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/sched/signal.h>
  31 #include <linux/mm_types.h>
  32 #include <linux/syscalls.h>
  33 #include <linux/ratelimit.h>
  34
  35 #include <asm/vsyscall.h>
  36 #include <asm/unistd.h>
  37 #include <asm/fixmap.h>
  38 #include <asm/traps.h>
  39
  40 #define CREATE_TRACE_POINTS
  41 #include "vsyscall_trace.h"
  42
  43 static enum { EMULATE, NATIVE, NONE } vsyscall_mode =
  44 #if defined(CONFIG_LEGACY_VSYSCALL_NATIVE)
  45         NATIVE;
  46 #elif defined(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("native", str))
  58                         vsyscall_mode = NATIVE;
  59                 else if (!strcmp("none", str))
  60                         vsyscall_mode = NONE;
  61                 else
  62                         return -EINVAL;
  63
  64                 return 0;
  65         }
  66
  67         return -EINVAL;
  68 }
  69 early_param("vsyscall", vsyscall_setup);
  70
  71 static void warn_bad_vsyscall(const char *level, struct pt_regs *regs,
  72                               const char *message)
  73 {
  74         if (!show_unhandled_signals)
  75                 return;
  76
  77         printk_ratelimited("%s%s[%d] %s ip:%lx cs:%lx sp:%lx ax:%lx si:%lx di:%lx\n",
  78                            level, current->comm, task_pid_nr(current),
  79                            message, regs->ip, regs->cs,
  80                            regs->sp, regs->ax, regs->si, regs->di);
  81 }
  82
  83 static int addr_to_vsyscall_nr(unsigned long addr)
  84 {
  85         int nr;
  86
  87         if ((addr & ~0xC00UL) != VSYSCALL_ADDR)
  88                 return -EINVAL;
  89
  90         nr = (addr & 0xC00UL) >> 10;
  91         if (nr >= 3)
  92                 return -EINVAL;
  93
  94         return nr;
  95 }
  96
  97 static bool write_ok_or_segv(unsigned long ptr, size_t size)
  98 {
  99         /*
 100          * XXX: if access_ok, get_user, and put_user handled
 101          * sig_on_uaccess_err, this could go away.
 102          */
 103
 104         if (!access_ok(VERIFY_WRITE, (void __user *)ptr, size)) {
 105                 siginfo_t info;
 106                 struct thread_struct *thread = &current->thread;
 107
 108                 thread->error_code      = 6;  /* user fault, no page, write */
 109                 thread->cr2             = ptr;
 110                 thread->trap_nr         = X86_TRAP_PF;
 111
 112                 memset(&info, 0, sizeof(info));
 113                 info.si_signo           = SIGSEGV;
 114                 info.si_errno           = 0;
 115                 info.si_code            = SEGV_MAPERR;
 116                 info.si_addr            = (void __user *)ptr;
 117
 118                 force_sig_info(SIGSEGV, &info, current);
 119                 return false;
 120         } else {
 121                 return true;
 122         }
 123 }
 124
 125 bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
 126 {
 127         struct task_struct *tsk;
 128         unsigned long caller;
 129         int vsyscall_nr, syscall_nr, tmp;
 130         int prev_sig_on_uaccess_err;
 131         long ret;
 132
 133         /*
 134          * No point in checking CS -- the only way to get here is a user mode
 135          * trap to a high address, which means that we're in 64-bit user code.
 136          */
 137
 138         WARN_ON_ONCE(address != regs->ip);
 139
 140         if (vsyscall_mode == NONE) {
 141                 warn_bad_vsyscall(KERN_INFO, regs,
 142                                   "vsyscall attempted with vsyscall=none");
 143                 return false;
 144         }
 145
 146         vsyscall_nr = addr_to_vsyscall_nr(address);
 147
 148         trace_emulate_vsyscall(vsyscall_nr);
 149
 150         if (vsyscall_nr < 0) {
 151                 warn_bad_vsyscall(KERN_WARNING, regs,
 152                                   "misaligned vsyscall (exploit attempt or buggy program) -- look up the vsyscall kernel parameter if you need a workaround");
 153                 goto sigsegv;
 154         }
 155
 156         if (get_user(caller, (unsigned long __user *)regs->sp) != 0) {
 157                 warn_bad_vsyscall(KERN_WARNING, regs,
 158                                   "vsyscall with bad stack (exploit attempt?)");
 159                 goto sigsegv;
 160         }
 161
 162         tsk = current;
 163
 164         /*
 165          * Check for access_ok violations and find the syscall nr.
 166          *
 167          * NULL is a valid user pointer (in the access_ok sense) on 32-bit and
 168          * 64-bit, so we don't need to special-case it here.  For all the
 169          * vsyscalls, NULL means "don't write anything" not "write it at
 170          * address 0".
 171          */
 172         switch (vsyscall_nr) {
 173         case 0:
 174                 if (!write_ok_or_segv(regs->di, sizeof(struct timeval)) ||
 175                     !write_ok_or_segv(regs->si, sizeof(struct timezone))) {
 176                         ret = -EFAULT;
 177                         goto check_fault;
 178                 }
 179
 180                 syscall_nr = __NR_gettimeofday;
 181                 break;
 182
 183         case 1:
 184                 if (!write_ok_or_segv(regs->di, sizeof(time_t))) {
 185                         ret = -EFAULT;
 186                         goto check_fault;
 187                 }
 188
 189                 syscall_nr = __NR_time;
 190                 break;
 191
 192         case 2:
 193                 if (!write_ok_or_segv(regs->di, sizeof(unsigned)) ||
 194                     !write_ok_or_segv(regs->si, sizeof(unsigned))) {
 195                         ret = -EFAULT;
 196                         goto check_fault;
 197                 }
 198
 199                 syscall_nr = __NR_getcpu;
 200                 break;
 201         }
 202
 203         /*
 204          * Handle seccomp.  regs->ip must be the original value.
 205          * See seccomp_send_sigsys and Documentation/prctl/seccomp_filter.txt.
 206          *
 207          * We could optimize the seccomp disabled case, but performance
 208          * here doesn't matter.
 209          */
 210         regs->orig_ax = syscall_nr;
 211         regs->ax = -ENOSYS;
 212         tmp = secure_computing(NULL);
 213         if ((!tmp && regs->orig_ax != syscall_nr) || regs->ip != address) {
 214                 warn_bad_vsyscall(KERN_DEBUG, regs,
 215                                   "seccomp tried to change syscall nr or ip");
 216                 do_exit(SIGSYS);
 217         }
 218         regs->orig_ax = -1;
 219         if (tmp)
 220                 goto do_ret;  /* skip requested */
 221
 222         /*
 223          * With a real vsyscall, page faults cause SIGSEGV.  We want to
 224          * preserve that behavior to make writing exploits harder.
 225          */
 226         prev_sig_on_uaccess_err = current->thread.sig_on_uaccess_err;
 227         current->thread.sig_on_uaccess_err = 1;
 228
 229         ret = -EFAULT;
 230         switch (vsyscall_nr) {
 231         case 0:
 232                 ret = sys_gettimeofday(
 233                         (struct timeval __user *)regs->di,
 234                         (struct timezone __user *)regs->si);
 235                 break;
 236
 237         case 1:
 238                 ret = sys_time((time_t __user *)regs->di);
 239                 break;
 240
 241         case 2:
 242                 ret = sys_getcpu((unsigned __user *)regs->di,
 243                                  (unsigned __user *)regs->si,
 244                                  NULL);
 245                 break;
 246         }
 247
 248         current->thread.sig_on_uaccess_err = prev_sig_on_uaccess_err;
 249
 250 check_fault:
 251         if (ret == -EFAULT) {
 252                 /* Bad news -- userspace fed a bad pointer to a vsyscall. */
 253                 warn_bad_vsyscall(KERN_INFO, regs,
 254                                   "vsyscall fault (exploit attempt?)");
 255
 256                 /*
 257                  * If we failed to generate a signal for any reason,
 258                  * generate one here.  (This should be impossible.)
 259                  */
 260                 if (WARN_ON_ONCE(!sigismember(&tsk->pending.signal, SIGBUS) &&
 261                                  !sigismember(&tsk->pending.signal, SIGSEGV)))
 262                         goto sigsegv;
 263
 264                 return true;  /* Don't emulate the ret. */
 265         }
 266
 267         regs->ax = ret;
 268
 269 do_ret:
 270         /* Emulate a ret instruction. */
 271         regs->ip = caller;
 272         regs->sp += 8;
 273         return true;
 274
 275 sigsegv:
 276         force_sig(SIGSEGV, current);
 277         return true;
 278 }
 279
 280 /*
 281  * A pseudo VMA to allow ptrace access for the vsyscall page.  This only
 282  * covers the 64bit vsyscall page now. 32bit has a real VMA now and does
 283  * not need special handling anymore:
 284  */
 285 static const char *gate_vma_name(struct vm_area_struct *vma)
 286 {
 287         return "[vsyscall]";
 288 }
 289 static const struct vm_operations_struct gate_vma_ops = {
 290         .name = gate_vma_name,
 291 };
 292 static struct vm_area_struct gate_vma = {
 293         .vm_start       = VSYSCALL_ADDR,
 294         .vm_end         = VSYSCALL_ADDR + PAGE_SIZE,
 295         .vm_page_prot   = PAGE_READONLY_EXEC,
 296         .vm_flags       = VM_READ | VM_EXEC,
 297         .vm_ops         = &gate_vma_ops,
 298 };
 299
 300 struct vm_area_struct *get_gate_vma(struct mm_struct *mm)
 301 {
 302 #ifdef CONFIG_COMPAT
 303         if (!mm || mm->context.ia32_compat)
 304                 return NULL;
 305 #endif
 306         if (vsyscall_mode == NONE)
 307                 return NULL;
 308         return &gate_vma;
 309 }
 310
 311 int in_gate_area(struct mm_struct *mm, unsigned long addr)
 312 {
 313         struct vm_area_struct *vma = get_gate_vma(mm);
 314
 315         if (!vma)
 316                 return 0;
 317
 318         return (addr >= vma->vm_start) && (addr < vma->vm_end);
 319 }
 320
 321 /*
 322  * Use this when you have no reliable mm, typically from interrupt
 323  * context. It is less reliable than using a task's mm and may give
 324  * false positives.
 325  */
 326 int in_gate_area_no_mm(unsigned long addr)
 327 {
 328         return vsyscall_mode != NONE && (addr & PAGE_MASK) == VSYSCALL_ADDR;
 329 }
 330
 331 void __init map_vsyscall(void)
 332 {
 333         extern char __vsyscall_page;
 334         unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);
 335
 336         if (vsyscall_mode != NONE)
 337                 __set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
 338                              vsyscall_mode == NATIVE
 339                              ? PAGE_KERNEL_VSYSCALL
 340                              : PAGE_KERNEL_VVAR);
 341
 342         BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
 343                      (unsigned long)VSYSCALL_ADDR);
 344 }