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
45static 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 #error VSYSCALL config is broken
52#endif
53
54static 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}
71early_param("vsyscall", vsyscall_setup);
72
73static 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:%x 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
85static 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
99static bool write_ok_or_segv(unsigned long ptr, size_t size)
100{
101 if (!access_ok((void __user *)ptr, size)) {
102 struct thread_struct *thread = &current->thread;
103
104 thread->error_code = X86_PF_USER | X86_PF_WRITE;
105 thread->cr2 = ptr;
106 thread->trap_nr = X86_TRAP_PF;
107
108 force_sig_fault(SIGSEGV, SEGV_MAPERR, addr: (void __user *)ptr);
109 return false;
110 } else {
111 return true;
112 }
113}
114
115bool emulate_vsyscall(unsigned long error_code,
116 struct pt_regs *regs, unsigned long address)
117{
118 unsigned long caller;
119 int vsyscall_nr, syscall_nr, tmp;
120 long ret;
121 unsigned long orig_dx;
122
123 /* Write faults or kernel-privilege faults never get fixed up. */
124 if ((error_code & (X86_PF_WRITE | X86_PF_USER)) != X86_PF_USER)
125 return false;
126
127 /*
128 * Assume that faults at regs->ip are because of an
129 * instruction fetch. Return early and avoid
130 * emulation for faults during data accesses:
131 */
132 if (address != regs->ip) {
133 /* Failed vsyscall read */
134 if (vsyscall_mode == EMULATE)
135 return false;
136
137 /*
138 * User code tried and failed to read the vsyscall page.
139 */
140 warn_bad_vsyscall(KERN_INFO, regs, message: "vsyscall read attempt denied -- look up the vsyscall kernel parameter if you need a workaround");
141 return false;
142 }
143
144 /*
145 * X86_PF_INSTR is only set when NX is supported. When
146 * available, use it to double-check that the emulation code
147 * is only being used for instruction fetches:
148 */
149 if (cpu_feature_enabled(X86_FEATURE_NX))
150 WARN_ON_ONCE(!(error_code & X86_PF_INSTR));
151
152 /*
153 * No point in checking CS -- the only way to get here is a user mode
154 * trap to a high address, which means that we're in 64-bit user code.
155 */
156
157 if (vsyscall_mode == NONE) {
158 warn_bad_vsyscall(KERN_INFO, regs,
159 message: "vsyscall attempted with vsyscall=none");
160 return false;
161 }
162
163 vsyscall_nr = addr_to_vsyscall_nr(addr: address);
164
165 trace_emulate_vsyscall(nr: vsyscall_nr);
166
167 if (vsyscall_nr < 0) {
168 warn_bad_vsyscall(KERN_WARNING, regs,
169 message: "misaligned vsyscall (exploit attempt or buggy program) -- look up the vsyscall kernel parameter if you need a workaround");
170 goto sigsegv;
171 }
172
173 if (get_user(caller, (unsigned long __user *)regs->sp) != 0) {
174 warn_bad_vsyscall(KERN_WARNING, regs,
175 message: "vsyscall with bad stack (exploit attempt?)");
176 goto sigsegv;
177 }
178
179 /*
180 * Check for access_ok violations and find the syscall nr.
181 *
182 * NULL is a valid user pointer (in the access_ok sense) on 32-bit and
183 * 64-bit, so we don't need to special-case it here. For all the
184 * vsyscalls, NULL means "don't write anything" not "write it at
185 * address 0".
186 */
187 switch (vsyscall_nr) {
188 case 0:
189 if (!write_ok_or_segv(ptr: regs->di, size: sizeof(struct __kernel_old_timeval)) ||
190 !write_ok_or_segv(ptr: regs->si, size: sizeof(struct timezone))) {
191 ret = -EFAULT;
192 goto check_fault;
193 }
194
195 syscall_nr = __NR_gettimeofday;
196 break;
197
198 case 1:
199 if (!write_ok_or_segv(ptr: regs->di, size: sizeof(__kernel_old_time_t))) {
200 ret = -EFAULT;
201 goto check_fault;
202 }
203
204 syscall_nr = __NR_time;
205 break;
206
207 case 2:
208 if (!write_ok_or_segv(ptr: regs->di, size: sizeof(unsigned)) ||
209 !write_ok_or_segv(ptr: regs->si, size: sizeof(unsigned))) {
210 ret = -EFAULT;
211 goto check_fault;
212 }
213
214 syscall_nr = __NR_getcpu;
215 break;
216 }
217
218 /*
219 * Handle seccomp. regs->ip must be the original value.
220 * See seccomp_send_sigsys and Documentation/userspace-api/seccomp_filter.rst.
221 *
222 * We could optimize the seccomp disabled case, but performance
223 * here doesn't matter.
224 */
225 regs->orig_ax = syscall_nr;
226 regs->ax = -ENOSYS;
227 tmp = secure_computing();
228 if ((!tmp && regs->orig_ax != syscall_nr) || regs->ip != address) {
229 warn_bad_vsyscall(KERN_DEBUG, regs,
230 message: "seccomp tried to change syscall nr or ip");
231 force_exit_sig(SIGSYS);
232 return true;
233 }
234 regs->orig_ax = -1;
235 if (tmp)
236 goto do_ret; /* skip requested */
237
238 /*
239 * With a real vsyscall, page faults cause SIGSEGV.
240 */
241 ret = -EFAULT;
242 switch (vsyscall_nr) {
243 case 0:
244 /* this decodes regs->di and regs->si on its own */
245 ret = __x64_sys_gettimeofday(regs);
246 break;
247
248 case 1:
249 /* this decodes regs->di on its own */
250 ret = __x64_sys_time(regs);
251 break;
252
253 case 2:
254 /* while we could clobber regs->dx, we didn't in the past... */
255 orig_dx = regs->dx;
256 regs->dx = 0;
257 /* this decodes regs->di, regs->si and regs->dx on its own */
258 ret = __x64_sys_getcpu(regs);
259 regs->dx = orig_dx;
260 break;
261 }
262
263check_fault:
264 if (ret == -EFAULT) {
265 /* Bad news -- userspace fed a bad pointer to a vsyscall. */
266 warn_bad_vsyscall(KERN_INFO, regs,
267 message: "vsyscall fault (exploit attempt?)");
268 goto sigsegv;
269 }
270
271 regs->ax = ret;
272
273do_ret:
274 /* Emulate a ret instruction. */
275 regs->ip = caller;
276 regs->sp += 8;
277 return true;
278
279sigsegv:
280 force_sig(SIGSEGV);
281 return true;
282}
283
284/*
285 * A pseudo VMA to allow ptrace access for the vsyscall page. This only
286 * covers the 64bit vsyscall page now. 32bit has a real VMA now and does
287 * not need special handling anymore:
288 */
289static const char *gate_vma_name(struct vm_area_struct *vma)
290{
291 return "[vsyscall]";
292}
293static const struct vm_operations_struct gate_vma_ops = {
294 .name = gate_vma_name,
295};
296static struct vm_area_struct gate_vma __ro_after_init = {
297 .vm_start = VSYSCALL_ADDR,
298 .vm_end = VSYSCALL_ADDR + PAGE_SIZE,
299 .vm_page_prot = PAGE_READONLY_EXEC,
300 .vm_flags = VM_READ | VM_EXEC,
301 .vm_ops = &gate_vma_ops,
302};
303
304struct vm_area_struct *get_gate_vma(struct mm_struct *mm)
305{
306#ifdef CONFIG_COMPAT
307 if (!mm || !test_bit(MM_CONTEXT_HAS_VSYSCALL, &mm->context.flags))
308 return NULL;
309#endif
310 if (vsyscall_mode == NONE)
311 return NULL;
312 return &gate_vma;
313}
314
315int in_gate_area(struct mm_struct *mm, unsigned long addr)
316{
317 struct vm_area_struct *vma = get_gate_vma(mm);
318
319 if (!vma)
320 return 0;
321
322 return (addr >= vma->vm_start) && (addr < vma->vm_end);
323}
324
325/*
326 * Use this when you have no reliable mm, typically from interrupt
327 * context. It is less reliable than using a task's mm and may give
328 * false positives.
329 */
330int in_gate_area_no_mm(unsigned long addr)
331{
332 return vsyscall_mode != NONE && (addr & PAGE_MASK) == VSYSCALL_ADDR;
333}
334
335/*
336 * The VSYSCALL page is the only user-accessible page in the kernel address
337 * range. Normally, the kernel page tables can have _PAGE_USER clear, but
338 * the tables covering VSYSCALL_ADDR need _PAGE_USER set if vsyscalls
339 * are enabled.
340 *
341 * Some day we may create a "minimal" vsyscall mode in which we emulate
342 * vsyscalls but leave the page not present. If so, we skip calling
343 * this.
344 */
345void __init set_vsyscall_pgtable_user_bits(pgd_t *root)
346{
347 pgd_t *pgd;
348 p4d_t *p4d;
349 pud_t *pud;
350 pmd_t *pmd;
351
352 pgd = pgd_offset_pgd(pgd: root, VSYSCALL_ADDR);
353 set_pgd(pgd, __pgd(pgd_val(*pgd) | _PAGE_USER));
354 p4d = p4d_offset(pgd, VSYSCALL_ADDR);
355 set_p4d(p4dp: p4d, p4d: __p4d(val: p4d_val(p4d: *p4d) | _PAGE_USER));
356 pud = pud_offset(p4d, VSYSCALL_ADDR);
357 set_pud(pudp: pud, pud: __pud(val: pud_val(pud: *pud) | _PAGE_USER));
358 pmd = pmd_offset(pud, VSYSCALL_ADDR);
359 set_pmd(pmdp: pmd, pmd: __pmd(val: pmd_val(pmd: *pmd) | _PAGE_USER));
360}
361
362void __init map_vsyscall(void)
363{
364 extern char __vsyscall_page;
365 unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);
366
367 /*
368 * For full emulation, the page needs to exist for real. In
369 * execute-only mode, there is no PTE at all backing the vsyscall
370 * page.
371 */
372 if (vsyscall_mode == EMULATE) {
373 __set_fixmap(idx: VSYSCALL_PAGE, phys: physaddr_vsyscall,
374 PAGE_KERNEL_VVAR);
375 set_vsyscall_pgtable_user_bits(swapper_pg_dir);
376 }
377
378 if (vsyscall_mode == XONLY)
379 vm_flags_init(vma: &gate_vma, VM_EXEC);
380
381 BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
382 (unsigned long)VSYSCALL_ADDR);
383}
384

source code of linux/arch/x86/entry/vsyscall/vsyscall_64.c