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// 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 = ¤t->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();
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
|