// SPDX-License-Identifier: GPL-2.0
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/*
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* Copyright (c) 2012-2014 Andy Lutomirski <luto@amacapital.net>
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*
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* Based on the original implementation which is:
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* Copyright (C) 2001 Andrea Arcangeli <andrea@suse.de> SuSE
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* Copyright 2003 Andi Kleen, SuSE Labs.
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*
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* Parts of the original code have been moved to arch/x86/vdso/vma.c
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*
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* This file implements vsyscall emulation. vsyscalls are a legacy ABI:
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* Userspace can request certain kernel services by calling fixed
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* addresses. This concept is problematic:
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*
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* - It interferes with ASLR.
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* - It's awkward to write code that lives in kernel addresses but is
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* callable by userspace at fixed addresses.
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* - The whole concept is impossible for 32-bit compat userspace.
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* - UML cannot easily virtualize a vsyscall.
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*
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* As of mid-2014, I believe that there is no new userspace code that
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* will use a vsyscall if the vDSO is present. I hope that there will
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* soon be no new userspace code that will ever use a vsyscall.
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*
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* The code in this file emulates vsyscalls when notified of a page
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* fault to a vsyscall address.
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*/
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#include <linux/kernel.h>
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#include <linux/timer.h>
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#include <linux/sched/signal.h>
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#include <linux/mm_types.h>
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#include <linux/syscalls.h>
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#include <linux/ratelimit.h>
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#include <asm/vsyscall.h>
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#include <asm/unistd.h>
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#include <asm/fixmap.h>
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#include <asm/traps.h>
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#include <asm/paravirt.h>
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#define CREATE_TRACE_POINTS
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#include "vsyscall_trace.h"
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static enum { EMULATE, NONE } vsyscall_mode =
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#ifdef CONFIG_LEGACY_VSYSCALL_NONE
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NONE;
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#else
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EMULATE;
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#endif
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static int __init vsyscall_setup(char *str)
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{
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if (str) {
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if (!strcmp("emulate", str))
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vsyscall_mode = EMULATE;
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else if (!strcmp("none", str))
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vsyscall_mode = NONE;
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else
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return -EINVAL;
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return 0;
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}
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return -EINVAL;
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}
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early_param("vsyscall", vsyscall_setup);
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static void warn_bad_vsyscall(const char *level, struct pt_regs *regs,
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const char *message)
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{
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if (!show_unhandled_signals)
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return;
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printk_ratelimited("%s%s[%d] %s ip:%lx cs:%lx sp:%lx ax:%lx si:%lx di:%lx\n",
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level, current->comm, task_pid_nr(current),
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message, regs->ip, regs->cs,
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regs->sp, regs->ax, regs->si, regs->di);
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}
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static int addr_to_vsyscall_nr(unsigned long addr)
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{
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int nr;
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if ((addr & ~0xC00UL) != VSYSCALL_ADDR)
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return -EINVAL;
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nr = (addr & 0xC00UL) >> 10;
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if (nr >= 3)
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return -EINVAL;
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return nr;
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}
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static bool write_ok_or_segv(unsigned long ptr, size_t size)
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{
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/*
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* XXX: if access_ok, get_user, and put_user handled
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* sig_on_uaccess_err, this could go away.
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*/
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if (!access_ok(VERIFY_WRITE, (void __user *)ptr, size)) {
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siginfo_t info;
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struct thread_struct *thread = ¤t->thread;
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thread->error_code = 6; /* user fault, no page, write */
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thread->cr2 = ptr;
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thread->trap_nr = X86_TRAP_PF;
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clear_siginfo(&info);
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info.si_signo = SIGSEGV;
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info.si_errno = 0;
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info.si_code = SEGV_MAPERR;
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info.si_addr = (void __user *)ptr;
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force_sig_info(SIGSEGV, &info, current);
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return false;
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} else {
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return true;
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}
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}
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bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
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{
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struct task_struct *tsk;
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unsigned long caller;
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int vsyscall_nr, syscall_nr, tmp;
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int prev_sig_on_uaccess_err;
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long ret;
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unsigned long orig_dx;
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/*
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* No point in checking CS -- the only way to get here is a user mode
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* trap to a high address, which means that we're in 64-bit user code.
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*/
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WARN_ON_ONCE(address != regs->ip);
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if (vsyscall_mode == NONE) {
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warn_bad_vsyscall(KERN_INFO, regs,
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"vsyscall attempted with vsyscall=none");
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return false;
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}
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vsyscall_nr = addr_to_vsyscall_nr(address);
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trace_emulate_vsyscall(vsyscall_nr);
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if (vsyscall_nr < 0) {
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warn_bad_vsyscall(KERN_WARNING, regs,
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"misaligned vsyscall (exploit attempt or buggy program) -- look up the vsyscall kernel parameter if you need a workaround");
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goto sigsegv;
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}
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if (get_user(caller, (unsigned long __user *)regs->sp) != 0) {
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warn_bad_vsyscall(KERN_WARNING, regs,
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"vsyscall with bad stack (exploit attempt?)");
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goto sigsegv;
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}
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tsk = current;
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/*
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* Check for access_ok violations and find the syscall nr.
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*
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* NULL is a valid user pointer (in the access_ok sense) on 32-bit and
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* 64-bit, so we don't need to special-case it here. For all the
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* vsyscalls, NULL means "don't write anything" not "write it at
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* address 0".
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*/
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switch (vsyscall_nr) {
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case 0:
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if (!write_ok_or_segv(regs->di, sizeof(struct timeval)) ||
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!write_ok_or_segv(regs->si, sizeof(struct timezone))) {
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ret = -EFAULT;
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goto check_fault;
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}
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syscall_nr = __NR_gettimeofday;
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break;
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case 1:
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if (!write_ok_or_segv(regs->di, sizeof(time_t))) {
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ret = -EFAULT;
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goto check_fault;
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}
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syscall_nr = __NR_time;
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break;
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case 2:
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if (!write_ok_or_segv(regs->di, sizeof(unsigned)) ||
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!write_ok_or_segv(regs->si, sizeof(unsigned))) {
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ret = -EFAULT;
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goto check_fault;
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}
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syscall_nr = __NR_getcpu;
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break;
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}
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/*
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* Handle seccomp. regs->ip must be the original value.
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* See seccomp_send_sigsys and Documentation/userspace-api/seccomp_filter.rst.
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*
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* We could optimize the seccomp disabled case, but performance
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* here doesn't matter.
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*/
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regs->orig_ax = syscall_nr;
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regs->ax = -ENOSYS;
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tmp = secure_computing(NULL);
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if ((!tmp && regs->orig_ax != syscall_nr) || regs->ip != address) {
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warn_bad_vsyscall(KERN_DEBUG, regs,
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"seccomp tried to change syscall nr or ip");
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do_exit(SIGSYS);
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}
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regs->orig_ax = -1;
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if (tmp)
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goto do_ret; /* skip requested */
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/*
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* With a real vsyscall, page faults cause SIGSEGV. We want to
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* preserve that behavior to make writing exploits harder.
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*/
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prev_sig_on_uaccess_err = current->thread.sig_on_uaccess_err;
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current->thread.sig_on_uaccess_err = 1;
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ret = -EFAULT;
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switch (vsyscall_nr) {
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case 0:
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/* this decodes regs->di and regs->si on its own */
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ret = __x64_sys_gettimeofday(regs);
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break;
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case 1:
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/* this decodes regs->di on its own */
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ret = __x64_sys_time(regs);
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break;
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case 2:
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/* while we could clobber regs->dx, we didn't in the past... */
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orig_dx = regs->dx;
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regs->dx = 0;
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/* this decodes regs->di, regs->si and regs->dx on its own */
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ret = __x64_sys_getcpu(regs);
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regs->dx = orig_dx;
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break;
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}
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current->thread.sig_on_uaccess_err = prev_sig_on_uaccess_err;
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check_fault:
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if (ret == -EFAULT) {
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/* Bad news -- userspace fed a bad pointer to a vsyscall. */
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warn_bad_vsyscall(KERN_INFO, regs,
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"vsyscall fault (exploit attempt?)");
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/*
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* If we failed to generate a signal for any reason,
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* generate one here. (This should be impossible.)
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*/
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if (WARN_ON_ONCE(!sigismember(&tsk->pending.signal, SIGBUS) &&
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!sigismember(&tsk->pending.signal, SIGSEGV)))
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goto sigsegv;
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return true; /* Don't emulate the ret. */
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}
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regs->ax = ret;
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do_ret:
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/* Emulate a ret instruction. */
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regs->ip = caller;
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regs->sp += 8;
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return true;
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sigsegv:
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force_sig(SIGSEGV, current);
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return true;
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}
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/*
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* A pseudo VMA to allow ptrace access for the vsyscall page. This only
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* covers the 64bit vsyscall page now. 32bit has a real VMA now and does
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* not need special handling anymore:
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*/
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static const char *gate_vma_name(struct vm_area_struct *vma)
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{
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return "[vsyscall]";
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}
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static const struct vm_operations_struct gate_vma_ops = {
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.name = gate_vma_name,
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};
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static struct vm_area_struct gate_vma = {
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.vm_start = VSYSCALL_ADDR,
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.vm_end = VSYSCALL_ADDR + PAGE_SIZE,
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.vm_page_prot = PAGE_READONLY_EXEC,
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.vm_flags = VM_READ | VM_EXEC,
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.vm_ops = &gate_vma_ops,
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};
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struct vm_area_struct *get_gate_vma(struct mm_struct *mm)
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{
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#ifdef CONFIG_COMPAT
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if (!mm || mm->context.ia32_compat)
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return NULL;
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#endif
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if (vsyscall_mode == NONE)
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return NULL;
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return &gate_vma;
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}
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int in_gate_area(struct mm_struct *mm, unsigned long addr)
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{
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struct vm_area_struct *vma = get_gate_vma(mm);
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if (!vma)
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return 0;
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return (addr >= vma->vm_start) && (addr < vma->vm_end);
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}
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/*
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* Use this when you have no reliable mm, typically from interrupt
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* context. It is less reliable than using a task's mm and may give
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* false positives.
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*/
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int in_gate_area_no_mm(unsigned long addr)
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{
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return vsyscall_mode != NONE && (addr & PAGE_MASK) == VSYSCALL_ADDR;
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}
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/*
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* The VSYSCALL page is the only user-accessible page in the kernel address
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* range. Normally, the kernel page tables can have _PAGE_USER clear, but
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* the tables covering VSYSCALL_ADDR need _PAGE_USER set if vsyscalls
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* are enabled.
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*
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* Some day we may create a "minimal" vsyscall mode in which we emulate
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* vsyscalls but leave the page not present. If so, we skip calling
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* this.
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*/
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void __init set_vsyscall_pgtable_user_bits(pgd_t *root)
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{
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pgd_t *pgd;
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p4d_t *p4d;
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pud_t *pud;
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pmd_t *pmd;
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pgd = pgd_offset_pgd(root, VSYSCALL_ADDR);
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set_pgd(pgd, __pgd(pgd_val(*pgd) | _PAGE_USER));
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p4d = p4d_offset(pgd, VSYSCALL_ADDR);
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#if CONFIG_PGTABLE_LEVELS >= 5
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set_p4d(p4d, __p4d(p4d_val(*p4d) | _PAGE_USER));
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#endif
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pud = pud_offset(p4d, VSYSCALL_ADDR);
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set_pud(pud, __pud(pud_val(*pud) | _PAGE_USER));
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pmd = pmd_offset(pud, VSYSCALL_ADDR);
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set_pmd(pmd, __pmd(pmd_val(*pmd) | _PAGE_USER));
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}
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void __init map_vsyscall(void)
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{
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extern char __vsyscall_page;
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unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);
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if (vsyscall_mode != NONE) {
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__set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
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PAGE_KERNEL_VVAR);
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set_vsyscall_pgtable_user_bits(swapper_pg_dir);
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}
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BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
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(unsigned long)VSYSCALL_ADDR);
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}
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