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// SPDX-License-Identifier: GPL-2.0-only
/*
* common.c - C code for kernel entry and exit
* Copyright (c) 2015 Andrew Lutomirski
*
* Based on asm and ptrace code by many authors. The code here originated
* in ptrace.c and signal.c.
*/
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/sched/task_stack.h>
#include <linux/entry-common.h>
#include <linux/mm.h>
#include <linux/smp.h>
#include <linux/errno.h>
#include <linux/ptrace.h>
#include <linux/export.h>
#include <linux/nospec.h>
#include <linux/syscalls.h>
#include <linux/uaccess.h>
#include <linux/init.h>
#ifdef CONFIG_XEN_PV
#include <xen/xen-ops.h>
#include <xen/events.h>
#endif
#include <asm/apic.h>
#include <asm/desc.h>
#include <asm/traps.h>
#include <asm/vdso.h>
#include <asm/cpufeature.h>
#include <asm/fpu/api.h>
#include <asm/nospec-branch.h>
#include <asm/io_bitmap.h>
#include <asm/syscall.h>
#include <asm/irq_stack.h>
#ifdef CONFIG_X86_64
static __always_inline bool do_syscall_x64(struct pt_regs *regs, int nr)
{
/*
* Convert negative numbers to very high and thus out of range
* numbers for comparisons.
*/
unsigned int unr = nr;
if (likely(unr < NR_syscalls)) {
unr = array_index_nospec(unr, NR_syscalls);
regs->ax = sys_call_table[unr](regs);
return true;
}
return false;
}
static __always_inline bool do_syscall_x32(struct pt_regs *regs, int nr)
{
/*
* Adjust the starting offset of the table, and convert numbers
* < __X32_SYSCALL_BIT to very high and thus out of range
* numbers for comparisons.
*/
unsigned int xnr = nr - __X32_SYSCALL_BIT;
if (IS_ENABLED(CONFIG_X86_X32_ABI) && likely(xnr < X32_NR_syscalls)) {
xnr = array_index_nospec(xnr, X32_NR_syscalls);
regs->ax = x32_sys_call_table[xnr](regs);
return true;
}
return false;
}
/* Returns true to return using SYSRET, or false to use IRET */
__visible noinstr bool do_syscall_64(struct pt_regs *regs, int nr)
{
add_random_kstack_offset();
nr = syscall_enter_from_user_mode(regs, nr);
instrumentation_begin();
if (!do_syscall_x64(regs, nr) && !do_syscall_x32(regs, nr) && nr != -1) {
/* Invalid system call, but still a system call. */
regs->ax = __x64_sys_ni_syscall(regs);
}
instrumentation_end();
syscall_exit_to_user_mode(regs);
/*
* Check that the register state is valid for using SYSRET to exit
* to userspace. Otherwise use the slower but fully capable IRET
* exit path.
*/
/* XEN PV guests always use the IRET path */
if (cpu_feature_enabled(X86_FEATURE_XENPV))
return false;
/* SYSRET requires RCX == RIP and R11 == EFLAGS */
if (unlikely(regs->cx != regs->ip || regs->r11 != regs->flags))
return false;
/* CS and SS must match the values set in MSR_STAR */
if (unlikely(regs->cs != __USER_CS || regs->ss != __USER_DS))
return false;
/*
* On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
* in kernel space. This essentially lets the user take over
* the kernel, since userspace controls RSP.
*
* TASK_SIZE_MAX covers all user-accessible addresses other than
* the deprecated vsyscall page.
*/
if (unlikely(regs->ip >= TASK_SIZE_MAX))
return false;
/*
* SYSRET cannot restore RF. It can restore TF, but unlike IRET,
* restoring TF results in a trap from userspace immediately after
* SYSRET.
*/
if (unlikely(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF)))
return false;
/* Use SYSRET to exit to userspace */
return true;
}
#endif
#if defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION)
static __always_inline int syscall_32_enter(struct pt_regs *regs)
{
if (IS_ENABLED(CONFIG_IA32_EMULATION))
current_thread_info()->status |= TS_COMPAT;
return (int)regs->orig_ax;
}
#ifdef CONFIG_IA32_EMULATION
bool __ia32_enabled __ro_after_init = !IS_ENABLED(CONFIG_IA32_EMULATION_DEFAULT_DISABLED);
static int ia32_emulation_override_cmdline(char *arg)
{
return kstrtobool(arg, &__ia32_enabled);
}
early_param("ia32_emulation", ia32_emulation_override_cmdline);
#endif
/*
* Invoke a 32-bit syscall. Called with IRQs on in CONTEXT_KERNEL.
*/
static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs, int nr)
{
/*
* Convert negative numbers to very high and thus out of range
* numbers for comparisons.
*/
unsigned int unr = nr;
if (likely(unr < IA32_NR_syscalls)) {
unr = array_index_nospec(unr, IA32_NR_syscalls);
regs->ax = ia32_sys_call_table[unr](regs);
} else if (nr != -1) {
regs->ax = __ia32_sys_ni_syscall(regs);
}
}
#ifdef CONFIG_IA32_EMULATION
static __always_inline bool int80_is_external(void)
{
const unsigned int offs = (0x80 / 32) * 0x10;
const u32 bit = BIT(0x80 % 32);
/* The local APIC on XENPV guests is fake */
if (cpu_feature_enabled(X86_FEATURE_XENPV))
return false;
/*
* If vector 0x80 is set in the APIC ISR then this is an external
* interrupt. Either from broken hardware or injected by a VMM.
*
* Note: In guest mode this is only valid for secure guests where
* the secure module fully controls the vAPIC exposed to the guest.
*/
return apic_read(APIC_ISR + offs) & bit;
}
/**
* int80_emulation - 32-bit legacy syscall entry
*
* This entry point can be used by 32-bit and 64-bit programs to perform
* 32-bit system calls. Instances of INT $0x80 can be found inline in
* various programs and libraries. It is also used by the vDSO's
* __kernel_vsyscall fallback for hardware that doesn't support a faster
* entry method. Restarted 32-bit system calls also fall back to INT
* $0x80 regardless of what instruction was originally used to do the
* system call.
*
* This is considered a slow path. It is not used by most libc
* implementations on modern hardware except during process startup.
*
* The arguments for the INT $0x80 based syscall are on stack in the
* pt_regs structure:
* eax: system call number
* ebx, ecx, edx, esi, edi, ebp: arg1 - arg 6
*/
DEFINE_IDTENTRY_RAW(int80_emulation)
{
int nr;
/* Kernel does not use INT $0x80! */
if (unlikely(!user_mode(regs))) {
irqentry_enter(regs);
instrumentation_begin();
panic("Unexpected external interrupt 0x80\n");
}
/*
* Establish kernel context for instrumentation, including for
* int80_is_external() below which calls into the APIC driver.
* Identical for soft and external interrupts.
*/
enter_from_user_mode(regs);
instrumentation_begin();
add_random_kstack_offset();
/* Validate that this is a soft interrupt to the extent possible */
if (unlikely(int80_is_external()))
panic("Unexpected external interrupt 0x80\n");
/*
* The low level idtentry code pushed -1 into regs::orig_ax
* and regs::ax contains the syscall number.
*
* User tracing code (ptrace or signal handlers) might assume
* that the regs::orig_ax contains a 32-bit number on invoking
* a 32-bit syscall.
*
* Establish the syscall convention by saving the 32bit truncated
* syscall number in regs::orig_ax and by invalidating regs::ax.
*/
regs->orig_ax = regs->ax & GENMASK(31, 0);
regs->ax = -ENOSYS;
nr = syscall_32_enter(regs);
local_irq_enable();
nr = syscall_enter_from_user_mode_work(regs, nr);
do_syscall_32_irqs_on(regs, nr);
instrumentation_end();
syscall_exit_to_user_mode(regs);
}
#else /* CONFIG_IA32_EMULATION */
/* Handles int $0x80 on a 32bit kernel */
__visible noinstr void do_int80_syscall_32(struct pt_regs *regs)
{
int nr = syscall_32_enter(regs);
add_random_kstack_offset();
/*
* Subtlety here: if ptrace pokes something larger than 2^31-1 into
* orig_ax, the int return value truncates it. This matches
* the semantics of syscall_get_nr().
*/
nr = syscall_enter_from_user_mode(regs, nr);
instrumentation_begin();
do_syscall_32_irqs_on(regs, nr);
instrumentation_end();
syscall_exit_to_user_mode(regs);
}
#endif /* !CONFIG_IA32_EMULATION */
static noinstr bool __do_fast_syscall_32(struct pt_regs *regs)
{
int nr = syscall_32_enter(regs);
int res;
add_random_kstack_offset();
/*
* This cannot use syscall_enter_from_user_mode() as it has to
* fetch EBP before invoking any of the syscall entry work
* functions.
*/
syscall_enter_from_user_mode_prepare(regs);
instrumentation_begin();
/* Fetch EBP from where the vDSO stashed it. */
if (IS_ENABLED(CONFIG_X86_64)) {
/*
* Micro-optimization: the pointer we're following is
* explicitly 32 bits, so it can't be out of range.
*/
res = __get_user(*(u32 *)&regs->bp,
(u32 __user __force *)(unsigned long)(u32)regs->sp);
} else {
res = get_user(*(u32 *)&regs->bp,
(u32 __user __force *)(unsigned long)(u32)regs->sp);
}
if (res) {
/* User code screwed up. */
regs->ax = -EFAULT;
local_irq_disable();
instrumentation_end();
irqentry_exit_to_user_mode(regs);
return false;
}
nr = syscall_enter_from_user_mode_work(regs, nr);
/* Now this is just like a normal syscall. */
do_syscall_32_irqs_on(regs, nr);
instrumentation_end();
syscall_exit_to_user_mode(regs);
return true;
}
/* Returns true to return using SYSEXIT/SYSRETL, or false to use IRET */
__visible noinstr bool do_fast_syscall_32(struct pt_regs *regs)
{
/*
* Called using the internal vDSO SYSENTER/SYSCALL32 calling
* convention. Adjust regs so it looks like we entered using int80.
*/
unsigned long landing_pad = (unsigned long)current->mm->context.vdso +
vdso_image_32.sym_int80_landing_pad;
/*
* SYSENTER loses EIP, and even SYSCALL32 needs us to skip forward
* so that 'regs->ip -= 2' lands back on an int $0x80 instruction.
* Fix it up.
*/
regs->ip = landing_pad;
/* Invoke the syscall. If it failed, keep it simple: use IRET. */
if (!__do_fast_syscall_32(regs))
return false;
/*
* Check that the register state is valid for using SYSRETL/SYSEXIT
* to exit to userspace. Otherwise use the slower but fully capable
* IRET exit path.
*/
/* XEN PV guests always use the IRET path */
if (cpu_feature_enabled(X86_FEATURE_XENPV))
return false;
/* EIP must point to the VDSO landing pad */
if (unlikely(regs->ip != landing_pad))
return false;
/* CS and SS must match the values set in MSR_STAR */
if (unlikely(regs->cs != __USER32_CS || regs->ss != __USER_DS))
return false;
/* If the TF, RF, or VM flags are set, use IRET */
if (unlikely(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF | X86_EFLAGS_VM)))
return false;
/* Use SYSRETL/SYSEXIT to exit to userspace */
return true;
}
/* Returns true to return using SYSEXIT/SYSRETL, or false to use IRET */
__visible noinstr bool do_SYSENTER_32(struct pt_regs *regs)
{
/* SYSENTER loses RSP, but the vDSO saved it in RBP. */
regs->sp = regs->bp;
/* SYSENTER clobbers EFLAGS.IF. Assume it was set in usermode. */
regs->flags |= X86_EFLAGS_IF;
return do_fast_syscall_32(regs);
}
#endif
SYSCALL_DEFINE0(ni_syscall)
{
return -ENOSYS;
}
#ifdef CONFIG_XEN_PV
#ifndef CONFIG_PREEMPTION
/*
* Some hypercalls issued by the toolstack can take many 10s of
* seconds. Allow tasks running hypercalls via the privcmd driver to
* be voluntarily preempted even if full kernel preemption is
* disabled.
*
* Such preemptible hypercalls are bracketed by
* xen_preemptible_hcall_begin() and xen_preemptible_hcall_end()
* calls.
*/
DEFINE_PER_CPU(bool, xen_in_preemptible_hcall);
EXPORT_SYMBOL_GPL(xen_in_preemptible_hcall);
/*
* In case of scheduling the flag must be cleared and restored after
* returning from schedule as the task might move to a different CPU.
*/
static __always_inline bool get_and_clear_inhcall(void)
{
bool inhcall = __this_cpu_read(xen_in_preemptible_hcall);
__this_cpu_write(xen_in_preemptible_hcall, false);
return inhcall;
}
static __always_inline void restore_inhcall(bool inhcall)
{
__this_cpu_write(xen_in_preemptible_hcall, inhcall);
}
#else
static __always_inline bool get_and_clear_inhcall(void) { return false; }
static __always_inline void restore_inhcall(bool inhcall) { }
#endif
static void __xen_pv_evtchn_do_upcall(struct pt_regs *regs)
{
struct pt_regs *old_regs = set_irq_regs(regs);
inc_irq_stat(irq_hv_callback_count);
xen_evtchn_do_upcall();
set_irq_regs(old_regs);
}
__visible noinstr void xen_pv_evtchn_do_upcall(struct pt_regs *regs)
{
irqentry_state_t state = irqentry_enter(regs);
bool inhcall;
instrumentation_begin();
run_sysvec_on_irqstack_cond(__xen_pv_evtchn_do_upcall, regs);
inhcall = get_and_clear_inhcall();
if (inhcall && !WARN_ON_ONCE(state.exit_rcu)) {
irqentry_exit_cond_resched();
instrumentation_end();
restore_inhcall(inhcall);
} else {
instrumentation_end();
irqentry_exit(regs, state);
}
}
#endif /* CONFIG_XEN_PV */