linux/arch/x86/kernel/vsyscall_64.c

/*
 *  Copyright (C) 2001 Andrea Arcangeli <andrea@suse.de> SuSE
 *  Copyright 2003 Andi Kleen, SuSE Labs.
 *
 *  [ NOTE: this mechanism is now deprecated in favor of the vDSO. ]
 *
 *  Thanks to hpa@transmeta.com for some useful hint.
 *  Special thanks to Ingo Molnar for his early experience with
 *  a different vsyscall implementation for Linux/IA32 and for the name.
 *
 *  vsyscall 1 is located at -10Mbyte, vsyscall 2 is located
 *  at virtual address -10Mbyte+1024bytes etc... There are at max 4
 *  vsyscalls. One vsyscall can reserve more than 1 slot to avoid
 *  jumping out of line if necessary. We cannot add more with this
 *  mechanism because older kernels won't return -ENOSYS.
 *
 *  Note: the concept clashes with user mode linux.  UML users should
 *  use the vDSO.
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/time.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/timer.h>
#include <linux/seqlock.h>
#include <linux/jiffies.h>
#include <linux/sysctl.h>
#include <linux/topology.h>
#include <linux/timekeeper_internal.h>
#include <linux/getcpu.h>
#include <linux/cpu.h>
#include <linux/smp.h>
#include <linux/notifier.h>
#include <linux/syscalls.h>
#include <linux/ratelimit.h>

#include <asm/vsyscall.h>
#include <asm/pgtable.h>
#include <asm/compat.h>
#include <asm/page.h>
#include <asm/unistd.h>
#include <asm/fixmap.h>
#include <asm/errno.h>
#include <asm/io.h>
#include <asm/segment.h>
#include <asm/desc.h>
#include <asm/topology.h>
#include <asm/traps.h>

#define CREATE_TRACE_POINTS
#include "vsyscall_trace.h"

DEFINE_VVAR(int, vgetcpu_mode);

static enum { EMULATE, NATIVE, NONE } vsyscall_mode = EMULATE;

static int __init vsyscall_setup(char *str)
{
	if (str) {
		if (!strcmp("emulate", str))
			vsyscall_mode = EMULATE;
		else if (!strcmp("native", str))
			vsyscall_mode = NATIVE;
		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_error, this could go away.
	 */

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

		thread->error_code	= 6;  /* user fault, no page, write */
		thread->cr2		= ptr;
		thread->trap_nr		= X86_TRAP_PF;

		memset(&info, 0, sizeof(info));
		info.si_signo		= SIGSEGV;
		info.si_errno		= 0;
		info.si_code		= SEGV_MAPERR;
		info.si_addr		= (void __user *)ptr;

		force_sig_info(SIGSEGV, &info, current);
		return false;
	} else {
		return true;
	}
}

bool emulate_vsyscall(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_error;
	long ret;

	/*
	 * 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/prctl/seccomp_filter.txt.
	 *
	 * We could optimize the seccomp disabled case, but performance
	 * here doesn't matter.
	 */
	regs->orig_ax = syscall_nr;
	regs->ax = -ENOSYS;
	tmp = secure_computing();
	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);
	}
	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_error = current_thread_info()->sig_on_uaccess_error;
	current_thread_info()->sig_on_uaccess_error = 1;

	ret = -EFAULT;
	switch (vsyscall_nr) {
	case 0:
		ret = sys_gettimeofday(
			(struct timeval __user *)regs->di,
			(struct timezone __user *)regs->si);
		break;

	case 1:
		ret = sys_time((time_t __user *)regs->di);
		break;

	case 2:
		ret = sys_getcpu((unsigned __user *)regs->di,
				 (unsigned __user *)regs->si,
				 NULL);
		break;
	}

	current_thread_info()->sig_on_uaccess_error = prev_sig_on_uaccess_error;

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, current);
	return true;
}

/*
 * Assume __initcall executes before all user space. Hopefully kmod
 * doesn't violate that. We'll find out if it does.
 */
static void vsyscall_set_cpu(int cpu)
{
	unsigned long d;
	unsigned long node = 0;
#ifdef CONFIG_NUMA
	node = cpu_to_node(cpu);
#endif
	if (cpu_has(&cpu_data(cpu), X86_FEATURE_RDTSCP))
		write_rdtscp_aux((node << 12) | cpu);

	/*
	 * Store cpu number in limit so that it can be loaded quickly
	 * in user space in vgetcpu. (12 bits for the CPU and 8 bits for the node)
	 */
	d = 0x0f40000000000ULL;
	d |= cpu;
	d |= (node & 0xf) << 12;
	d |= (node >> 4) << 48;

	write_gdt_entry(get_cpu_gdt_table(cpu), GDT_ENTRY_PER_CPU, &d, DESCTYPE_S);
}

static void cpu_vsyscall_init(void *arg)
{
	/* preemption should be already off */
	vsyscall_set_cpu(raw_smp_processor_id());
}

static int
cpu_vsyscall_notifier(struct notifier_block *n, unsigned long action, void *arg)
{
	long cpu = (long)arg;

	if (action == CPU_ONLINE || action == CPU_ONLINE_FROZEN)
		smp_call_function_single(cpu, cpu_vsyscall_init, NULL, 1);

	return NOTIFY_DONE;
}

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

	__set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
		     vsyscall_mode == NATIVE
		     ? PAGE_KERNEL_VSYSCALL
		     : PAGE_KERNEL_VVAR);
	BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
		     (unsigned long)VSYSCALL_ADDR);
}

static int __init vsyscall_init(void)
{
	cpu_notifier_register_begin();

	on_each_cpu(cpu_vsyscall_init, NULL, 1);
	/* notifier priority > KVM */
	__hotcpu_notifier(cpu_vsyscall_notifier, 30);

	cpu_notifier_register_done();

	return 0;
}
__initcall(vsyscall_init);