arch/x86/entry/vsyscall/vsyscall_64.c

   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_err, 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_err;
 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(NULL);
 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_err = current->thread.sig_on_uaccess_err;
 225         current->thread.sig_on_uaccess_err = 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.sig_on_uaccess_err = prev_sig_on_uaccess_err;
 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 }