode = NATIVE;
 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_error, this could go away.
100          */
101 
102         if (!access_ok(VERIFY_WRITE, (void __user *)ptr, size)) {
103                 siginfo_t info;
104                 struct thread_struct *thread = &current->thread;
105 
106                 thread->error_code      = 6;  /* user fault, no page, write */
107                 thread->cr2             = ptr;
108                 thread->trap_nr         = X86_TRAP_PF;
109 
110                 memset(&info, 0, sizeof(info));
111                 info.si_signo           = SIGSEGV;
112                 info.si_errno           = 0;
113                 info.si_code            = SEGV_MAPERR;
114                 info.si_addr            = (void __user *)ptr;
115 
116                 force_sig_info(SIGSEGV, &info, current);
117                 return false;
118         } else {
119                 return true;
120         }
121 }
122 
123 bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
124 {
125         struct task_struct *tsk;
126         unsigned long caller;
127         int vsyscall_nr, syscall_nr, tmp;
128         int prev_sig_on_uaccess_error;
129         long ret;
130 
131         /*
132          * No point in checking CS -- the only way to get here is a user mode
133          * trap to a high address, which means that we're in 64-bit user code.
134          */
135 
136         WARN_ON_ONCE(address != regs->ip);
137 
138         if (vsyscall_mode == NONE) {
139                 warn_bad_vsyscall(KERN_INFO, regs,
140                                   "vsyscall attempted with vsyscall=none");
141                 return false;
142         }
143 
144         vsyscall_nr = addr_to_vsyscall_nr(address);
145 
146         trace_emulate_vsyscall(vsyscall_nr);
147 
148         if (vsyscall_nr < 0) {
149                 warn_bad_vsyscall(KERN_WARNING, regs,
150                                   "misaligned vsyscall (exploit attempt or buggy program) -- look up the vsyscall kernel parameter if you need a workaround");
151                 goto sigsegv;
152         }
153 
154         if (get_user(caller, (unsigned long __user *)regs->sp) != 0) {
155                 warn_bad_vsyscall(KERN_WARNING, regs,
156                                   "vsyscall with bad stack (exploit attempt?)");
157                 goto sigsegv;
158         }
159 
160         tsk = current;
161 
162         /*
163          * Check for access_ok violations and find the syscall nr.
164          *
165          * NULL is a valid user pointer (in the access_ok sense) on 32-bit and
166          * 64-bit, so we don't need to special-case it here.  For all the
167          * vsyscalls, NULL means "don't write anything" not "write it at
168          * address 0".
169          */
170         switch (vsyscall_nr) {
171         case 0:
172                 if (!write_ok_or_segv(regs->di, sizeof(struct timeval)) ||
173                     !write_ok_or_segv(regs->si, sizeof(struct timezone))) {
174                         ret = -EFAULT;
175                         goto check_fault;
176                 }
177 
178                 syscall_nr = __NR_gettimeofday;
179                 break;
180 
181         case 1:
182                 if (!write_ok_or_segv(regs->di, sizeof(time_t))) {
183                         ret = -EFAULT;
184                         goto check_fault;
185                 }
186 
187                 syscall_nr = __NR_time;
188                 break;
189 
190         case 2:
191                 if (!write_ok_or_segv(regs->di, sizeof(unsigned)) ||
192                     !write_ok_or_segv(regs->si, sizeof(unsigned))) {
193                         ret = -EFAULT;
194                         goto check_fault;
195                 }
196 
197                 syscall_nr = __NR_getcpu;
198                 break;
199         }
200 
201         /*
202          * Handle seccomp.  regs->ip must be the original value.
203          * See seccomp_send_sigsys and Documentation/prctl/seccomp_filter.txt.
204          *
205          * We could optimize the seccomp disabled case, but performance
206          * here doesn't matter.
207          */
208         regs->orig_ax = syscall_nr;
209         regs->ax = -ENOSYS;
210         tmp = secure_computing();
211         if ((!tmp && regs->orig_ax != syscall_nr) || regs->ip != address) {
212                 warn_bad_vsyscall(KERN_DEBUG, regs,
213                                   "seccomp tried to change syscall nr or ip");
214                 do_exit(SIGSYS);
215         }
216         regs->orig_ax = -1;
217         if (tmp)
218                 goto do_ret;  /* skip requested */
219 
220         /*
221          * With a real vsyscall, page faults cause SIGSEGV.  We want to
222          * preserve that behavior to make writing exploits harder.
223          */
224         prev_sig_on_uaccess_error = current_thread_info()->sig_on_uaccess_error;
225         current_thread_info()->sig_on_uaccess_error = 1;
226 
227         ret = -EFAULT;
228         switch (vsyscall_nr) {
229         case 0:
230                 ret = sys_gettimeofday(
231                         (struct timeval __user *)regs->di,
232                         (struct timezone __user *)regs->si);
233                 break;
234 
235         case 1:
236                 ret = sys_time((time_t __user *)regs->di);
237                 break;
238 
239         case 2:
240                 ret = sys_getcpu((unsigned __user *)regs->di,
241                                  (unsigned __user *)regs->si,
242                                  NULL);
243                 break;
244         }
245 
246         current_thread_info()->sig_on_uaccess_error = prev_sig_on_uaccess_error;
247 
248 check_fault:
249         if (ret == -EFAULT) {
250                 /* Bad news -- userspace fed a bad pointer to a vsyscall. */
251                 warn_bad_vsyscall(KERN_INFO, regs,
252                                   "vsyscall fault (exploit attempt?)");
253 
254                 /*
255                  * If we failed to generate a signal for any reason,
256                  * generate one here.  (This should be impossible.)
257                  */
258                 if (WARN_ON_ONCE(!sigismember(&tsk->pending.signal, SIGBUS) &&
259                                  !sigismember(&tsk->pending.signal, SIGSEGV)))
260                         goto sigsegv;
261 
262                 return true;  /* Don't emulate the ret. */
263         }
264 
265         regs->ax = ret;
266 
267 do_ret:
268         /* Emulate a ret instruction. */
269         regs->ip = caller;
270         regs->sp += 8;
271         return true;
272 
273 sigsegv:
274         force_sig(SIGSEGV, current);
275         return true;
276 }
277 
278 /*
279  * A pseudo VMA to allow ptrace access for the vsyscall page.  This only
280  * covers the 64bit vsyscall page now. 32bit has a real VMA now and does
281  * not need special handling anymore:
282  */
283 static const char *gate_vma_name(struct vm_area_struct *vma)
284 {
285         return "[vsyscall]";
286 }
287 static const struct vm_operations_struct gate_vma_ops = {
288         .name = gate_vma_name,
289 };
290 static struct vm_area_struct gate_vma = {
291         .vm_start       = VSYSCALL_ADDR,
292         .vm_end         = VSYSCALL_ADDR + PAGE_SIZE,
293         .vm_page_prot   = PAGE_READONLY_EXEC,
294         .vm_flags       = VM_READ | VM_EXEC,
295         .vm_ops         = &gate_vma_ops,
296 };
297 
298 struct vm_area_struct *get_gate_vma(struct mm_struct *mm)
299 {
300 #ifdef CONFIG_COMPAT
301         if (!mm || mm->context.ia32_compat)
302                 return NULL;
303 #endif
304         if (vsyscall_mode == NONE)
305                 return NULL;
306         return &gate_vma;
307 }
308 
309 int in_gate_area(struct mm_struct *mm, unsigned long addr)
310 {
311         struct vm_area_struct *vma = get_gate_vma(mm);
312 
313         if (!vma)
314                 return 0;
315 
316         return (addr >= vma->vm_start) && (addr < vma->vm_end);
317 }
318 
319 /*
320  * Use this when you have no reliable mm, typically from interrupt
321  * context. It is less reliable than using a task's mm and may give
322  * false positives.
323  */
324 int in_gate_area_no_mm(unsigned long addr)
325 {
326         return vsyscall_mode != NONE && (addr & PAGE_MASK) == VSYSCALL_ADDR;
327 }
328 
329 void __init map_vsyscall(void)
330 {
331         extern char __vsyscall_page;
332         unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);
333 
334         if (vsyscall_mode != NONE)
335                 __set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
336                              vsyscall_mode == NATIVE
337                              ? PAGE_KERNEL_VSYSCALL
338                              : PAGE_KERNEL_VVAR);
339 
340         BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
341                      (unsigned long)VSYSCALL_ADDR);
342 }
343 

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