arch/x86/mm/extable.c - linux - Git at Google

 #include <linux/extable.h>
 #include <linux/uaccess.h>
 #include <linux/sched/debug.h>
 #include <xen/xen.h>

 #include <asm/fpu/internal.h>
 #include <asm/traps.h>
 #include <asm/kdebug.h>

 typedef bool (*ex_handler_t)(const struct exception_table_entry *,
 			    struct pt_regs *, int, unsigned long,
 			    unsigned long);

 static inline unsigned long
 ex_fixup_addr(const struct exception_table_entry *x)
 {
 	return (unsigned long)&x->fixup + x->fixup;
 }
 static inline ex_handler_t
 ex_fixup_handler(const struct exception_table_entry *x)
 {
 	return (ex_handler_t)((unsigned long)&x->handler + x->handler);
 }

 __visible bool ex_handler_default(const struct exception_table_entry *fixup,
 				  struct pt_regs *regs, int trapnr,
 				  unsigned long error_code,
 				  unsigned long fault_addr)
 {
 	regs->ip = ex_fixup_addr(fixup);
 	return true;
 }
 EXPORT_SYMBOL(ex_handler_default);

 __visible bool ex_handler_fault(const struct exception_table_entry *fixup,
 				struct pt_regs *regs, int trapnr,
 				unsigned long error_code,
 				unsigned long fault_addr)
 {
 	regs->ip = ex_fixup_addr(fixup);
 	regs->ax = trapnr;
 	return true;
 }
 EXPORT_SYMBOL_GPL(ex_handler_fault);

 /*
  * Handler for UD0 exception following a failed test against the
  * result of a refcount inc/dec/add/sub.
  */
 __visible bool ex_handler_refcount(const struct exception_table_entry *fixup,
 				   struct pt_regs *regs, int trapnr,
 				   unsigned long error_code,
 				   unsigned long fault_addr)
 {
 	/* First unconditionally saturate the refcount. */
 	*(int *)regs->cx = INT_MIN / 2;

 	/*
 	 * Strictly speaking, this reports the fixup destination, not
 	 * the fault location, and not the actually overflowing
 	 * instruction, which is the instruction before the "js", but
 	 * since that instruction could be a variety of lengths, just
 	 * report the location after the overflow, which should be close
 	 * enough for finding the overflow, as it's at least back in
 	 * the function, having returned from .text.unlikely.
 	 */
 	regs->ip = ex_fixup_addr(fixup);

 	/*
 	 * This function has been called because either a negative refcount
 	 * value was seen by any of the refcount functions, or a zero
 	 * refcount value was seen by refcount_dec().
 	 *
 	 * If we crossed from INT_MAX to INT_MIN, OF (Overflow Flag: result
 	 * wrapped around) will be set. Additionally, seeing the refcount
 	 * reach 0 will set ZF (Zero Flag: result was zero). In each of
 	 * these cases we want a report, since it's a boundary condition.
 	 * The SF case is not reported since it indicates post-boundary
 	 * manipulations below zero or above INT_MAX. And if none of the
 	 * flags are set, something has gone very wrong, so report it.
 	 */
 	if (regs->flags & (X86_EFLAGS_OF | X86_EFLAGS_ZF)) {
 		bool zero = regs->flags & X86_EFLAGS_ZF;

 		refcount_error_report(regs, zero ? "hit zero" : "overflow");
 	} else if ((regs->flags & X86_EFLAGS_SF) == 0) {
 		/* Report if none of OF, ZF, nor SF are set. */
 		refcount_error_report(regs, "unexpected saturation");
 	}

 	return true;
 }
 EXPORT_SYMBOL(ex_handler_refcount);

 /*
  * Handler for when we fail to restore a task's FPU state.  We should never get
  * here because the FPU state of a task using the FPU (task->thread.fpu.state)
  * should always be valid.  However, past bugs have allowed userspace to set
  * reserved bits in the XSAVE area using PTRACE_SETREGSET or sys_rt_sigreturn().
  * These caused XRSTOR to fail when switching to the task, leaking the FPU
  * registers of the task previously executing on the CPU.  Mitigate this class
  * of vulnerability by restoring from the initial state (essentially, zeroing
  * out all the FPU registers) if we can't restore from the task's FPU state.
  */
 __visible bool ex_handler_fprestore(const struct exception_table_entry *fixup,
 				    struct pt_regs *regs, int trapnr,
 				    unsigned long error_code,
 				    unsigned long fault_addr)
 {
 	regs->ip = ex_fixup_addr(fixup);

 	WARN_ONCE(1, "Bad FPU state detected at %pB, reinitializing FPU registers.",
 		  (void *)instruction_pointer(regs));

 	__copy_kernel_to_fpregs(&init_fpstate, -1);
 	return true;
 }
 EXPORT_SYMBOL_GPL(ex_handler_fprestore);

 /* Helper to check whether a uaccess fault indicates a kernel bug. */
 static bool bogus_uaccess(struct pt_regs *regs, int trapnr,
 			  unsigned long fault_addr)
 {
 	/* This is the normal case: #PF with a fault address in userspace. */
 	if (trapnr == X86_TRAP_PF && fault_addr < TASK_SIZE_MAX)
 		return false;

 	/*
 	 * This code can be reached for machine checks, but only if the #MC
 	 * handler has already decided that it looks like a candidate for fixup.
 	 * This e.g. happens when attempting to access userspace memory which
 	 * the CPU can't access because of uncorrectable bad memory.
 	 */
 	if (trapnr == X86_TRAP_MC)
 		return false;

 	/*
 	 * There are two remaining exception types we might encounter here:
 	 *  - #PF for faulting accesses to kernel addresses
 	 *  - #GP for faulting accesses to noncanonical addresses
 	 * Complain about anything else.
 	 */
 	if (trapnr != X86_TRAP_PF && trapnr != X86_TRAP_GP) {
 		WARN(1, "unexpected trap %d in uaccess\n", trapnr);
 		return false;
 	}

 	/*
 	 * This is a faulting memory access in kernel space, on a kernel
 	 * address, in a usercopy function. This can e.g. be caused by improper
 	 * use of helpers like __put_user and by improper attempts to access
 	 * userspace addresses in KERNEL_DS regions.
 	 * The one (semi-)legitimate exception are probe_kernel_{read,write}(),
 	 * which can be invoked from places like kgdb, /dev/mem (for reading)
 	 * and privileged BPF code (for reading).
 	 * The probe_kernel_*() functions set the kernel_uaccess_faults_ok flag
 	 * to tell us that faulting on kernel addresses, and even noncanonical
 	 * addresses, in a userspace accessor does not necessarily imply a
 	 * kernel bug, root might just be doing weird stuff.
 	 */
 	if (current->kernel_uaccess_faults_ok)
 		return false;

 	/* This is bad. Refuse the fixup so that we go into die(). */
 	if (trapnr == X86_TRAP_PF) {
 		pr_emerg("BUG: pagefault on kernel address 0x%lx in non-whitelisted uaccess\n",
 			 fault_addr);
 	} else {
 		pr_emerg("BUG: GPF in non-whitelisted uaccess (non-canonical address?)\n");
 	}
 	return true;
 }

 __visible bool ex_handler_uaccess(const struct exception_table_entry *fixup,
 				  struct pt_regs *regs, int trapnr,
 				  unsigned long error_code,
 				  unsigned long fault_addr)
 {
 	if (bogus_uaccess(regs, trapnr, fault_addr))
 		return false;
 	regs->ip = ex_fixup_addr(fixup);
 	return true;
 }
 EXPORT_SYMBOL(ex_handler_uaccess);

 __visible bool ex_handler_ext(const struct exception_table_entry *fixup,
 			      struct pt_regs *regs, int trapnr,
 			      unsigned long error_code,
 			      unsigned long fault_addr)
 {
 	if (bogus_uaccess(regs, trapnr, fault_addr))
 		return false;
 	/* Special hack for uaccess_err */
 	current->thread.uaccess_err = 1;
 	regs->ip = ex_fixup_addr(fixup);
 	return true;
 }
 EXPORT_SYMBOL(ex_handler_ext);

 __visible bool ex_handler_rdmsr_unsafe(const struct exception_table_entry *fixup,
 				       struct pt_regs *regs, int trapnr,
 				       unsigned long error_code,
 				       unsigned long fault_addr)
 {
 	if (pr_warn_once("unchecked MSR access error: RDMSR from 0x%x at rIP: 0x%lx (%pF)\n",
 			 (unsigned int)regs->cx, regs->ip, (void *)regs->ip))
 		show_stack_regs(regs);

 	/* Pretend that the read succeeded and returned 0. */
 	regs->ip = ex_fixup_addr(fixup);
 	regs->ax = 0;
 	regs->dx = 0;
 	return true;
 }
 EXPORT_SYMBOL(ex_handler_rdmsr_unsafe);

 __visible bool ex_handler_wrmsr_unsafe(const struct exception_table_entry *fixup,
 				       struct pt_regs *regs, int trapnr,
 				       unsigned long error_code,
 				       unsigned long fault_addr)
 {
 	if (pr_warn_once("unchecked MSR access error: WRMSR to 0x%x (tried to write 0x%08x%08x) at rIP: 0x%lx (%pF)\n",
 			 (unsigned int)regs->cx, (unsigned int)regs->dx,
 			 (unsigned int)regs->ax,  regs->ip, (void *)regs->ip))
 		show_stack_regs(regs);

 	/* Pretend that the write succeeded. */
 	regs->ip = ex_fixup_addr(fixup);
 	return true;
 }
 EXPORT_SYMBOL(ex_handler_wrmsr_unsafe);

 __visible bool ex_handler_clear_fs(const struct exception_table_entry *fixup,
 				   struct pt_regs *regs, int trapnr,
 				   unsigned long error_code,
 				   unsigned long fault_addr)
 {
 	if (static_cpu_has(X86_BUG_NULL_SEG))
 		asm volatile ("mov %0, %%fs" : : "rm" (__USER_DS));
 	asm volatile ("mov %0, %%fs" : : "rm" (0));
 	return ex_handler_default(fixup, regs, trapnr, error_code, fault_addr);
 }
 EXPORT_SYMBOL(ex_handler_clear_fs);

 __visible bool ex_has_fault_handler(unsigned long ip)
 {
 	const struct exception_table_entry *e;
 	ex_handler_t handler;

 	e = search_exception_tables(ip);
 	if (!e)
 		return false;
 	handler = ex_fixup_handler(e);

 	return handler == ex_handler_fault;
 }

 int fixup_exception(struct pt_regs *regs, int trapnr, unsigned long error_code,
 		    unsigned long fault_addr)
 {
 	const struct exception_table_entry *e;
 	ex_handler_t handler;

 #ifdef CONFIG_PNPBIOS
 	if (unlikely(SEGMENT_IS_PNP_CODE(regs->cs))) {
 		extern u32 pnp_bios_fault_eip, pnp_bios_fault_esp;
 		extern u32 pnp_bios_is_utter_crap;
 		pnp_bios_is_utter_crap = 1;
 		printk(KERN_CRIT "PNPBIOS fault.. attempting recovery.\n");
 		__asm__ volatile(
 			"movl %0, %%esp\n\t"
 			"jmp *%1\n\t"
 			: : "g" (pnp_bios_fault_esp), "g" (pnp_bios_fault_eip));
 		panic("do_trap: can't hit this");
 	}
 #endif

 	e = search_exception_tables(regs->ip);
 	if (!e)
 		return 0;

 	handler = ex_fixup_handler(e);
 	return handler(e, regs, trapnr, error_code, fault_addr);
 }

 extern unsigned int early_recursion_flag;

 /* Restricted version used during very early boot */
 void __init early_fixup_exception(struct pt_regs *regs, int trapnr)
 {
 	/* Ignore early NMIs. */
 	if (trapnr == X86_TRAP_NMI)
 		return;

 	if (early_recursion_flag > 2)
 		goto halt_loop;

 	/*
 	 * Old CPUs leave the high bits of CS on the stack
 	 * undefined.  I'm not sure which CPUs do this, but at least
 	 * the 486 DX works this way.
 	 * Xen pv domains are not using the default __KERNEL_CS.
 	 */
 	if (!xen_pv_domain() && regs->cs != __KERNEL_CS)
 		goto fail;

 	/*
 	 * The full exception fixup machinery is available as soon as
 	 * the early IDT is loaded.  This means that it is the
 	 * responsibility of extable users to either function correctly
 	 * when handlers are invoked early or to simply avoid causing
 	 * exceptions before they're ready to handle them.
 	 *
 	 * This is better than filtering which handlers can be used,
 	 * because refusing to call a handler here is guaranteed to
 	 * result in a hard-to-debug panic.
 	 *
 	 * Keep in mind that not all vectors actually get here.  Early
 	 * page faults, for example, are special.
 	 */
 	if (fixup_exception(regs, trapnr, regs->orig_ax, 0))
 		return;

 	if (fixup_bug(regs, trapnr))
 		return;

 fail:
 	early_printk("PANIC: early exception 0x%02x IP %lx:%lx error %lx cr2 0x%lx\n",
 		     (unsigned)trapnr, (unsigned long)regs->cs, regs->ip,
 		     regs->orig_ax, read_cr2());

 	show_regs(regs);

 halt_loop:
 	while (true)
 		halt();
 }
	#include <linux/extable.h>
	#include <linux/uaccess.h>
	#include <linux/sched/debug.h>
	#include <xen/xen.h>

	#include <asm/fpu/internal.h>
	#include <asm/traps.h>
	#include <asm/kdebug.h>

	typedef bool (ex_handler_t)(const struct exception_table_entry ,
	struct pt_regs *, int, unsigned long,
	unsigned long);

	static inline unsigned long
	ex_fixup_addr(const struct exception_table_entry *x)
	{
	return (unsigned long)&x->fixup + x->fixup;
	}
	static inline ex_handler_t
	ex_fixup_handler(const struct exception_table_entry *x)
	{
	return (ex_handler_t)((unsigned long)&x->handler + x->handler);
	}

	__visible bool ex_handler_default(const struct exception_table_entry *fixup,
	struct pt_regs *regs, int trapnr,
	unsigned long error_code,
	unsigned long fault_addr)
	{
	regs->ip = ex_fixup_addr(fixup);
	return true;
	}
	EXPORT_SYMBOL(ex_handler_default);

	__visible bool ex_handler_fault(const struct exception_table_entry *fixup,
	struct pt_regs *regs, int trapnr,
	unsigned long error_code,
	unsigned long fault_addr)
	{
	regs->ip = ex_fixup_addr(fixup);
	regs->ax = trapnr;
	return true;
	}
	EXPORT_SYMBOL_GPL(ex_handler_fault);

	/*
	* Handler for UD0 exception following a failed test against the
	* result of a refcount inc/dec/add/sub.
	*/
	__visible bool ex_handler_refcount(const struct exception_table_entry *fixup,
	struct pt_regs *regs, int trapnr,
	unsigned long error_code,
	unsigned long fault_addr)
	{
	/* First unconditionally saturate the refcount. */
	(int )regs->cx = INT_MIN / 2;

	/*
	* Strictly speaking, this reports the fixup destination, not
	* the fault location, and not the actually overflowing
	* instruction, which is the instruction before the "js", but
	* since that instruction could be a variety of lengths, just
	* report the location after the overflow, which should be close
	* enough for finding the overflow, as it's at least back in
	* the function, having returned from .text.unlikely.
	*/
	regs->ip = ex_fixup_addr(fixup);

	/*
	* This function has been called because either a negative refcount
	* value was seen by any of the refcount functions, or a zero
	* refcount value was seen by refcount_dec().
	*
	* If we crossed from INT_MAX to INT_MIN, OF (Overflow Flag: result
	* wrapped around) will be set. Additionally, seeing the refcount
	* reach 0 will set ZF (Zero Flag: result was zero). In each of
	* these cases we want a report, since it's a boundary condition.
	* The SF case is not reported since it indicates post-boundary
	* manipulations below zero or above INT_MAX. And if none of the
	* flags are set, something has gone very wrong, so report it.
	*/
	if (regs->flags & (X86_EFLAGS_OF \| X86_EFLAGS_ZF)) {
	bool zero = regs->flags & X86_EFLAGS_ZF;

	refcount_error_report(regs, zero ? "hit zero" : "overflow");
	} else if ((regs->flags & X86_EFLAGS_SF) == 0) {
	/* Report if none of OF, ZF, nor SF are set. */
	refcount_error_report(regs, "unexpected saturation");
	}

	return true;
	}
	EXPORT_SYMBOL(ex_handler_refcount);

	/*
	* Handler for when we fail to restore a task's FPU state. We should never get
	* here because the FPU state of a task using the FPU (task->thread.fpu.state)
	* should always be valid. However, past bugs have allowed userspace to set
	* reserved bits in the XSAVE area using PTRACE_SETREGSET or sys_rt_sigreturn().
	* These caused XRSTOR to fail when switching to the task, leaking the FPU
	* registers of the task previously executing on the CPU. Mitigate this class
	* of vulnerability by restoring from the initial state (essentially, zeroing
	* out all the FPU registers) if we can't restore from the task's FPU state.
	*/
	__visible bool ex_handler_fprestore(const struct exception_table_entry *fixup,
	struct pt_regs *regs, int trapnr,
	unsigned long error_code,
	unsigned long fault_addr)
	{
	regs->ip = ex_fixup_addr(fixup);

	WARN_ONCE(1, "Bad FPU state detected at %pB, reinitializing FPU registers.",
	(void *)instruction_pointer(regs));

	__copy_kernel_to_fpregs(&init_fpstate, -1);
	return true;
	}
	EXPORT_SYMBOL_GPL(ex_handler_fprestore);

	/* Helper to check whether a uaccess fault indicates a kernel bug. */
	static bool bogus_uaccess(struct pt_regs *regs, int trapnr,
	unsigned long fault_addr)
	{
	/* This is the normal case: #PF with a fault address in userspace. */
	if (trapnr == X86_TRAP_PF && fault_addr < TASK_SIZE_MAX)
	return false;

	/*
	* This code can be reached for machine checks, but only if the #MC
	* handler has already decided that it looks like a candidate for fixup.
	* This e.g. happens when attempting to access userspace memory which
	* the CPU can't access because of uncorrectable bad memory.
	*/
	if (trapnr == X86_TRAP_MC)
	return false;

	/*
	* There are two remaining exception types we might encounter here:
	* - #PF for faulting accesses to kernel addresses
	* - #GP for faulting accesses to noncanonical addresses
	* Complain about anything else.
	*/
	if (trapnr != X86_TRAP_PF && trapnr != X86_TRAP_GP) {
	WARN(1, "unexpected trap %d in uaccess\n", trapnr);
	return false;
	}

	/*
	* This is a faulting memory access in kernel space, on a kernel
	* address, in a usercopy function. This can e.g. be caused by improper
	* use of helpers like __put_user and by improper attempts to access
	* userspace addresses in KERNEL_DS regions.
	* The one (semi-)legitimate exception are probe_kernel_{read,write}(),
	* which can be invoked from places like kgdb, /dev/mem (for reading)
	* and privileged BPF code (for reading).
	* The probe_kernel_*() functions set the kernel_uaccess_faults_ok flag
	* to tell us that faulting on kernel addresses, and even noncanonical
	* addresses, in a userspace accessor does not necessarily imply a
	* kernel bug, root might just be doing weird stuff.
	*/
	if (current->kernel_uaccess_faults_ok)
	return false;

	/* This is bad. Refuse the fixup so that we go into die(). */
	if (trapnr == X86_TRAP_PF) {
	pr_emerg("BUG: pagefault on kernel address 0x%lx in non-whitelisted uaccess\n",
	fault_addr);
	} else {
	pr_emerg("BUG: GPF in non-whitelisted uaccess (non-canonical address?)\n");
	}
	return true;
	}

	__visible bool ex_handler_uaccess(const struct exception_table_entry *fixup,
	struct pt_regs *regs, int trapnr,
	unsigned long error_code,
	unsigned long fault_addr)
	{
	if (bogus_uaccess(regs, trapnr, fault_addr))
	return false;
	regs->ip = ex_fixup_addr(fixup);
	return true;
	}
	EXPORT_SYMBOL(ex_handler_uaccess);

	__visible bool ex_handler_ext(const struct exception_table_entry *fixup,
	struct pt_regs *regs, int trapnr,
	unsigned long error_code,
	unsigned long fault_addr)
	{
	if (bogus_uaccess(regs, trapnr, fault_addr))
	return false;
	/* Special hack for uaccess_err */
	current->thread.uaccess_err = 1;
	regs->ip = ex_fixup_addr(fixup);
	return true;
	}
	EXPORT_SYMBOL(ex_handler_ext);

	__visible bool ex_handler_rdmsr_unsafe(const struct exception_table_entry *fixup,
	struct pt_regs *regs, int trapnr,
	unsigned long error_code,
	unsigned long fault_addr)
	{
	if (pr_warn_once("unchecked MSR access error: RDMSR from 0x%x at rIP: 0x%lx (%pF)\n",
	(unsigned int)regs->cx, regs->ip, (void *)regs->ip))
	show_stack_regs(regs);

	/* Pretend that the read succeeded and returned 0. */
	regs->ip = ex_fixup_addr(fixup);
	regs->ax = 0;
	regs->dx = 0;
	return true;
	}
	EXPORT_SYMBOL(ex_handler_rdmsr_unsafe);

	__visible bool ex_handler_wrmsr_unsafe(const struct exception_table_entry *fixup,
	struct pt_regs *regs, int trapnr,
	unsigned long error_code,
	unsigned long fault_addr)
	{
	if (pr_warn_once("unchecked MSR access error: WRMSR to 0x%x (tried to write 0x%08x%08x) at rIP: 0x%lx (%pF)\n",
	(unsigned int)regs->cx, (unsigned int)regs->dx,
	(unsigned int)regs->ax, regs->ip, (void *)regs->ip))
	show_stack_regs(regs);

	/* Pretend that the write succeeded. */
	regs->ip = ex_fixup_addr(fixup);
	return true;
	}
	EXPORT_SYMBOL(ex_handler_wrmsr_unsafe);

	__visible bool ex_handler_clear_fs(const struct exception_table_entry *fixup,
	struct pt_regs *regs, int trapnr,
	unsigned long error_code,
	unsigned long fault_addr)
	{
	if (static_cpu_has(X86_BUG_NULL_SEG))
	asm volatile ("mov %0, %%fs" : : "rm" (__USER_DS));
	asm volatile ("mov %0, %%fs" : : "rm" (0));
	return ex_handler_default(fixup, regs, trapnr, error_code, fault_addr);
	}
	EXPORT_SYMBOL(ex_handler_clear_fs);

	__visible bool ex_has_fault_handler(unsigned long ip)
	{
	const struct exception_table_entry *e;
	ex_handler_t handler;

	e = search_exception_tables(ip);
	if (!e)
	return false;
	handler = ex_fixup_handler(e);

	return handler == ex_handler_fault;
	}

	int fixup_exception(struct pt_regs *regs, int trapnr, unsigned long error_code,
	unsigned long fault_addr)
	{
	const struct exception_table_entry *e;
	ex_handler_t handler;

	#ifdef CONFIG_PNPBIOS
	if (unlikely(SEGMENT_IS_PNP_CODE(regs->cs))) {
	extern u32 pnp_bios_fault_eip, pnp_bios_fault_esp;
	extern u32 pnp_bios_is_utter_crap;
	pnp_bios_is_utter_crap = 1;
	printk(KERN_CRIT "PNPBIOS fault.. attempting recovery.\n");
	__asm__ volatile(
	"movl %0, %%esp\n\t"
	"jmp *%1\n\t"
	: : "g" (pnp_bios_fault_esp), "g" (pnp_bios_fault_eip));
	panic("do_trap: can't hit this");
	}
	#endif

	e = search_exception_tables(regs->ip);
	if (!e)
	return 0;

	handler = ex_fixup_handler(e);
	return handler(e, regs, trapnr, error_code, fault_addr);
	}

	extern unsigned int early_recursion_flag;

	/* Restricted version used during very early boot */
	void __init early_fixup_exception(struct pt_regs *regs, int trapnr)
	{
	/* Ignore early NMIs. */
	if (trapnr == X86_TRAP_NMI)
	return;

	if (early_recursion_flag > 2)
	goto halt_loop;

	/*
	* Old CPUs leave the high bits of CS on the stack
	* undefined. I'm not sure which CPUs do this, but at least
	* the 486 DX works this way.
	* Xen pv domains are not using the default __KERNEL_CS.
	*/
	if (!xen_pv_domain() && regs->cs != __KERNEL_CS)
	goto fail;

	/*
	* The full exception fixup machinery is available as soon as
	* the early IDT is loaded. This means that it is the
	* responsibility of extable users to either function correctly
	* when handlers are invoked early or to simply avoid causing
	* exceptions before they're ready to handle them.
	*
	* This is better than filtering which handlers can be used,
	* because refusing to call a handler here is guaranteed to
	* result in a hard-to-debug panic.
	*
	* Keep in mind that not all vectors actually get here. Early
	* page faults, for example, are special.
	*/
	if (fixup_exception(regs, trapnr, regs->orig_ax, 0))
	return;

	if (fixup_bug(regs, trapnr))
	return;

	fail:
	early_printk("PANIC: early exception 0x%02x IP %lx:%lx error %lx cr2 0x%lx\n",
	(unsigned)trapnr, (unsigned long)regs->cs, regs->ip,
	regs->orig_ax, read_cr2());

	show_regs(regs);

	halt_loop:
	while (true)
	halt();
	}