vsyscall_64.c

  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, (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, "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				  "vsyscall attempted with vsyscall=none");
160		return false;
161	}
162
163	vsyscall_nr = addr_to_vsyscall_nr(address);
164
165	trace_emulate_vsyscall(vsyscall_nr);
166
167	if (vsyscall_nr < 0) {
168		warn_bad_vsyscall(KERN_WARNING, regs,
169				  "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				  "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(regs->di, sizeof(struct __kernel_old_timeval)) ||
190		    !write_ok_or_segv(regs->si, 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(regs->di, 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(regs->di, sizeof(unsigned)) ||
209		    !write_ok_or_segv(regs->si, 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				  "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				  "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(root, VSYSCALL_ADDR);
353	set_pgd(pgd, __pgd(pgd_val(*pgd) | _PAGE_USER));
354	p4d = p4d_offset(pgd, VSYSCALL_ADDR);
355	set_p4d(p4d, __p4d(p4d_val(*p4d) | _PAGE_USER));
356	pud = pud_offset(p4d, VSYSCALL_ADDR);
357	set_pud(pud, __pud(pud_val(*pud) | _PAGE_USER));
358	pmd = pmd_offset(pud, VSYSCALL_ADDR);
359	set_pmd(pmd, __pmd(pmd_val(*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(VSYSCALL_PAGE, 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(&gate_vma, VM_EXEC);
380
381	BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
382		     (unsigned long)VSYSCALL_ADDR);
383}