text: // SPDX-License-Identifier: GPL-2.0
  /*
   * Copyright (c) 2012-2014 Andy Lutomirski <luto@amacapital.net>
   *
   * Based on the original implementation which is:
   *  Copyright (C) 2001 Andrea Arcangeli <andrea@suse.de> 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 __kernel_old_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(__kernel_old_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();
  	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);
  }
  