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

DEFINITIONS

1. vsyscall_setup
2. warn_bad_vsyscall
3. addr_to_vsyscall_nr
4. write_ok_or_segv
5. emulate_vsyscall
6. gate_vma_name
7. get_gate_vma
8. in_gate_area
9. in_gate_area_no_mm
10. set_vsyscall_pgtable_user_bits
11. map_vsyscall

   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, XONLY, NONE } vsyscall_mode __ro_after_init =
  46 #ifdef CONFIG_LEGACY_VSYSCALL_NONE
  47         NONE;
  48 #elif defined(CONFIG_LEGACY_VSYSCALL_XONLY)
  49         XONLY;
  50 #else
  51         EMULATE;
  52 #endif
  53 
  54 static int __init vsyscall_setup(char *str)
  55 {
  56         if (str) {
  57                 if (!strcmp("emulate", str))
  58                         vsyscall_mode = EMULATE;
  59                 else if (!strcmp("xonly", str))
  60                         vsyscall_mode = XONLY;
  61                 else if (!strcmp("none", str))
  62                         vsyscall_mode = NONE;
  63                 else
  64                         return -EINVAL;
  65 
  66                 return 0;
  67         }
  68 
  69         return -EINVAL;
  70 }
  71 early_param("vsyscall", vsyscall_setup);
  72 
  73 static void warn_bad_vsyscall(const char *level, struct pt_regs *regs,
  74                               const char *message)
  75 {
  76         if (!show_unhandled_signals)
  77                 return;
  78 
  79         printk_ratelimited("%s%s[%d] %s ip:%lx cs:%lx sp:%lx ax:%lx si:%lx di:%lx\n",
  80                            level, current->comm, task_pid_nr(current),
  81                            message, regs->ip, regs->cs,
  82                            regs->sp, regs->ax, regs->si, regs->di);
  83 }
  84 
  85 static int addr_to_vsyscall_nr(unsigned long addr)
  86 {
  87         int nr;
  88 
  89         if ((addr & ~0xC00UL) != VSYSCALL_ADDR)
  90                 return -EINVAL;
  91 
  92         nr = (addr & 0xC00UL) >> 10;
  93         if (nr >= 3)
  94                 return -EINVAL;
  95 
  96         return nr;
  97 }
  98 
  99 static bool write_ok_or_segv(unsigned long ptr, size_t size)
 100 {
 101         /*
 102          * XXX: if access_ok, get_user, and put_user handled
 103          * sig_on_uaccess_err, this could go away.
 104          */
 105 
 106         if (!access_ok((void __user *)ptr, size)) {
 107                 struct thread_struct *thread = &current->thread;
 108 
 109                 thread->error_code      = X86_PF_USER | X86_PF_WRITE;
 110                 thread->cr2             = ptr;
 111                 thread->trap_nr         = X86_TRAP_PF;
 112 
 113                 force_sig_fault(SIGSEGV, SEGV_MAPERR, (void __user *)ptr);
 114                 return false;
 115         } else {
 116                 return true;
 117         }
 118 }
 119 
 120 bool emulate_vsyscall(unsigned long error_code,
 121                       struct pt_regs *regs, unsigned long address)
 122 {
 123         struct task_struct *tsk;
 124         unsigned long caller;
 125         int vsyscall_nr, syscall_nr, tmp;
 126         int prev_sig_on_uaccess_err;
 127         long ret;
 128         unsigned long orig_dx;
 129 
 130         /* Write faults or kernel-privilege faults never get fixed up. */
 131         if ((error_code & (X86_PF_WRITE | X86_PF_USER)) != X86_PF_USER)
 132                 return false;
 133 
 134         if (!(error_code & X86_PF_INSTR)) {
 135                 /* Failed vsyscall read */
 136                 if (vsyscall_mode == EMULATE)
 137                         return false;
 138 
 139                 /*
 140                  * User code tried and failed to read the vsyscall page.
 141                  */
 142                 warn_bad_vsyscall(KERN_INFO, regs, "vsyscall read attempt denied -- look up the vsyscall kernel parameter if you need a workaround");
 143                 return false;
 144         }
 145 
 146         /*
 147          * No point in checking CS -- the only way to get here is a user mode
 148          * trap to a high address, which means that we're in 64-bit user code.
 149          */
 150 
 151         WARN_ON_ONCE(address != regs->ip);
 152 
 153         if (vsyscall_mode == NONE) {
 154                 warn_bad_vsyscall(KERN_INFO, regs,
 155                                   "vsyscall attempted with vsyscall=none");
 156                 return false;
 157         }
 158 
 159         vsyscall_nr = addr_to_vsyscall_nr(address);
 160 
 161         trace_emulate_vsyscall(vsyscall_nr);
 162 
 163         if (vsyscall_nr < 0) {
 164                 warn_bad_vsyscall(KERN_WARNING, regs,
 165                                   "misaligned vsyscall (exploit attempt or buggy program) -- look up the vsyscall kernel parameter if you need a workaround");
 166                 goto sigsegv;
 167         }
 168 
 169         if (get_user(caller, (unsigned long __user *)regs->sp) != 0) {
 170                 warn_bad_vsyscall(KERN_WARNING, regs,
 171                                   "vsyscall with bad stack (exploit attempt?)");
 172                 goto sigsegv;
 173         }
 174 
 175         tsk = current;
 176 
 177         /*
 178          * Check for access_ok violations and find the syscall nr.
 179          *
 180          * NULL is a valid user pointer (in the access_ok sense) on 32-bit and
 181          * 64-bit, so we don't need to special-case it here.  For all the
 182          * vsyscalls, NULL means "don't write anything" not "write it at
 183          * address 0".
 184          */
 185         switch (vsyscall_nr) {
 186         case 0:
 187                 if (!write_ok_or_segv(regs->di, sizeof(struct timeval)) ||
 188                     !write_ok_or_segv(regs->si, sizeof(struct timezone))) {
 189                         ret = -EFAULT;
 190                         goto check_fault;
 191                 }
 192 
 193                 syscall_nr = __NR_gettimeofday;
 194                 break;
 195 
 196         case 1:
 197                 if (!write_ok_or_segv(regs->di, sizeof(time_t))) {
 198                         ret = -EFAULT;
 199                         goto check_fault;
 200                 }
 201 
 202                 syscall_nr = __NR_time;
 203                 break;
 204 
 205         case 2:
 206                 if (!write_ok_or_segv(regs->di, sizeof(unsigned)) ||
 207                     !write_ok_or_segv(regs->si, sizeof(unsigned))) {
 208                         ret = -EFAULT;
 209                         goto check_fault;
 210                 }
 211 
 212                 syscall_nr = __NR_getcpu;
 213                 break;
 214         }
 215 
 216         /*
 217          * Handle seccomp.  regs->ip must be the original value.
 218          * See seccomp_send_sigsys and Documentation/userspace-api/seccomp_filter.rst.
 219          *
 220          * We could optimize the seccomp disabled case, but performance
 221          * here doesn't matter.
 222          */
 223         regs->orig_ax = syscall_nr;
 224         regs->ax = -ENOSYS;
 225         tmp = secure_computing(NULL);
 226         if ((!tmp && regs->orig_ax != syscall_nr) || regs->ip != address) {
 227                 warn_bad_vsyscall(KERN_DEBUG, regs,
 228                                   "seccomp tried to change syscall nr or ip");
 229                 do_exit(SIGSYS);
 230         }
 231         regs->orig_ax = -1;
 232         if (tmp)
 233                 goto do_ret;  /* skip requested */
 234 
 235         /*
 236          * With a real vsyscall, page faults cause SIGSEGV.  We want to
 237          * preserve that behavior to make writing exploits harder.
 238          */
 239         prev_sig_on_uaccess_err = current->thread.sig_on_uaccess_err;
 240         current->thread.sig_on_uaccess_err = 1;
 241 
 242         ret = -EFAULT;
 243         switch (vsyscall_nr) {
 244         case 0:
 245                 /* this decodes regs->di and regs->si on its own */
 246                 ret = __x64_sys_gettimeofday(regs);
 247                 break;
 248 
 249         case 1:
 250                 /* this decodes regs->di on its own */
 251                 ret = __x64_sys_time(regs);
 252                 break;
 253 
 254         case 2:
 255                 /* while we could clobber regs->dx, we didn't in the past... */
 256                 orig_dx = regs->dx;
 257                 regs->dx = 0;
 258                 /* this decodes regs->di, regs->si and regs->dx on its own */
 259                 ret = __x64_sys_getcpu(regs);
 260                 regs->dx = orig_dx;
 261                 break;
 262         }
 263 
 264         current->thread.sig_on_uaccess_err = prev_sig_on_uaccess_err;
 265 
 266 check_fault:
 267         if (ret == -EFAULT) {
 268                 /* Bad news -- userspace fed a bad pointer to a vsyscall. */
 269                 warn_bad_vsyscall(KERN_INFO, regs,
 270                                   "vsyscall fault (exploit attempt?)");
 271 
 272                 /*
 273                  * If we failed to generate a signal for any reason,
 274                  * generate one here.  (This should be impossible.)
 275                  */
 276                 if (WARN_ON_ONCE(!sigismember(&tsk->pending.signal, SIGBUS) &&
 277                                  !sigismember(&tsk->pending.signal, SIGSEGV)))
 278                         goto sigsegv;
 279 
 280                 return true;  /* Don't emulate the ret. */
 281         }
 282 
 283         regs->ax = ret;
 284 
 285 do_ret:
 286         /* Emulate a ret instruction. */
 287         regs->ip = caller;
 288         regs->sp += 8;
 289         return true;
 290 
 291 sigsegv:
 292         force_sig(SIGSEGV);
 293         return true;
 294 }
 295 
 296 /*
 297  * A pseudo VMA to allow ptrace access for the vsyscall page.  This only
 298  * covers the 64bit vsyscall page now. 32bit has a real VMA now and does
 299  * not need special handling anymore:
 300  */
 301 static const char *gate_vma_name(struct vm_area_struct *vma)
 302 {
 303         return "[vsyscall]";
 304 }
 305 static const struct vm_operations_struct gate_vma_ops = {
 306         .name = gate_vma_name,
 307 };
 308 static struct vm_area_struct gate_vma __ro_after_init = {
 309         .vm_start       = VSYSCALL_ADDR,
 310         .vm_end         = VSYSCALL_ADDR + PAGE_SIZE,
 311         .vm_page_prot   = PAGE_READONLY_EXEC,
 312         .vm_flags       = VM_READ | VM_EXEC,
 313         .vm_ops         = &gate_vma_ops,
 314 };
 315 
 316 struct vm_area_struct *get_gate_vma(struct mm_struct *mm)
 317 {
 318 #ifdef CONFIG_COMPAT
 319         if (!mm || mm->context.ia32_compat)
 320                 return NULL;
 321 #endif
 322         if (vsyscall_mode == NONE)
 323                 return NULL;
 324         return &gate_vma;
 325 }
 326 
 327 int in_gate_area(struct mm_struct *mm, unsigned long addr)
 328 {
 329         struct vm_area_struct *vma = get_gate_vma(mm);
 330 
 331         if (!vma)
 332                 return 0;
 333 
 334         return (addr >= vma->vm_start) && (addr < vma->vm_end);
 335 }
 336 
 337 /*
 338  * Use this when you have no reliable mm, typically from interrupt
 339  * context. It is less reliable than using a task's mm and may give
 340  * false positives.
 341  */
 342 int in_gate_area_no_mm(unsigned long addr)
 343 {
 344         return vsyscall_mode != NONE && (addr & PAGE_MASK) == VSYSCALL_ADDR;
 345 }
 346 
 347 /*
 348  * The VSYSCALL page is the only user-accessible page in the kernel address
 349  * range.  Normally, the kernel page tables can have _PAGE_USER clear, but
 350  * the tables covering VSYSCALL_ADDR need _PAGE_USER set if vsyscalls
 351  * are enabled.
 352  *
 353  * Some day we may create a "minimal" vsyscall mode in which we emulate
 354  * vsyscalls but leave the page not present.  If so, we skip calling
 355  * this.
 356  */
 357 void __init set_vsyscall_pgtable_user_bits(pgd_t *root)
 358 {
 359         pgd_t *pgd;
 360         p4d_t *p4d;
 361         pud_t *pud;
 362         pmd_t *pmd;
 363 
 364         pgd = pgd_offset_pgd(root, VSYSCALL_ADDR);
 365         set_pgd(pgd, __pgd(pgd_val(*pgd) | _PAGE_USER));
 366         p4d = p4d_offset(pgd, VSYSCALL_ADDR);
 367 #if CONFIG_PGTABLE_LEVELS >= 5
 368         set_p4d(p4d, __p4d(p4d_val(*p4d) | _PAGE_USER));
 369 #endif
 370         pud = pud_offset(p4d, VSYSCALL_ADDR);
 371         set_pud(pud, __pud(pud_val(*pud) | _PAGE_USER));
 372         pmd = pmd_offset(pud, VSYSCALL_ADDR);
 373         set_pmd(pmd, __pmd(pmd_val(*pmd) | _PAGE_USER));
 374 }
 375 
 376 void __init map_vsyscall(void)
 377 {
 378         extern char __vsyscall_page;
 379         unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);
 380 
 381         /*
 382          * For full emulation, the page needs to exist for real.  In
 383          * execute-only mode, there is no PTE at all backing the vsyscall
 384          * page.
 385          */
 386         if (vsyscall_mode == EMULATE) {
 387                 __set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
 388                              PAGE_KERNEL_VVAR);
 389                 set_vsyscall_pgtable_user_bits(swapper_pg_dir);
 390         }
 391 
 392         if (vsyscall_mode == XONLY)
 393                 gate_vma.vm_flags = VM_EXEC;
 394 
 395         BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
 396                      (unsigned long)VSYSCALL_ADDR);
 397 }