mm: fix exec activate_mm vs TLB shootdown and lazy tlb switching race

[linux/fpc-iii.git] / arch / x86 / kernel / traps.c

/*
 *  Copyright (C) 1991, 1992  Linus Torvalds
 *  Copyright (C) 2000, 2001, 2002 Andi Kleen, SuSE Labs
 *
 *  Pentium III FXSR, SSE support
 *      Gareth Hughes <gareth@valinux.com>, May 2000
 */

/*
 * Handle hardware traps and faults.
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/context_tracking.h>
#include <linux/interrupt.h>
#include <linux/kallsyms.h>
#include <linux/spinlock.h>
#include <linux/kprobes.h>
#include <linux/uaccess.h>
#include <linux/kdebug.h>
#include <linux/kgdb.h>
#include <linux/kernel.h>
#include <linux/export.h>
#include <linux/ptrace.h>
#include <linux/uprobes.h>
#include <linux/string.h>
#include <linux/delay.h>
#include <linux/errno.h>
#include <linux/kexec.h>
#include <linux/sched.h>
#include <linux/sched/task_stack.h>
#include <linux/timer.h>
#include <linux/init.h>
#include <linux/bug.h>
#include <linux/nmi.h>
#include <linux/mm.h>
#include <linux/smp.h>
#include <linux/io.h>

#if defined(CONFIG_EDAC)
#include <linux/edac.h>
#endif

#include <asm/stacktrace.h>
#include <asm/processor.h>
#include <asm/debugreg.h>
#include <linux/atomic.h>
#include <asm/text-patching.h>
#include <asm/ftrace.h>
#include <asm/traps.h>
#include <asm/desc.h>
#include <asm/fpu/internal.h>
#include <asm/cpu_entry_area.h>
#include <asm/mce.h>
#include <asm/fixmap.h>
#include <asm/mach_traps.h>
#include <asm/alternative.h>
#include <asm/fpu/xstate.h>
#include <asm/trace/mpx.h>
#include <asm/mpx.h>
#include <asm/vm86.h>

#ifdef CONFIG_X86_64
#include <asm/x86_init.h>
#include <asm/pgalloc.h>
#include <asm/proto.h>
#else
#include <asm/processor-flags.h>
#include <asm/setup.h>
#include <asm/proto.h>
#endif

DECLARE_BITMAP(used_vectors, NR_VECTORS);

static inline void cond_local_irq_enable(struct pt_regs *regs)
{
        if (regs->flags & X86_EFLAGS_IF)
                local_irq_enable();
}

static inline void cond_local_irq_disable(struct pt_regs *regs)
{
        if (regs->flags & X86_EFLAGS_IF)
                local_irq_disable();
}

/*
 * In IST context, we explicitly disable preemption.  This serves two
 * purposes: it makes it much less likely that we would accidentally
 * schedule in IST context and it will force a warning if we somehow
 * manage to schedule by accident.
 */
void ist_enter(struct pt_regs *regs)
{
        if (user_mode(regs)) {
                RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
        } else {
                /*
                 * We might have interrupted pretty much anything.  In
                 * fact, if we're a machine check, we can even interrupt
                 * NMI processing.  We don't want in_nmi() to return true,
                 * but we need to notify RCU.
                 */
                rcu_nmi_enter();
        }

        preempt_disable();

        /* This code is a bit fragile.  Test it. */
        RCU_LOCKDEP_WARN(!rcu_is_watching(), "ist_enter didn't work");
}

void ist_exit(struct pt_regs *regs)
{
        preempt_enable_no_resched();

        if (!user_mode(regs))
                rcu_nmi_exit();
}

/**
 * ist_begin_non_atomic() - begin a non-atomic section in an IST exception
 * @regs:       regs passed to the IST exception handler
 *
 * IST exception handlers normally cannot schedule.  As a special
 * exception, if the exception interrupted userspace code (i.e.
 * user_mode(regs) would return true) and the exception was not
 * a double fault, it can be safe to schedule.  ist_begin_non_atomic()
 * begins a non-atomic section within an ist_enter()/ist_exit() region.
 * Callers are responsible for enabling interrupts themselves inside
 * the non-atomic section, and callers must call ist_end_non_atomic()
 * before ist_exit().
 */
void ist_begin_non_atomic(struct pt_regs *regs)
{
        BUG_ON(!user_mode(regs));

        /*
         * Sanity check: we need to be on the normal thread stack.  This
         * will catch asm bugs and any attempt to use ist_preempt_enable
         * from double_fault.
         */
        BUG_ON(!on_thread_stack());

        preempt_enable_no_resched();
}

/**
 * ist_end_non_atomic() - begin a non-atomic section in an IST exception
 *
 * Ends a non-atomic section started with ist_begin_non_atomic().
 */
void ist_end_non_atomic(void)
{
        preempt_disable();
}

int is_valid_bugaddr(unsigned long addr)
{
        unsigned short ud;

        if (addr < TASK_SIZE_MAX)
                return 0;

        if (probe_kernel_address((unsigned short *)addr, ud))
                return 0;

        return ud == INSN_UD0 || ud == INSN_UD2;
}

int fixup_bug(struct pt_regs *regs, int trapnr)
{
        if (trapnr != X86_TRAP_UD)
                return 0;

        switch (report_bug(regs->ip, regs)) {
        case BUG_TRAP_TYPE_NONE:
        case BUG_TRAP_TYPE_BUG:
                break;

        case BUG_TRAP_TYPE_WARN:
                regs->ip += LEN_UD2;
                return 1;
        }

        return 0;
}

static nokprobe_inline int
do_trap_no_signal(struct task_struct *tsk, int trapnr, char *str,
                  struct pt_regs *regs, long error_code)
{
        if (v8086_mode(regs)) {
                /*
                 * Traps 0, 1, 3, 4, and 5 should be forwarded to vm86.
                 * On nmi (interrupt 2), do_trap should not be called.
                 */
                if (trapnr < X86_TRAP_UD) {
                        if (!handle_vm86_trap((struct kernel_vm86_regs *) regs,
                                                error_code, trapnr))
                                return 0;
                }
                return -1;
        }

        if (!user_mode(regs)) {
                if (fixup_exception(regs, trapnr))
                        return 0;

                tsk->thread.error_code = error_code;
                tsk->thread.trap_nr = trapnr;
                die(str, regs, error_code);
        }

        return -1;
}

static siginfo_t *fill_trap_info(struct pt_regs *regs, int signr, int trapnr,
                                siginfo_t *info)
{
        unsigned long siaddr;
        int sicode;

        switch (trapnr) {
        default:
                return SEND_SIG_PRIV;

        case X86_TRAP_DE:
                sicode = FPE_INTDIV;
                siaddr = uprobe_get_trap_addr(regs);
                break;
        case X86_TRAP_UD:
                sicode = ILL_ILLOPN;
                siaddr = uprobe_get_trap_addr(regs);
                break;
        case X86_TRAP_AC:
                sicode = BUS_ADRALN;
                siaddr = 0;
                break;
        }

        info->si_signo = signr;
        info->si_errno = 0;
        info->si_code = sicode;
        info->si_addr = (void __user *)siaddr;
        return info;
}

static void
do_trap(int trapnr, int signr, char *str, struct pt_regs *regs,
        long error_code, siginfo_t *info)
{
        struct task_struct *tsk = current;


        if (!do_trap_no_signal(tsk, trapnr, str, regs, error_code))
                return;
        /*
         * We want error_code and trap_nr set for userspace faults and
         * kernelspace faults which result in die(), but not
         * kernelspace faults which are fixed up.  die() gives the
         * process no chance to handle the signal and notice the
         * kernel fault information, so that won't result in polluting
         * the information about previously queued, but not yet
         * delivered, faults.  See also do_general_protection below.
         */
        tsk->thread.error_code = error_code;
        tsk->thread.trap_nr = trapnr;

        if (show_unhandled_signals && unhandled_signal(tsk, signr) &&
            printk_ratelimit()) {
                pr_info("%s[%d] trap %s ip:%lx sp:%lx error:%lx",
                        tsk->comm, tsk->pid, str,
                        regs->ip, regs->sp, error_code);
                print_vma_addr(KERN_CONT " in ", regs->ip);
                pr_cont("\n");
        }

        force_sig_info(signr, info ?: SEND_SIG_PRIV, tsk);
}
NOKPROBE_SYMBOL(do_trap);

static void do_error_trap(struct pt_regs *regs, long error_code, char *str,
                          unsigned long trapnr, int signr)
{
        siginfo_t info;

        RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");

        /*
         * WARN*()s end up here; fix them up before we call the
         * notifier chain.
         */
        if (!user_mode(regs) && fixup_bug(regs, trapnr))
                return;

        if (notify_die(DIE_TRAP, str, regs, error_code, trapnr, signr) !=
                        NOTIFY_STOP) {
                cond_local_irq_enable(regs);
                do_trap(trapnr, signr, str, regs, error_code,
                        fill_trap_info(regs, signr, trapnr, &info));
        }
}

#define DO_ERROR(trapnr, signr, str, name)                              \
dotraplinkage void do_##name(struct pt_regs *regs, long error_code)     \
{                                                                       \
        do_error_trap(regs, error_code, str, trapnr, signr);            \
}

DO_ERROR(X86_TRAP_DE,     SIGFPE,  "divide error",              divide_error)
DO_ERROR(X86_TRAP_OF,     SIGSEGV, "overflow",                  overflow)
DO_ERROR(X86_TRAP_UD,     SIGILL,  "invalid opcode",            invalid_op)
DO_ERROR(X86_TRAP_OLD_MF, SIGFPE,  "coprocessor segment overrun",coprocessor_segment_overrun)
DO_ERROR(X86_TRAP_TS,     SIGSEGV, "invalid TSS",               invalid_TSS)
DO_ERROR(X86_TRAP_NP,     SIGBUS,  "segment not present",       segment_not_present)
DO_ERROR(X86_TRAP_SS,     SIGBUS,  "stack segment",             stack_segment)
DO_ERROR(X86_TRAP_AC,     SIGBUS,  "alignment check",           alignment_check)

#ifdef CONFIG_VMAP_STACK
__visible void __noreturn handle_stack_overflow(const char *message,
                                                struct pt_regs *regs,
                                                unsigned long fault_address)
{
        printk(KERN_EMERG "BUG: stack guard page was hit at %p (stack is %p..%p)\n",
                 (void *)fault_address, current->stack,
                 (char *)current->stack + THREAD_SIZE - 1);
        die(message, regs, 0);

        /* Be absolutely certain we don't return. */
        panic(message);
}
#endif

#ifdef CONFIG_X86_64
/* Runs on IST stack */
dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
{
        static const char str[] = "double fault";
        struct task_struct *tsk = current;
#ifdef CONFIG_VMAP_STACK
        unsigned long cr2;
#endif

#ifdef CONFIG_X86_ESPFIX64
        extern unsigned char native_irq_return_iret[];

        /*
         * If IRET takes a non-IST fault on the espfix64 stack, then we
         * end up promoting it to a doublefault.  In that case, take
         * advantage of the fact that we're not using the normal (TSS.sp0)
         * stack right now.  We can write a fake #GP(0) frame at TSS.sp0
         * and then modify our own IRET frame so that, when we return,
         * we land directly at the #GP(0) vector with the stack already
         * set up according to its expectations.
         *
         * The net result is that our #GP handler will think that we
         * entered from usermode with the bad user context.
         *
         * No need for ist_enter here because we don't use RCU.
         */
        if (((long)regs->sp >> P4D_SHIFT) == ESPFIX_PGD_ENTRY &&
                regs->cs == __KERNEL_CS &&
                regs->ip == (unsigned long)native_irq_return_iret)
        {
                struct pt_regs *gpregs = (struct pt_regs *)this_cpu_read(cpu_tss_rw.x86_tss.sp0) - 1;

                /*
                 * regs->sp points to the failing IRET frame on the
                 * ESPFIX64 stack.  Copy it to the entry stack.  This fills
                 * in gpregs->ss through gpregs->ip.
                 *
                 */
                memmove(&gpregs->ip, (void *)regs->sp, 5*8);
                gpregs->orig_ax = 0;  /* Missing (lost) #GP error code */

                /*
                 * Adjust our frame so that we return straight to the #GP
                 * vector with the expected RSP value.  This is safe because
                 * we won't enable interupts or schedule before we invoke
                 * general_protection, so nothing will clobber the stack
                 * frame we just set up.
                 */
                regs->ip = (unsigned long)general_protection;
                regs->sp = (unsigned long)&gpregs->orig_ax;

                return;
        }
#endif

        ist_enter(regs);
        notify_die(DIE_TRAP, str, regs, error_code, X86_TRAP_DF, SIGSEGV);

        tsk->thread.error_code = error_code;
        tsk->thread.trap_nr = X86_TRAP_DF;

#ifdef CONFIG_VMAP_STACK
        /*
         * If we overflow the stack into a guard page, the CPU will fail
         * to deliver #PF and will send #DF instead.  Similarly, if we
         * take any non-IST exception while too close to the bottom of
         * the stack, the processor will get a page fault while
         * delivering the exception and will generate a double fault.
         *
         * According to the SDM (footnote in 6.15 under "Interrupt 14 -
         * Page-Fault Exception (#PF):
         *
         *   Processors update CR2 whenever a page fault is detected. If a
         *   second page fault occurs while an earlier page fault is being
         *   delivered, the faulting linear address of the second fault will
         *   overwrite the contents of CR2 (replacing the previous
         *   address). These updates to CR2 occur even if the page fault
         *   results in a double fault or occurs during the delivery of a
         *   double fault.
         *
         * The logic below has a small possibility of incorrectly diagnosing
         * some errors as stack overflows.  For example, if the IDT or GDT
         * gets corrupted such that #GP delivery fails due to a bad descriptor
         * causing #GP and we hit this condition while CR2 coincidentally
         * points to the stack guard page, we'll think we overflowed the
         * stack.  Given that we're going to panic one way or another
         * if this happens, this isn't necessarily worth fixing.
         *
         * If necessary, we could improve the test by only diagnosing
         * a stack overflow if the saved RSP points within 47 bytes of
         * the bottom of the stack: if RSP == tsk_stack + 48 and we
         * take an exception, the stack is already aligned and there
         * will be enough room SS, RSP, RFLAGS, CS, RIP, and a
         * possible error code, so a stack overflow would *not* double
         * fault.  With any less space left, exception delivery could
         * fail, and, as a practical matter, we've overflowed the
         * stack even if the actual trigger for the double fault was
         * something else.
         */
        cr2 = read_cr2();
        if ((unsigned long)task_stack_page(tsk) - 1 - cr2 < PAGE_SIZE)
                handle_stack_overflow("kernel stack overflow (double-fault)", regs, cr2);
#endif

#ifdef CONFIG_DOUBLEFAULT
        df_debug(regs, error_code);
#endif
        /*
         * This is always a kernel trap and never fixable (and thus must
         * never return).
         */
        for (;;)
                die(str, regs, error_code);
}
#endif

dotraplinkage void do_bounds(struct pt_regs *regs, long error_code)
{
        const struct mpx_bndcsr *bndcsr;
        siginfo_t *info;

        RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
        if (notify_die(DIE_TRAP, "bounds", regs, error_code,
                        X86_TRAP_BR, SIGSEGV) == NOTIFY_STOP)
                return;
        cond_local_irq_enable(regs);

        if (!user_mode(regs))
                die("bounds", regs, error_code);

        if (!cpu_feature_enabled(X86_FEATURE_MPX)) {
                /* The exception is not from Intel MPX */
                goto exit_trap;
        }

        /*
         * We need to look at BNDSTATUS to resolve this exception.
         * A NULL here might mean that it is in its 'init state',
         * which is all zeros which indicates MPX was not
         * responsible for the exception.
         */
        bndcsr = get_xsave_field_ptr(XFEATURE_MASK_BNDCSR);
        if (!bndcsr)
                goto exit_trap;

        trace_bounds_exception_mpx(bndcsr);
        /*
         * The error code field of the BNDSTATUS register communicates status
         * information of a bound range exception #BR or operation involving
         * bound directory.
         */
        switch (bndcsr->bndstatus & MPX_BNDSTA_ERROR_CODE) {
        case 2: /* Bound directory has invalid entry. */
                if (mpx_handle_bd_fault())
                        goto exit_trap;
                break; /* Success, it was handled */
        case 1: /* Bound violation. */
                info = mpx_generate_siginfo(regs);
                if (IS_ERR(info)) {
                        /*
                         * We failed to decode the MPX instruction.  Act as if
                         * the exception was not caused by MPX.
                         */
                        goto exit_trap;
                }
                /*
                 * Success, we decoded the instruction and retrieved
                 * an 'info' containing the address being accessed
                 * which caused the exception.  This information
                 * allows and application to possibly handle the
                 * #BR exception itself.
                 */
                do_trap(X86_TRAP_BR, SIGSEGV, "bounds", regs, error_code, info);
                kfree(info);
                break;
        case 0: /* No exception caused by Intel MPX operations. */
                goto exit_trap;
        default:
                die("bounds", regs, error_code);
        }

        return;

exit_trap:
        /*
         * This path out is for all the cases where we could not
         * handle the exception in some way (like allocating a
         * table or telling userspace about it.  We will also end
         * up here if the kernel has MPX turned off at compile
         * time..
         */
        do_trap(X86_TRAP_BR, SIGSEGV, "bounds", regs, error_code, NULL);
}

dotraplinkage void
do_general_protection(struct pt_regs *regs, long error_code)
{
        struct task_struct *tsk;

        RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
        cond_local_irq_enable(regs);

        if (v8086_mode(regs)) {
                local_irq_enable();
                handle_vm86_fault((struct kernel_vm86_regs *) regs, error_code);
                return;
        }

        tsk = current;
        if (!user_mode(regs)) {
                if (fixup_exception(regs, X86_TRAP_GP))
                        return;

                tsk->thread.error_code = error_code;
                tsk->thread.trap_nr = X86_TRAP_GP;
                if (notify_die(DIE_GPF, "general protection fault", regs, error_code,
                               X86_TRAP_GP, SIGSEGV) != NOTIFY_STOP)
                        die("general protection fault", regs, error_code);
                return;
        }

        tsk->thread.error_code = error_code;
        tsk->thread.trap_nr = X86_TRAP_GP;

        if (show_unhandled_signals && unhandled_signal(tsk, SIGSEGV) &&
                        printk_ratelimit()) {
                pr_info("%s[%d] general protection ip:%lx sp:%lx error:%lx",
                        tsk->comm, task_pid_nr(tsk),
                        regs->ip, regs->sp, error_code);
                print_vma_addr(KERN_CONT " in ", regs->ip);
                pr_cont("\n");
        }

        force_sig_info(SIGSEGV, SEND_SIG_PRIV, tsk);
}
NOKPROBE_SYMBOL(do_general_protection);

dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code)
{
#ifdef CONFIG_DYNAMIC_FTRACE
        /*
         * ftrace must be first, everything else may cause a recursive crash.
         * See note by declaration of modifying_ftrace_code in ftrace.c
         */
        if (unlikely(atomic_read(&modifying_ftrace_code)) &&
            ftrace_int3_handler(regs))
                return;
#endif
        if (poke_int3_handler(regs))
                return;

        /*
         * Use ist_enter despite the fact that we don't use an IST stack.
         * We can be called from a kprobe in non-CONTEXT_KERNEL kernel
         * mode or even during context tracking state changes.
         *
         * This means that we can't schedule.  That's okay.
         */
        ist_enter(regs);
        RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
#ifdef CONFIG_KGDB_LOW_LEVEL_TRAP
        if (kgdb_ll_trap(DIE_INT3, "int3", regs, error_code, X86_TRAP_BP,
                                SIGTRAP) == NOTIFY_STOP)
                goto exit;
#endif /* CONFIG_KGDB_LOW_LEVEL_TRAP */

#ifdef CONFIG_KPROBES
        if (kprobe_int3_handler(regs))
                goto exit;
#endif

        if (notify_die(DIE_INT3, "int3", regs, error_code, X86_TRAP_BP,
                        SIGTRAP) == NOTIFY_STOP)
                goto exit;

        cond_local_irq_enable(regs);
        do_trap(X86_TRAP_BP, SIGTRAP, "int3", regs, error_code, NULL);
        cond_local_irq_disable(regs);

exit:
        ist_exit(regs);
}
NOKPROBE_SYMBOL(do_int3);

#ifdef CONFIG_X86_64
/*
 * Help handler running on a per-cpu (IST or entry trampoline) stack
 * to switch to the normal thread stack if the interrupted code was in
 * user mode. The actual stack switch is done in entry_64.S
 */
asmlinkage __visible notrace struct pt_regs *sync_regs(struct pt_regs *eregs)
{
        struct pt_regs *regs = (struct pt_regs *)this_cpu_read(cpu_current_top_of_stack) - 1;
        if (regs != eregs)
                *regs = *eregs;
        return regs;
}
NOKPROBE_SYMBOL(sync_regs);

struct bad_iret_stack {
        void *error_entry_ret;
        struct pt_regs regs;
};

asmlinkage __visible notrace
struct bad_iret_stack *fixup_bad_iret(struct bad_iret_stack *s)
{
        /*
         * This is called from entry_64.S early in handling a fault
         * caused by a bad iret to user mode.  To handle the fault
         * correctly, we want to move our stack frame to where it would
         * be had we entered directly on the entry stack (rather than
         * just below the IRET frame) and we want to pretend that the
         * exception came from the IRET target.
         */
        struct bad_iret_stack *new_stack =
                (struct bad_iret_stack *)this_cpu_read(cpu_tss_rw.x86_tss.sp0) - 1;

        /* Copy the IRET target to the new stack. */
        memmove(&new_stack->regs.ip, (void *)s->regs.sp, 5*8);

        /* Copy the remainder of the stack from the current stack. */
        memmove(new_stack, s, offsetof(struct bad_iret_stack, regs.ip));

        BUG_ON(!user_mode(&new_stack->regs));
        return new_stack;
}
NOKPROBE_SYMBOL(fixup_bad_iret);
#endif

static bool is_sysenter_singlestep(struct pt_regs *regs)
{
        /*
         * We don't try for precision here.  If we're anywhere in the region of
         * code that can be single-stepped in the SYSENTER entry path, then
         * assume that this is a useless single-step trap due to SYSENTER
         * being invoked with TF set.  (We don't know in advance exactly
         * which instructions will be hit because BTF could plausibly
         * be set.)
         */
#ifdef CONFIG_X86_32
        return (regs->ip - (unsigned long)__begin_SYSENTER_singlestep_region) <
                (unsigned long)__end_SYSENTER_singlestep_region -
                (unsigned long)__begin_SYSENTER_singlestep_region;
#elif defined(CONFIG_IA32_EMULATION)
        return (regs->ip - (unsigned long)entry_SYSENTER_compat) <
                (unsigned long)__end_entry_SYSENTER_compat -
                (unsigned long)entry_SYSENTER_compat;
#else
        return false;
#endif
}

/*
 * Our handling of the processor debug registers is non-trivial.
 * We do not clear them on entry and exit from the kernel. Therefore
 * it is possible to get a watchpoint trap here from inside the kernel.
 * However, the code in ./ptrace.c has ensured that the user can
 * only set watchpoints on userspace addresses. Therefore the in-kernel
 * watchpoint trap can only occur in code which is reading/writing
 * from user space. Such code must not hold kernel locks (since it
 * can equally take a page fault), therefore it is safe to call
 * force_sig_info even though that claims and releases locks.
 *
 * Code in ./signal.c ensures that the debug control register
 * is restored before we deliver any signal, and therefore that
 * user code runs with the correct debug control register even though
 * we clear it here.
 *
 * Being careful here means that we don't have to be as careful in a
 * lot of more complicated places (task switching can be a bit lazy
 * about restoring all the debug state, and ptrace doesn't have to
 * find every occurrence of the TF bit that could be saved away even
 * by user code)
 *
 * May run on IST stack.
 */
dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
{
        struct task_struct *tsk = current;
        int user_icebp = 0;
        unsigned long dr6;
        int si_code;

        ist_enter(regs);

        get_debugreg(dr6, 6);
        /*
         * The Intel SDM says:
         *
         *   Certain debug exceptions may clear bits 0-3. The remaining
         *   contents of the DR6 register are never cleared by the
         *   processor. To avoid confusion in identifying debug
         *   exceptions, debug handlers should clear the register before
         *   returning to the interrupted task.
         *
         * Keep it simple: clear DR6 immediately.
         */
        set_debugreg(0, 6);

        /* Filter out all the reserved bits which are preset to 1 */
        dr6 &= ~DR6_RESERVED;

        /*
         * The SDM says "The processor clears the BTF flag when it
         * generates a debug exception."  Clear TIF_BLOCKSTEP to keep
         * TIF_BLOCKSTEP in sync with the hardware BTF flag.
         */
        clear_tsk_thread_flag(tsk, TIF_BLOCKSTEP);

        if (unlikely(!user_mode(regs) && (dr6 & DR_STEP) &&
                     is_sysenter_singlestep(regs))) {
                dr6 &= ~DR_STEP;
                if (!dr6)
                        goto exit;
                /*
                 * else we might have gotten a single-step trap and hit a
                 * watchpoint at the same time, in which case we should fall
                 * through and handle the watchpoint.
                 */
        }

        /*
         * If dr6 has no reason to give us about the origin of this trap,
         * then it's very likely the result of an icebp/int01 trap.
         * User wants a sigtrap for that.
         */
        if (!dr6 && user_mode(regs))
                user_icebp = 1;

        /* Store the virtualized DR6 value */
        tsk->thread.debugreg6 = dr6;

#ifdef CONFIG_KPROBES
        if (kprobe_debug_handler(regs))
                goto exit;
#endif

        if (notify_die(DIE_DEBUG, "debug", regs, (long)&dr6, error_code,
                                                        SIGTRAP) == NOTIFY_STOP)
                goto exit;

        /*
         * Let others (NMI) know that the debug stack is in use
         * as we may switch to the interrupt stack.
         */
        debug_stack_usage_inc();

        /* It's safe to allow irq's after DR6 has been saved */
        cond_local_irq_enable(regs);

        if (v8086_mode(regs)) {
                handle_vm86_trap((struct kernel_vm86_regs *) regs, error_code,
                                        X86_TRAP_DB);
                cond_local_irq_disable(regs);
                debug_stack_usage_dec();
                goto exit;
        }

        if (WARN_ON_ONCE((dr6 & DR_STEP) && !user_mode(regs))) {
                /*
                 * Historical junk that used to handle SYSENTER single-stepping.
                 * This should be unreachable now.  If we survive for a while
                 * without anyone hitting this warning, we'll turn this into
                 * an oops.
                 */
                tsk->thread.debugreg6 &= ~DR_STEP;
                set_tsk_thread_flag(tsk, TIF_SINGLESTEP);
                regs->flags &= ~X86_EFLAGS_TF;
        }
        si_code = get_si_code(tsk->thread.debugreg6);
        if (tsk->thread.debugreg6 & (DR_STEP | DR_TRAP_BITS) || user_icebp)
                send_sigtrap(tsk, regs, error_code, si_code);
        cond_local_irq_disable(regs);
        debug_stack_usage_dec();

exit:
        ist_exit(regs);
}
NOKPROBE_SYMBOL(do_debug);

/*
 * Note that we play around with the 'TS' bit in an attempt to get
 * the correct behaviour even in the presence of the asynchronous
 * IRQ13 behaviour
 */
static void math_error(struct pt_regs *regs, int error_code, int trapnr)
{
        struct task_struct *task = current;
        struct fpu *fpu = &task->thread.fpu;
        siginfo_t info;
        char *str = (trapnr == X86_TRAP_MF) ? "fpu exception" :
                                                "simd exception";

        cond_local_irq_enable(regs);

        if (!user_mode(regs)) {
                if (fixup_exception(regs, trapnr))
                        return;

                task->thread.error_code = error_code;
                task->thread.trap_nr = trapnr;

                if (notify_die(DIE_TRAP, str, regs, error_code,
                                        trapnr, SIGFPE) != NOTIFY_STOP)
                        die(str, regs, error_code);
                return;
        }

        /*
         * Save the info for the exception handler and clear the error.
         */
        fpu__save(fpu);

        task->thread.trap_nr    = trapnr;
        task->thread.error_code = error_code;
        info.si_signo           = SIGFPE;
        info.si_errno           = 0;
        info.si_addr            = (void __user *)uprobe_get_trap_addr(regs);

        info.si_code = fpu__exception_code(fpu, trapnr);

        /* Retry when we get spurious exceptions: */
        if (!info.si_code)
                return;

        force_sig_info(SIGFPE, &info, task);
}

dotraplinkage void do_coprocessor_error(struct pt_regs *regs, long error_code)
{
        RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
        math_error(regs, error_code, X86_TRAP_MF);
}

dotraplinkage void
do_simd_coprocessor_error(struct pt_regs *regs, long error_code)
{
        RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
        math_error(regs, error_code, X86_TRAP_XF);
}

dotraplinkage void
do_spurious_interrupt_bug(struct pt_regs *regs, long error_code)
{
        cond_local_irq_enable(regs);
}

dotraplinkage void
do_device_not_available(struct pt_regs *regs, long error_code)
{
        unsigned long cr0;

        RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");

#ifdef CONFIG_MATH_EMULATION
        if (!boot_cpu_has(X86_FEATURE_FPU) && (read_cr0() & X86_CR0_EM)) {
                struct math_emu_info info = { };

                cond_local_irq_enable(regs);

                info.regs = regs;
                math_emulate(&info);
                return;
        }
#endif

        /* This should not happen. */
        cr0 = read_cr0();
        if (WARN(cr0 & X86_CR0_TS, "CR0.TS was set")) {
                /* Try to fix it up and carry on. */
                write_cr0(cr0 & ~X86_CR0_TS);
        } else {
                /*
                 * Something terrible happened, and we're better off trying
                 * to kill the task than getting stuck in a never-ending
                 * loop of #NM faults.
                 */
                die("unexpected #NM exception", regs, error_code);
        }
}
NOKPROBE_SYMBOL(do_device_not_available);

#ifdef CONFIG_X86_32
dotraplinkage void do_iret_error(struct pt_regs *regs, long error_code)
{
        siginfo_t info;

        RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
        local_irq_enable();

        info.si_signo = SIGILL;
        info.si_errno = 0;
        info.si_code = ILL_BADSTK;
        info.si_addr = NULL;
        if (notify_die(DIE_TRAP, "iret exception", regs, error_code,
                        X86_TRAP_IRET, SIGILL) != NOTIFY_STOP) {
                do_trap(X86_TRAP_IRET, SIGILL, "iret exception", regs, error_code,
                        &info);
        }
}
#endif

void __init trap_init(void)
{
        /* Init cpu_entry_area before IST entries are set up */
        setup_cpu_entry_areas();

        idt_setup_traps();

        /*
         * Set the IDT descriptor to a fixed read-only location, so that the
         * "sidt" instruction will not leak the location of the kernel, and
         * to defend the IDT against arbitrary memory write vulnerabilities.
         * It will be reloaded in cpu_init() */
        cea_set_pte(CPU_ENTRY_AREA_RO_IDT_VADDR, __pa_symbol(idt_table),
                    PAGE_KERNEL_RO);
        idt_descr.address = CPU_ENTRY_AREA_RO_IDT;

        /*
         * Should be a barrier for any external CPU state:
         */
        cpu_init();

        idt_setup_ist_traps();

        x86_init.irqs.trap_init();

        idt_setup_debugidt_traps();
}