arch/x86/kernel/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 = EMULATE;
  42
  43 static int __init vsyscall_setup(char *str)
  44 {
  45         if (str) {
  46                 if (!strcmp("emulate", str))
  47                         vsyscall_mode = EMULATE;
  48                 else if (!strcmp("native", str))
  49                         vsyscall_mode = NATIVE;
  50                 else if (!strcmp("none", str))
  51                         vsyscall_mode = NONE;
  52                 else
  53                         return -EINVAL;
  54
  55                 return 0;
  56         }
  57
  58         return -EINVAL;
  59 }
  60 early_param("vsyscall", vsyscall_setup);
  61
  62 static void warn_bad_vsyscall(const char *level, struct pt_regs *regs,
  63                               const char *message)
  64 {
  65         if (!show_unhandled_signals)
  66                 return;
  67
  68         printk_ratelimited("%s%s[%d] %s ip:%lx cs:%lx sp:%lx ax:%lx si:%lx di:%lx\n",
  69                            level, current->comm, task_pid_nr(current),
  70                            message, regs->ip, regs->cs,
  71                            regs->sp, regs->ax, regs->si, regs->di);
  72 }
  73
  74 static int addr_to_vsyscall_nr(unsigned long addr)
  75 {
  76         int nr;
  77
  78         if ((addr & ~0xC00UL) != VSYSCALL_ADDR)
  79                 return -EINVAL;
  80
  81         nr = (addr & 0xC00UL) >> 10;
  82         if (nr >= 3)
  83                 return -EINVAL;
  84
  85         return nr;
  86 }
  87
  88 static bool write_ok_or_segv(unsigned long ptr, size_t size)
  89 {
  90         /*
  91          * XXX: if access_ok, get_user, and put_user handled
  92          * sig_on_uaccess_error, this could go away.
  93          */
  94
  95         if (!access_ok(VERIFY_WRITE, (void __user *)ptr, size)) {
  96                 siginfo_t info;
  97                 struct thread_struct *thread = &current->thread;
  98
  99                 thread->error_code      = 6;  /* user fault, no page, write */
 100                 thread->cr2             = ptr;
 101                 thread->trap_nr         = X86_TRAP_PF;
 102
 103                 memset(&info, 0, sizeof(info));
 104                 info.si_signo           = SIGSEGV;
 105                 info.si_errno           = 0;
 106                 info.si_code            = SEGV_MAPERR;
 107                 info.si_addr            = (void __user *)ptr;
 108
 109                 force_sig_info(SIGSEGV, &info, 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_error;
 122         long ret;
 123
 124         /*
 125          * No point in checking CS -- the only way to get here is a user mode
 126          * trap to a high address, which means that we're in 64-bit user code.
 127          */
 128
 129         WARN_ON_ONCE(address != regs->ip);
 130
 131         if (vsyscall_mode == NONE) {
 132                 warn_bad_vsyscall(KERN_INFO, regs,
 133                                   "vsyscall attempted with vsyscall=none");
 134                 return false;
 135         }
 136
 137         vsyscall_nr = addr_to_vsyscall_nr(address);
 138
 139         trace_emulate_vsyscall(vsyscall_nr);
 140
 141         if (vsyscall_nr < 0) {
 142                 warn_bad_vsyscall(KERN_WARNING, regs,
 143                                   "misaligned vsyscall (exploit attempt or buggy program) -- look up the vsyscall kernel parameter if you need a workaround");
 144                 goto sigsegv;
 145         }
 146
 147         if (get_user(caller, (unsigned long __user *)regs->sp) != 0) {
 148                 warn_bad_vsyscall(KERN_WARNING, regs,
 149                                   "vsyscall with bad stack (exploit attempt?)");
 150                 goto sigsegv;
 151         }
 152
 153         tsk = current;
 154
 155         /*
 156          * Check for access_ok violations and find the syscall nr.
 157          *
 158          * NULL is a valid user pointer (in the access_ok sense) on 32-bit and
 159          * 64-bit, so we don't need to special-case it here.  For all the
 160          * vsyscalls, NULL means "don't write anything" not "write it at
 161          * address 0".
 162          */
 163         switch (vsyscall_nr) {
 164         case 0:
 165                 if (!write_ok_or_segv(regs->di, sizeof(struct timeval)) ||
 166                     !write_ok_or_segv(regs->si, sizeof(struct timezone))) {
 167                         ret = -EFAULT;
 168                         goto check_fault;
 169                 }
 170
 171                 syscall_nr = __NR_gettimeofday;
 172                 break;
 173
 174         case 1:
 175                 if (!write_ok_or_segv(regs->di, sizeof(time_t))) {
 176                         ret = -EFAULT;
 177                         goto check_fault;
 178                 }
 179
 180                 syscall_nr = __NR_time;
 181                 break;
 182
 183         case 2:
 184                 if (!write_ok_or_segv(regs->di, sizeof(unsigned)) ||
 185                     !write_ok_or_segv(regs->si, sizeof(unsigned))) {
 186                         ret = -EFAULT;
 187                         goto check_fault;
 188                 }
 189
 190                 syscall_nr = __NR_getcpu;
 191                 break;
 192         }
 193
 194         /*
 195          * Handle seccomp.  regs->ip must be the original value.
 196          * See seccomp_send_sigsys and Documentation/prctl/seccomp_filter.txt.
 197          *
 198          * We could optimize the seccomp disabled case, but performance
 199          * here doesn't matter.
 200          */
 201         regs->orig_ax = syscall_nr;
 202         regs->ax = -ENOSYS;
 203         tmp = secure_computing();
 204         if ((!tmp && regs->orig_ax != syscall_nr) || regs->ip != address) {
 205                 warn_bad_vsyscall(KERN_DEBUG, regs,
 206                                   "seccomp tried to change syscall nr or ip");
 207                 do_exit(SIGSYS);
 208         }
 209         regs->orig_ax = -1;
 210         if (tmp)
 211                 goto do_ret;  /* skip requested */
 212
 213         /*
 214          * With a real vsyscall, page faults cause SIGSEGV.  We want to
 215          * preserve that behavior to make writing exploits harder.
 216          */
 217         prev_sig_on_uaccess_error = current_thread_info()->sig_on_uaccess_error;
 218         current_thread_info()->sig_on_uaccess_error = 1;
 219
 220         ret = -EFAULT;
 221         switch (vsyscall_nr) {
 222         case 0:
 223                 ret = sys_gettimeofday(
 224                         (struct timeval __user *)regs->di,
 225                         (struct timezone __user *)regs->si);
 226                 break;
 227
 228         case 1:
 229                 ret = sys_time((time_t __user *)regs->di);
 230                 break;
 231
 232         case 2:
 233                 ret = sys_getcpu((unsigned __user *)regs->di,
 234                                  (unsigned __user *)regs->si,
 235                                  NULL);
 236                 break;
 237         }
 238
 239         current_thread_info()->sig_on_uaccess_error = prev_sig_on_uaccess_error;
 240
 241 check_fault:
 242         if (ret == -EFAULT) {
 243                 /* Bad news -- userspace fed a bad pointer to a vsyscall. */
 244                 warn_bad_vsyscall(KERN_INFO, regs,
 245                                   "vsyscall fault (exploit attempt?)");
 246
 247                 /*
 248                  * If we failed to generate a signal for any reason,
 249                  * generate one here.  (This should be impossible.)
 250                  */
 251                 if (WARN_ON_ONCE(!sigismember(&tsk->pending.signal, SIGBUS) &&
 252                                  !sigismember(&tsk->pending.signal, SIGSEGV)))
 253                         goto sigsegv;
 254
 255                 return true;  /* Don't emulate the ret. */
 256         }
 257
 258         regs->ax = ret;
 259
 260 do_ret:
 261         /* Emulate a ret instruction. */
 262         regs->ip = caller;
 263         regs->sp += 8;
 264         return true;
 265
 266 sigsegv:
 267         force_sig(SIGSEGV, current);
 268         return true;
 269 }
 270
 271 /*
 272  * A pseudo VMA to allow ptrace access for the vsyscall page.  This only
 273  * covers the 64bit vsyscall page now. 32bit has a real VMA now and does
 274  * not need special handling anymore:
 275  */
 276 static const char *gate_vma_name(struct vm_area_struct *vma)
 277 {
 278         return "[vsyscall]";
 279 }
 280 static struct vm_operations_struct gate_vma_ops = {
 281         .name = gate_vma_name,
 282 };
 283 static struct vm_area_struct gate_vma = {
 284         .vm_start       = VSYSCALL_ADDR,
 285         .vm_end         = VSYSCALL_ADDR + PAGE_SIZE,
 286         .vm_page_prot   = PAGE_READONLY_EXEC,
 287         .vm_flags       = VM_READ | VM_EXEC,
 288         .vm_ops         = &gate_vma_ops,
 289 };
 290
 291 struct vm_area_struct *get_gate_vma(struct mm_struct *mm)
 292 {
 293 #ifdef CONFIG_IA32_EMULATION
 294         if (!mm || mm->context.ia32_compat)
 295                 return NULL;
 296 #endif
 297         if (vsyscall_mode == NONE)
 298                 return NULL;
 299         return &gate_vma;
 300 }
 301
 302 int in_gate_area(struct mm_struct *mm, unsigned long addr)
 303 {
 304         struct vm_area_struct *vma = get_gate_vma(mm);
 305
 306         if (!vma)
 307                 return 0;
 308
 309         return (addr >= vma->vm_start) && (addr < vma->vm_end);
 310 }
 311
 312 /*
 313  * Use this when you have no reliable mm, typically from interrupt
 314  * context. It is less reliable than using a task's mm and may give
 315  * false positives.
 316  */
 317 int in_gate_area_no_mm(unsigned long addr)
 318 {
 319         return vsyscall_mode != NONE && (addr & PAGE_MASK) == VSYSCALL_ADDR;
 320 }
 321
 322 void __init map_vsyscall(void)
 323 {
 324         extern char __vsyscall_page;
 325         unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);
 326
 327         if (vsyscall_mode != NONE)
 328                 __set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
 329                              vsyscall_mode == NATIVE
 330                              ? PAGE_KERNEL_VSYSCALL
 331                              : PAGE_KERNEL_VVAR);
 332
 333         BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
 334                      (unsigned long)VSYSCALL_ADDR);
 335 }