arch/x86/entry/vsyscall/vsyscall_64.c - kernel/common - Git at Google

 // SPDX-License-Identifier: GPL-2.0
 /*
  * Copyright (c) 2012-2014 Andy Lutomirski <[email protected]>
  *
  * Based on the original implementation which is:
  *  Copyright (C) 2001 Andrea Arcangeli <[email protected]> SuSE
  *  Copyright 2003 Andi Kleen, SuSE Labs.
  *
  *  Parts of the original code have been moved to arch/x86/vdso/vma.c
  *
  * This file implements vsyscall emulation.  vsyscalls are a legacy ABI:
  * Userspace can request certain kernel services by calling fixed
  * addresses.  This concept is problematic:
  *
  * - It interferes with ASLR.
  * - It's awkward to write code that lives in kernel addresses but is
  *   callable by userspace at fixed addresses.
  * - The whole concept is impossible for 32-bit compat userspace.
  * - UML cannot easily virtualize a vsyscall.
  *
  * As of mid-2014, I believe that there is no new userspace code that
  * will use a vsyscall if the vDSO is present.  I hope that there will
  * soon be no new userspace code that will ever use a vsyscall.
  *
  * The code in this file emulates vsyscalls when notified of a page
  * fault to a vsyscall address.
  */

 #include <linux/kernel.h>
 #include <linux/timer.h>
 #include <linux/sched/signal.h>
 #include <linux/mm_types.h>
 #include <linux/syscalls.h>
 #include <linux/ratelimit.h>

 #include <asm/vsyscall.h>
 #include <asm/unistd.h>
 #include <asm/fixmap.h>
 #include <asm/traps.h>
 #include <asm/paravirt.h>

 #define CREATE_TRACE_POINTS
 #include "vsyscall_trace.h"

 static enum { EMULATE, XONLY, NONE } vsyscall_mode __ro_after_init =
 #ifdef CONFIG_LEGACY_VSYSCALL_NONE
 	NONE;
 #elif defined(CONFIG_LEGACY_VSYSCALL_XONLY)
 	XONLY;
 #else
 	EMULATE;
 #endif

 static int __init vsyscall_setup(char *str)
 {
 	if (str) {
 		if (!strcmp("emulate", str))
 			vsyscall_mode = EMULATE;
 		else if (!strcmp("xonly", str))
 			vsyscall_mode = XONLY;
 		else if (!strcmp("none", str))
 			vsyscall_mode = NONE;
 		else
 			return -EINVAL;

 		return 0;
 	}

 	return -EINVAL;
 }
 early_param("vsyscall", vsyscall_setup);

 static void warn_bad_vsyscall(const char *level, struct pt_regs *regs,
 			      const char *message)
 {
 	if (!show_unhandled_signals)
 		return;

 	printk_ratelimited("%s%s[%d] %s ip:%lx cs:%lx sp:%lx ax:%lx si:%lx di:%lx\n",
 			   level, current->comm, task_pid_nr(current),
 			   message, regs->ip, regs->cs,
 			   regs->sp, regs->ax, regs->si, regs->di);
 }

 static int addr_to_vsyscall_nr(unsigned long addr)
 {
 	int nr;

 	if ((addr & ~0xC00UL) != VSYSCALL_ADDR)
 		return -EINVAL;

 	nr = (addr & 0xC00UL) >> 10;
 	if (nr >= 3)
 		return -EINVAL;

 	return nr;
 }

 static bool write_ok_or_segv(unsigned long ptr, size_t size)
 {
 	/*
 	 * XXX: if access_ok, get_user, and put_user handled
 	 * sig_on_uaccess_err, this could go away.
 	 */

 	if (!access_ok((void __user *)ptr, size)) {
 		struct thread_struct *thread = &current->thread;

 		thread->error_code	= X86_PF_USER | X86_PF_WRITE;
 		thread->cr2		= ptr;
 		thread->trap_nr		= X86_TRAP_PF;

 		force_sig_fault(SIGSEGV, SEGV_MAPERR, (void __user *)ptr);
 		return false;
 	} else {
 		return true;
 	}
 }

 bool emulate_vsyscall(unsigned long error_code,
 		      struct pt_regs *regs, unsigned long address)
 {
 	struct task_struct *tsk;
 	unsigned long caller;
 	int vsyscall_nr, syscall_nr, tmp;
 	int prev_sig_on_uaccess_err;
 	long ret;
 	unsigned long orig_dx;

 	/* Write faults or kernel-privilege faults never get fixed up. */
 	if ((error_code & (X86_PF_WRITE | X86_PF_USER)) != X86_PF_USER)
 		return false;

 	if (!(error_code & X86_PF_INSTR)) {
 		/* Failed vsyscall read */
 		if (vsyscall_mode == EMULATE)
 			return false;

 		/*
 		 * User code tried and failed to read the vsyscall page.
 		 */
 		warn_bad_vsyscall(KERN_INFO, regs, "vsyscall read attempt denied -- look up the vsyscall kernel parameter if you need a workaround");
 		return false;
 	}

 	/*
 	 * No point in checking CS -- the only way to get here is a user mode
 	 * trap to a high address, which means that we're in 64-bit user code.
 	 */

 	WARN_ON_ONCE(address != regs->ip);

 	if (vsyscall_mode == NONE) {
 		warn_bad_vsyscall(KERN_INFO, regs,
 				  "vsyscall attempted with vsyscall=none");
 		return false;
 	}

 	vsyscall_nr = addr_to_vsyscall_nr(address);

 	trace_emulate_vsyscall(vsyscall_nr);

 	if (vsyscall_nr < 0) {
 		warn_bad_vsyscall(KERN_WARNING, regs,
 				  "misaligned vsyscall (exploit attempt or buggy program) -- look up the vsyscall kernel parameter if you need a workaround");
 		goto sigsegv;
 	}

 	if (get_user(caller, (unsigned long __user *)regs->sp) != 0) {
 		warn_bad_vsyscall(KERN_WARNING, regs,
 				  "vsyscall with bad stack (exploit attempt?)");
 		goto sigsegv;
 	}

 	tsk = current;

 	/*
 	 * Check for access_ok violations and find the syscall nr.
 	 *
 	 * NULL is a valid user pointer (in the access_ok sense) on 32-bit and
 	 * 64-bit, so we don't need to special-case it here.  For all the
 	 * vsyscalls, NULL means "don't write anything" not "write it at
 	 * address 0".
 	 */
 	switch (vsyscall_nr) {
 	case 0:
 		if (!write_ok_or_segv(regs->di, sizeof(struct timeval)) ||
 		    !write_ok_or_segv(regs->si, sizeof(struct timezone))) {
 			ret = -EFAULT;
 			goto check_fault;
 		}

 		syscall_nr = __NR_gettimeofday;
 		break;

 	case 1:
 		if (!write_ok_or_segv(regs->di, sizeof(time_t))) {
 			ret = -EFAULT;
 			goto check_fault;
 		}

 		syscall_nr = __NR_time;
 		break;

 	case 2:
 		if (!write_ok_or_segv(regs->di, sizeof(unsigned)) ||
 		    !write_ok_or_segv(regs->si, sizeof(unsigned))) {
 			ret = -EFAULT;
 			goto check_fault;
 		}

 		syscall_nr = __NR_getcpu;
 		break;
 	}

 	/*
 	 * Handle seccomp.  regs->ip must be the original value.
 	 * See seccomp_send_sigsys and Documentation/userspace-api/seccomp_filter.rst.
 	 *
 	 * We could optimize the seccomp disabled case, but performance
 	 * here doesn't matter.
 	 */
 	regs->orig_ax = syscall_nr;
 	regs->ax = -ENOSYS;
 	tmp = secure_computing(NULL);
 	if ((!tmp && regs->orig_ax != syscall_nr) || regs->ip != address) {
 		warn_bad_vsyscall(KERN_DEBUG, regs,
 				  "seccomp tried to change syscall nr or ip");
 		do_exit(SIGSYS);
 	}
 	regs->orig_ax = -1;
 	if (tmp)
 		goto do_ret;  /* skip requested */

 	/*
 	 * With a real vsyscall, page faults cause SIGSEGV.  We want to
 	 * preserve that behavior to make writing exploits harder.
 	 */
 	prev_sig_on_uaccess_err = current->thread.sig_on_uaccess_err;
 	current->thread.sig_on_uaccess_err = 1;

 	ret = -EFAULT;
 	switch (vsyscall_nr) {
 	case 0:
 		/* this decodes regs->di and regs->si on its own */
 		ret = __x64_sys_gettimeofday(regs);
 		break;

 	case 1:
 		/* this decodes regs->di on its own */
 		ret = __x64_sys_time(regs);
 		break;

 	case 2:
 		/* while we could clobber regs->dx, we didn't in the past... */
 		orig_dx = regs->dx;
 		regs->dx = 0;
 		/* this decodes regs->di, regs->si and regs->dx on its own */
 		ret = __x64_sys_getcpu(regs);
 		regs->dx = orig_dx;
 		break;
 	}

 	current->thread.sig_on_uaccess_err = prev_sig_on_uaccess_err;

 check_fault:
 	if (ret == -EFAULT) {
 		/* Bad news -- userspace fed a bad pointer to a vsyscall. */
 		warn_bad_vsyscall(KERN_INFO, regs,
 				  "vsyscall fault (exploit attempt?)");

 		/*
 		 * If we failed to generate a signal for any reason,
 		 * generate one here.  (This should be impossible.)
 		 */
 		if (WARN_ON_ONCE(!sigismember(&tsk->pending.signal, SIGBUS) &&
 				 !sigismember(&tsk->pending.signal, SIGSEGV)))
 			goto sigsegv;

 		return true;  /* Don't emulate the ret. */
 	}

 	regs->ax = ret;

 do_ret:
 	/* Emulate a ret instruction. */
 	regs->ip = caller;
 	regs->sp += 8;
 	return true;

 sigsegv:
 	force_sig(SIGSEGV);
 	return true;
 }

 /*
  * A pseudo VMA to allow ptrace access for the vsyscall page.  This only
  * covers the 64bit vsyscall page now. 32bit has a real VMA now and does
  * not need special handling anymore:
  */
 static const char *gate_vma_name(struct vm_area_struct *vma)
 {
 	return "[vsyscall]";
 }
 static const struct vm_operations_struct gate_vma_ops = {
 	.name = gate_vma_name,
 };
 static struct vm_area_struct gate_vma __ro_after_init = {
 	.vm_start	= VSYSCALL_ADDR,
 	.vm_end		= VSYSCALL_ADDR + PAGE_SIZE,
 	.vm_page_prot	= PAGE_READONLY_EXEC,
 	.vm_flags	= VM_READ | VM_EXEC,
 	.vm_ops		= &gate_vma_ops,
 };

 struct vm_area_struct *get_gate_vma(struct mm_struct *mm)
 {
 #ifdef CONFIG_COMPAT
 	if (!mm || mm->context.ia32_compat)
 		return NULL;
 #endif
 	if (vsyscall_mode == NONE)
 		return NULL;
 	return &gate_vma;
 }

 int in_gate_area(struct mm_struct *mm, unsigned long addr)
 {
 	struct vm_area_struct *vma = get_gate_vma(mm);

 	if (!vma)
 		return 0;

 	return (addr >= vma->vm_start) && (addr < vma->vm_end);
 }

 /*
  * Use this when you have no reliable mm, typically from interrupt
  * context. It is less reliable than using a task's mm and may give
  * false positives.
  */
 int in_gate_area_no_mm(unsigned long addr)
 {
 	return vsyscall_mode != NONE && (addr & PAGE_MASK) == VSYSCALL_ADDR;
 }

 /*
  * The VSYSCALL page is the only user-accessible page in the kernel address
  * range.  Normally, the kernel page tables can have _PAGE_USER clear, but
  * the tables covering VSYSCALL_ADDR need _PAGE_USER set if vsyscalls
  * are enabled.
  *
  * Some day we may create a "minimal" vsyscall mode in which we emulate
  * vsyscalls but leave the page not present.  If so, we skip calling
  * this.
  */
 void __init set_vsyscall_pgtable_user_bits(pgd_t *root)
 {
 	pgd_t *pgd;
 	p4d_t *p4d;
 	pud_t *pud;
 	pmd_t *pmd;

 	pgd = pgd_offset_pgd(root, VSYSCALL_ADDR);
 	set_pgd(pgd, __pgd(pgd_val(*pgd) | _PAGE_USER));
 	p4d = p4d_offset(pgd, VSYSCALL_ADDR);
 #if CONFIG_PGTABLE_LEVELS >= 5
 	set_p4d(p4d, __p4d(p4d_val(*p4d) | _PAGE_USER));
 #endif
 	pud = pud_offset(p4d, VSYSCALL_ADDR);
 	set_pud(pud, __pud(pud_val(*pud) | _PAGE_USER));
 	pmd = pmd_offset(pud, VSYSCALL_ADDR);
 	set_pmd(pmd, __pmd(pmd_val(*pmd) | _PAGE_USER));
 }

 void __init map_vsyscall(void)
 {
 	extern char __vsyscall_page;
 	unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);

 	/*
 	 * For full emulation, the page needs to exist for real.  In
 	 * execute-only mode, there is no PTE at all backing the vsyscall
 	 * page.
 	 */
 	if (vsyscall_mode == EMULATE) {
 		__set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
 			     PAGE_KERNEL_VVAR);
 		set_vsyscall_pgtable_user_bits(swapper_pg_dir);
 	}

 	if (vsyscall_mode == XONLY)
 		gate_vma.vm_flags = VM_EXEC;

 	BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
 		     (unsigned long)VSYSCALL_ADDR);
 }
	// SPDX-License-Identifier: GPL-2.0
	/*
	* Copyright (c) 2012-2014 Andy Lutomirski <[email protected]>
	*
	* Based on the original implementation which is:
	* Copyright (C) 2001 Andrea Arcangeli <[email protected]> SuSE
	* Copyright 2003 Andi Kleen, SuSE Labs.
	*
	* Parts of the original code have been moved to arch/x86/vdso/vma.c
	*
	* This file implements vsyscall emulation. vsyscalls are a legacy ABI:
	* Userspace can request certain kernel services by calling fixed
	* addresses. This concept is problematic:
	*
	* - It interferes with ASLR.
	* - It's awkward to write code that lives in kernel addresses but is
	* callable by userspace at fixed addresses.
	* - The whole concept is impossible for 32-bit compat userspace.
	* - UML cannot easily virtualize a vsyscall.
	*
	* As of mid-2014, I believe that there is no new userspace code that
	* will use a vsyscall if the vDSO is present. I hope that there will
	* soon be no new userspace code that will ever use a vsyscall.
	*
	* The code in this file emulates vsyscalls when notified of a page
	* fault to a vsyscall address.
	*/

	#include <linux/kernel.h>
	#include <linux/timer.h>
	#include <linux/sched/signal.h>
	#include <linux/mm_types.h>
	#include <linux/syscalls.h>
	#include <linux/ratelimit.h>

	#include <asm/vsyscall.h>
	#include <asm/unistd.h>
	#include <asm/fixmap.h>
	#include <asm/traps.h>
	#include <asm/paravirt.h>

	#define CREATE_TRACE_POINTS
	#include "vsyscall_trace.h"

	static enum { EMULATE, XONLY, NONE } vsyscall_mode __ro_after_init =
	#ifdef CONFIG_LEGACY_VSYSCALL_NONE
	NONE;
	#elif defined(CONFIG_LEGACY_VSYSCALL_XONLY)
	XONLY;
	#else
	EMULATE;
	#endif

	static int __init vsyscall_setup(char *str)
	{
	if (str) {
	if (!strcmp("emulate", str))
	vsyscall_mode = EMULATE;
	else if (!strcmp("xonly", str))
	vsyscall_mode = XONLY;
	else if (!strcmp("none", str))
	vsyscall_mode = NONE;
	else
	return -EINVAL;

	return 0;
	}

	return -EINVAL;
	}
	early_param("vsyscall", vsyscall_setup);

	static void warn_bad_vsyscall(const char level, struct pt_regs regs,
	const char *message)
	{
	if (!show_unhandled_signals)
	return;

	printk_ratelimited("%s%s[%d] %s ip:%lx cs:%lx sp:%lx ax:%lx si:%lx di:%lx\n",
	level, current->comm, task_pid_nr(current),
	message, regs->ip, regs->cs,
	regs->sp, regs->ax, regs->si, regs->di);
	}

	static int addr_to_vsyscall_nr(unsigned long addr)
	{
	int nr;

	if ((addr & ~0xC00UL) != VSYSCALL_ADDR)
	return -EINVAL;

	nr = (addr & 0xC00UL) >> 10;
	if (nr >= 3)
	return -EINVAL;

	return nr;
	}

	static bool write_ok_or_segv(unsigned long ptr, size_t size)
	{
	/*
	* XXX: if access_ok, get_user, and put_user handled
	* sig_on_uaccess_err, this could go away.
	*/

	if (!access_ok((void __user *)ptr, size)) {
	struct thread_struct *thread = &current->thread;

	thread->error_code = X86_PF_USER \| X86_PF_WRITE;
	thread->cr2 = ptr;
	thread->trap_nr = X86_TRAP_PF;

	force_sig_fault(SIGSEGV, SEGV_MAPERR, (void __user *)ptr);
	return false;
	} else {
	return true;
	}
	}

	bool emulate_vsyscall(unsigned long error_code,
	struct pt_regs *regs, unsigned long address)
	{
	struct task_struct *tsk;
	unsigned long caller;
	int vsyscall_nr, syscall_nr, tmp;
	int prev_sig_on_uaccess_err;
	long ret;
	unsigned long orig_dx;

	/* Write faults or kernel-privilege faults never get fixed up. */
	if ((error_code & (X86_PF_WRITE \| X86_PF_USER)) != X86_PF_USER)
	return false;

	if (!(error_code & X86_PF_INSTR)) {
	/* Failed vsyscall read */
	if (vsyscall_mode == EMULATE)
	return false;

	/*
	* User code tried and failed to read the vsyscall page.
	*/
	warn_bad_vsyscall(KERN_INFO, regs, "vsyscall read attempt denied -- look up the vsyscall kernel parameter if you need a workaround");
	return false;
	}

	/*
	* No point in checking CS -- the only way to get here is a user mode
	* trap to a high address, which means that we're in 64-bit user code.
	*/

	WARN_ON_ONCE(address != regs->ip);

	if (vsyscall_mode == NONE) {
	warn_bad_vsyscall(KERN_INFO, regs,
	"vsyscall attempted with vsyscall=none");
	return false;
	}

	vsyscall_nr = addr_to_vsyscall_nr(address);

	trace_emulate_vsyscall(vsyscall_nr);

	if (vsyscall_nr < 0) {
	warn_bad_vsyscall(KERN_WARNING, regs,
	"misaligned vsyscall (exploit attempt or buggy program) -- look up the vsyscall kernel parameter if you need a workaround");
	goto sigsegv;
	}

	if (get_user(caller, (unsigned long __user *)regs->sp) != 0) {
	warn_bad_vsyscall(KERN_WARNING, regs,
	"vsyscall with bad stack (exploit attempt?)");
	goto sigsegv;
	}

	tsk = current;

	/*
	* Check for access_ok violations and find the syscall nr.
	*
	* NULL is a valid user pointer (in the access_ok sense) on 32-bit and
	* 64-bit, so we don't need to special-case it here. For all the
	* vsyscalls, NULL means "don't write anything" not "write it at
	* address 0".
	*/
	switch (vsyscall_nr) {
	case 0:
	if (!write_ok_or_segv(regs->di, sizeof(struct timeval)) \|\|
	!write_ok_or_segv(regs->si, sizeof(struct timezone))) {
	ret = -EFAULT;
	goto check_fault;
	}

	syscall_nr = __NR_gettimeofday;
	break;

	case 1:
	if (!write_ok_or_segv(regs->di, sizeof(time_t))) {
	ret = -EFAULT;
	goto check_fault;
	}

	syscall_nr = __NR_time;
	break;

	case 2:
	if (!write_ok_or_segv(regs->di, sizeof(unsigned)) \|\|
	!write_ok_or_segv(regs->si, sizeof(unsigned))) {
	ret = -EFAULT;
	goto check_fault;
	}

	syscall_nr = __NR_getcpu;
	break;
	}

	/*
	* Handle seccomp. regs->ip must be the original value.
	* See seccomp_send_sigsys and Documentation/userspace-api/seccomp_filter.rst.
	*
	* We could optimize the seccomp disabled case, but performance
	* here doesn't matter.
	*/
	regs->orig_ax = syscall_nr;
	regs->ax = -ENOSYS;
	tmp = secure_computing(NULL);
	if ((!tmp && regs->orig_ax != syscall_nr) \|\| regs->ip != address) {
	warn_bad_vsyscall(KERN_DEBUG, regs,
	"seccomp tried to change syscall nr or ip");
	do_exit(SIGSYS);
	}
	regs->orig_ax = -1;
	if (tmp)
	goto do_ret; /* skip requested */

	/*
	* With a real vsyscall, page faults cause SIGSEGV. We want to
	* preserve that behavior to make writing exploits harder.
	*/
	prev_sig_on_uaccess_err = current->thread.sig_on_uaccess_err;
	current->thread.sig_on_uaccess_err = 1;

	ret = -EFAULT;
	switch (vsyscall_nr) {
	case 0:
	/* this decodes regs->di and regs->si on its own */
	ret = __x64_sys_gettimeofday(regs);
	break;

	case 1:
	/* this decodes regs->di on its own */
	ret = __x64_sys_time(regs);
	break;

	case 2:
	/* while we could clobber regs->dx, we didn't in the past... */
	orig_dx = regs->dx;
	regs->dx = 0;
	/* this decodes regs->di, regs->si and regs->dx on its own */
	ret = __x64_sys_getcpu(regs);
	regs->dx = orig_dx;
	break;
	}

	current->thread.sig_on_uaccess_err = prev_sig_on_uaccess_err;

	check_fault:
	if (ret == -EFAULT) {
	/* Bad news -- userspace fed a bad pointer to a vsyscall. */
	warn_bad_vsyscall(KERN_INFO, regs,
	"vsyscall fault (exploit attempt?)");

	/*
	* If we failed to generate a signal for any reason,
	* generate one here. (This should be impossible.)
	*/
	if (WARN_ON_ONCE(!sigismember(&tsk->pending.signal, SIGBUS) &&
	!sigismember(&tsk->pending.signal, SIGSEGV)))
	goto sigsegv;

	return true; /* Don't emulate the ret. */
	}

	regs->ax = ret;

	do_ret:
	/* Emulate a ret instruction. */
	regs->ip = caller;
	regs->sp += 8;
	return true;

	sigsegv:
	force_sig(SIGSEGV);
	return true;
	}

	/*
	* A pseudo VMA to allow ptrace access for the vsyscall page. This only
	* covers the 64bit vsyscall page now. 32bit has a real VMA now and does
	* not need special handling anymore:
	*/
	static const char gate_vma_name(struct vm_area_struct vma)
	{
	return "[vsyscall]";
	}
	static const struct vm_operations_struct gate_vma_ops = {
	.name = gate_vma_name,
	};
	static struct vm_area_struct gate_vma __ro_after_init = {
	.vm_start = VSYSCALL_ADDR,
	.vm_end = VSYSCALL_ADDR + PAGE_SIZE,
	.vm_page_prot = PAGE_READONLY_EXEC,
	.vm_flags = VM_READ \| VM_EXEC,
	.vm_ops = &gate_vma_ops,
	};

	struct vm_area_struct get_gate_vma(struct mm_struct mm)
	{
	#ifdef CONFIG_COMPAT
	if (!mm \|\| mm->context.ia32_compat)
	return NULL;
	#endif
	if (vsyscall_mode == NONE)
	return NULL;
	return &gate_vma;
	}

	int in_gate_area(struct mm_struct *mm, unsigned long addr)
	{
	struct vm_area_struct *vma = get_gate_vma(mm);

	if (!vma)
	return 0;

	return (addr >= vma->vm_start) && (addr < vma->vm_end);
	}

	/*
	* Use this when you have no reliable mm, typically from interrupt
	* context. It is less reliable than using a task's mm and may give
	* false positives.
	*/
	int in_gate_area_no_mm(unsigned long addr)
	{
	return vsyscall_mode != NONE && (addr & PAGE_MASK) == VSYSCALL_ADDR;
	}

	/*
	* The VSYSCALL page is the only user-accessible page in the kernel address
	* range. Normally, the kernel page tables can have _PAGE_USER clear, but
	* the tables covering VSYSCALL_ADDR need _PAGE_USER set if vsyscalls
	* are enabled.
	*
	* Some day we may create a "minimal" vsyscall mode in which we emulate
	* vsyscalls but leave the page not present. If so, we skip calling
	* this.
	*/
	void __init set_vsyscall_pgtable_user_bits(pgd_t *root)
	{
	pgd_t *pgd;
	p4d_t *p4d;
	pud_t *pud;
	pmd_t *pmd;

	pgd = pgd_offset_pgd(root, VSYSCALL_ADDR);
	set_pgd(pgd, __pgd(pgd_val(*pgd) \| _PAGE_USER));
	p4d = p4d_offset(pgd, VSYSCALL_ADDR);
	#if CONFIG_PGTABLE_LEVELS >= 5
	set_p4d(p4d, __p4d(p4d_val(*p4d) \| _PAGE_USER));
	#endif
	pud = pud_offset(p4d, VSYSCALL_ADDR);
	set_pud(pud, __pud(pud_val(*pud) \| _PAGE_USER));
	pmd = pmd_offset(pud, VSYSCALL_ADDR);
	set_pmd(pmd, __pmd(pmd_val(*pmd) \| _PAGE_USER));
	}

	void __init map_vsyscall(void)
	{
	extern char __vsyscall_page;
	unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);

	/*
	* For full emulation, the page needs to exist for real. In
	* execute-only mode, there is no PTE at all backing the vsyscall
	* page.
	*/
	if (vsyscall_mode == EMULATE) {
	__set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
	PAGE_KERNEL_VVAR);
	set_vsyscall_pgtable_user_bits(swapper_pg_dir);
	}

	if (vsyscall_mode == XONLY)
	gate_vma.vm_flags = VM_EXEC;

	BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
	(unsigned long)VSYSCALL_ADDR);
	}