x86/entry: Fixup bad_iret vs noinstr

[linux-2.6-microblaze.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>
#include <linux/hardirq.h>
#include <linux/atomic.h>

#include <asm/stacktrace.h>
#include <asm/processor.h>
#include <asm/debugreg.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.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/vm86.h>
#include <asm/umip.h>
#include <asm/insn.h>
#include <asm/insn-eval.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(system_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();
}

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;
}

static nokprobe_inline int
do_trap_no_signal(struct task_struct *tsk, int trapnr, const 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;
                }
        } else if (!user_mode(regs)) {
                if (fixup_exception(regs, trapnr, error_code, 0))
                        return 0;

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

        /*
         * 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 exc_general_protection below.
         */
        tsk->thread.error_code = error_code;
        tsk->thread.trap_nr = trapnr;

        return -1;
}

static void show_signal(struct task_struct *tsk, int signr,
                        const char *type, const char *desc,
                        struct pt_regs *regs, long error_code)
{
        if (show_unhandled_signals && unhandled_signal(tsk, signr) &&
            printk_ratelimit()) {
                pr_info("%s[%d] %s%s ip:%lx sp:%lx error:%lx",
                        tsk->comm, task_pid_nr(tsk), type, desc,
                        regs->ip, regs->sp, error_code);
                print_vma_addr(KERN_CONT " in ", regs->ip);
                pr_cont("\n");
        }
}

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

        if (!do_trap_no_signal(tsk, trapnr, str, regs, error_code))
                return;

        show_signal(tsk, signr, "trap ", str, regs, error_code);

        if (!sicode)
                force_sig(signr);
        else
                force_sig_fault(signr, sicode, addr);
}
NOKPROBE_SYMBOL(do_trap);

static void do_error_trap(struct pt_regs *regs, long error_code, char *str,
        unsigned long trapnr, int signr, int sicode, void __user *addr)
{
        RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");

        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, sicode, addr);
                cond_local_irq_disable(regs);
        }
}

/*
 * Posix requires to provide the address of the faulting instruction for
 * SIGILL (#UD) and SIGFPE (#DE) in the si_addr member of siginfo_t.
 *
 * This address is usually regs->ip, but when an uprobe moved the code out
 * of line then regs->ip points to the XOL code which would confuse
 * anything which analyzes the fault address vs. the unmodified binary. If
 * a trap happened in XOL code then uprobe maps regs->ip back to the
 * original instruction address.
 */
static __always_inline void __user *error_get_trap_addr(struct pt_regs *regs)
{
        return (void __user *)uprobe_get_trap_addr(regs);
}

DEFINE_IDTENTRY(exc_divide_error)
{
        do_error_trap(regs, 0, "divide_error", X86_TRAP_DE, SIGFPE,
                      FPE_INTDIV, error_get_trap_addr(regs));
}

DEFINE_IDTENTRY(exc_overflow)
{
        do_error_trap(regs, 0, "overflow", X86_TRAP_OF, SIGSEGV, 0, NULL);
}

#ifdef CONFIG_X86_F00F_BUG
void handle_invalid_op(struct pt_regs *regs)
#else
static inline void handle_invalid_op(struct pt_regs *regs)
#endif
{
        do_error_trap(regs, 0, "invalid opcode", X86_TRAP_UD, SIGILL,
                      ILL_ILLOPN, error_get_trap_addr(regs));
}

DEFINE_IDTENTRY_RAW(exc_invalid_op)
{
        bool rcu_exit;

        /*
         * Handle BUG/WARN like NMIs instead of like normal idtentries:
         * if we bugged/warned in a bad RCU context, for example, the last
         * thing we want is to BUG/WARN again in the idtentry code, ad
         * infinitum.
         */
        if (!user_mode(regs) && is_valid_bugaddr(regs->ip)) {
                enum bug_trap_type type;

                nmi_enter();
                instrumentation_begin();
                trace_hardirqs_off_finish();
                type = report_bug(regs->ip, regs);
                if (regs->flags & X86_EFLAGS_IF)
                        trace_hardirqs_on_prepare();
                instrumentation_end();
                nmi_exit();

                if (type == BUG_TRAP_TYPE_WARN) {
                        /* Skip the ud2. */
                        regs->ip += LEN_UD2;
                        return;
                }

                /*
                 * Else, if this was a BUG and report_bug returns or if this
                 * was just a normal #UD, we want to continue onward and
                 * crash.
                 */
        }

        rcu_exit = idtentry_enter_cond_rcu(regs);
        instrumentation_begin();
        handle_invalid_op(regs);
        instrumentation_end();
        idtentry_exit_cond_rcu(regs, rcu_exit);
}

DEFINE_IDTENTRY(exc_coproc_segment_overrun)
{
        do_error_trap(regs, 0, "coprocessor segment overrun",
                      X86_TRAP_OLD_MF, SIGFPE, 0, NULL);
}

DEFINE_IDTENTRY_ERRORCODE(exc_invalid_tss)
{
        do_error_trap(regs, error_code, "invalid TSS", X86_TRAP_TS, SIGSEGV,
                      0, NULL);
}

DEFINE_IDTENTRY_ERRORCODE(exc_segment_not_present)
{
        do_error_trap(regs, error_code, "segment not present", X86_TRAP_NP,
                      SIGBUS, 0, NULL);
}

DEFINE_IDTENTRY_ERRORCODE(exc_stack_segment)
{
        do_error_trap(regs, error_code, "stack segment", X86_TRAP_SS, SIGBUS,
                      0, NULL);
}

DEFINE_IDTENTRY_ERRORCODE(exc_alignment_check)
{
        char *str = "alignment check";

        if (notify_die(DIE_TRAP, str, regs, error_code, X86_TRAP_AC, SIGBUS) == NOTIFY_STOP)
                return;

        if (!user_mode(regs))
                die("Split lock detected\n", regs, error_code);

        local_irq_enable();

        if (handle_user_split_lock(regs, error_code))
                return;

        do_trap(X86_TRAP_AC, SIGBUS, "alignment check", regs,
                error_code, BUS_ADRALN, NULL);
}

#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("%s", message);
}
#endif

/*
 * Runs on an IST stack for x86_64 and on a special task stack for x86_32.
 *
 * On x86_64, this is more or less a normal kernel entry.  Notwithstanding the
 * SDM's warnings about double faults being unrecoverable, returning works as
 * expected.  Presumably what the SDM actually means is that the CPU may get
 * the register state wrong on entry, so returning could be a bad idea.
 *
 * Various CPU engineers have promised that double faults due to an IRET fault
 * while the stack is read-only are, in fact, recoverable.
 *
 * On x86_32, this is entered through a task gate, and regs are synthesized
 * from the TSS.  Returning is, in principle, okay, but changes to regs will
 * be lost.  If, for some reason, we need to return to a context with modified
 * regs, the shim code could be adjusted to synchronize the registers.
 *
 * The 32bit #DF shim provides CR2 already as an argument. On 64bit it needs
 * to be read before doing anything else.
 */
DEFINE_IDTENTRY_DF(exc_double_fault)
{
        static const char str[] = "double fault";
        struct task_struct *tsk = current;

#ifdef CONFIG_VMAP_STACK
        unsigned long address = read_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 nmi_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;
                unsigned long *p = (unsigned long *)regs->sp;

                /*
                 * 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.
                 *
                 */
                gpregs->ip      = p[0];
                gpregs->cs      = p[1];
                gpregs->flags   = p[2];
                gpregs->sp      = p[3];
                gpregs->ss      = p[4];
                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.
                 *
                 * We will enter general_protection with kernel GSBASE,
                 * which is what the stub expects, given that the faulting
                 * RIP will be the IRET instruction.
                 */
                regs->ip = (unsigned long)asm_exc_general_protection;
                regs->sp = (unsigned long)&gpregs->orig_ax;

                return;
        }
#endif

        nmi_enter();
        instrumentation_begin();
        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.
         */
        if ((unsigned long)task_stack_page(tsk) - 1 - address < PAGE_SIZE) {
                handle_stack_overflow("kernel stack overflow (double-fault)",
                                      regs, address);
        }
#endif

        pr_emerg("PANIC: double fault, error_code: 0x%lx\n", error_code);
        die("double fault", regs, error_code);
        panic("Machine halted.");
        instrumentation_end();
}

DEFINE_IDTENTRY(exc_bounds)
{
        if (notify_die(DIE_TRAP, "bounds", regs, 0,
                        X86_TRAP_BR, SIGSEGV) == NOTIFY_STOP)
                return;
        cond_local_irq_enable(regs);

        if (!user_mode(regs))
                die("bounds", regs, 0);

        do_trap(X86_TRAP_BR, SIGSEGV, "bounds", regs, 0, 0, NULL);

        cond_local_irq_disable(regs);
}

enum kernel_gp_hint {
        GP_NO_HINT,
        GP_NON_CANONICAL,
        GP_CANONICAL
};

/*
 * When an uncaught #GP occurs, try to determine the memory address accessed by
 * the instruction and return that address to the caller. Also, try to figure
 * out whether any part of the access to that address was non-canonical.
 */
static enum kernel_gp_hint get_kernel_gp_address(struct pt_regs *regs,
                                                 unsigned long *addr)
{
        u8 insn_buf[MAX_INSN_SIZE];
        struct insn insn;

        if (probe_kernel_read(insn_buf, (void *)regs->ip, MAX_INSN_SIZE))
                return GP_NO_HINT;

        kernel_insn_init(&insn, insn_buf, MAX_INSN_SIZE);
        insn_get_modrm(&insn);
        insn_get_sib(&insn);

        *addr = (unsigned long)insn_get_addr_ref(&insn, regs);
        if (*addr == -1UL)
                return GP_NO_HINT;

#ifdef CONFIG_X86_64
        /*
         * Check that:
         *  - the operand is not in the kernel half
         *  - the last byte of the operand is not in the user canonical half
         */
        if (*addr < ~__VIRTUAL_MASK &&
            *addr + insn.opnd_bytes - 1 > __VIRTUAL_MASK)
                return GP_NON_CANONICAL;
#endif

        return GP_CANONICAL;
}

#define GPFSTR "general protection fault"

DEFINE_IDTENTRY_ERRORCODE(exc_general_protection)
{
        char desc[sizeof(GPFSTR) + 50 + 2*sizeof(unsigned long) + 1] = GPFSTR;
        enum kernel_gp_hint hint = GP_NO_HINT;
        struct task_struct *tsk;
        unsigned long gp_addr;
        int ret;

        cond_local_irq_enable(regs);

        if (static_cpu_has(X86_FEATURE_UMIP)) {
                if (user_mode(regs) && fixup_umip_exception(regs))
                        goto exit;
        }

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

        tsk = current;

        if (user_mode(regs)) {
                tsk->thread.error_code = error_code;
                tsk->thread.trap_nr = X86_TRAP_GP;

                show_signal(tsk, SIGSEGV, "", desc, regs, error_code);
                force_sig(SIGSEGV);
                goto exit;
        }

        if (fixup_exception(regs, X86_TRAP_GP, error_code, 0))
                goto exit;

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

        /*
         * To be potentially processing a kprobe fault and to trust the result
         * from kprobe_running(), we have to be non-preemptible.
         */
        if (!preemptible() &&
            kprobe_running() &&
            kprobe_fault_handler(regs, X86_TRAP_GP))
                goto exit;

        ret = notify_die(DIE_GPF, desc, regs, error_code, X86_TRAP_GP, SIGSEGV);
        if (ret == NOTIFY_STOP)
                goto exit;

        if (error_code)
                snprintf(desc, sizeof(desc), "segment-related " GPFSTR);
        else
                hint = get_kernel_gp_address(regs, &gp_addr);

        if (hint != GP_NO_HINT)
                snprintf(desc, sizeof(desc), GPFSTR ", %s 0x%lx",
                         (hint == GP_NON_CANONICAL) ? "probably for non-canonical address"
                                                    : "maybe for address",
                         gp_addr);

        /*
         * KASAN is interested only in the non-canonical case, clear it
         * otherwise.
         */
        if (hint != GP_NON_CANONICAL)
                gp_addr = 0;

        die_addr(desc, regs, error_code, gp_addr);

exit:
        cond_local_irq_disable(regs);
}

static bool do_int3(struct pt_regs *regs)
{
        int res;

#ifdef CONFIG_KGDB_LOW_LEVEL_TRAP
        if (kgdb_ll_trap(DIE_INT3, "int3", regs, 0, X86_TRAP_BP,
                         SIGTRAP) == NOTIFY_STOP)
                return true;
#endif /* CONFIG_KGDB_LOW_LEVEL_TRAP */

#ifdef CONFIG_KPROBES
        if (kprobe_int3_handler(regs))
                return true;
#endif
        res = notify_die(DIE_INT3, "int3", regs, 0, X86_TRAP_BP, SIGTRAP);

        return res == NOTIFY_STOP;
}

static void do_int3_user(struct pt_regs *regs)
{
        if (do_int3(regs))
                return;

        cond_local_irq_enable(regs);
        do_trap(X86_TRAP_BP, SIGTRAP, "int3", regs, 0, 0, NULL);
        cond_local_irq_disable(regs);
}

DEFINE_IDTENTRY_RAW(exc_int3)
{
        /*
         * poke_int3_handler() is completely self contained code; it does (and
         * must) *NOT* call out to anything, lest it hits upon yet another
         * INT3.
         */
        if (poke_int3_handler(regs))
                return;

        /*
         * idtentry_enter_user() uses static_branch_{,un}likely() and therefore
         * can trigger INT3, hence poke_int3_handler() must be done
         * before. If the entry came from kernel mode, then use nmi_enter()
         * because the INT3 could have been hit in any context including
         * NMI.
         */
        if (user_mode(regs)) {
                idtentry_enter_user(regs);
                instrumentation_begin();
                do_int3_user(regs);
                instrumentation_end();
                idtentry_exit_user(regs);
        } else {
                nmi_enter();
                instrumentation_begin();
                trace_hardirqs_off_finish();
                if (!do_int3(regs))
                        die("int3", regs, 0);
                if (regs->flags & X86_EFLAGS_IF)
                        trace_hardirqs_on_prepare();
                instrumentation_end();
                nmi_exit();
        }
}

#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 noinstr 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;
}

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

asmlinkage __visible noinstr
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 tmp, *new_stack =
                (struct bad_iret_stack *)__this_cpu_read(cpu_tss_rw.x86_tss.sp0) - 1;

        /* Copy the IRET target to the temporary storage. */
        __memcpy(&tmp.regs.ip, (void *)s->regs.sp, 5*8);

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

        /* Update the entry stack */
        __memcpy(new_stack, &tmp, sizeof(tmp));

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

static __always_inline void debug_enter(unsigned long *dr6, unsigned long *dr7)
{
        /*
         * Disable breakpoints during exception handling; recursive exceptions
         * are exceedingly 'fun'.
         *
         * Since this function is NOKPROBE, and that also applies to
         * HW_BREAKPOINT_X, we can't hit a breakpoint before this (XXX except a
         * HW_BREAKPOINT_W on our stack)
         *
         * Entry text is excluded for HW_BP_X and cpu_entry_area, which
         * includes the entry stack is excluded for everything.
         */
        *dr7 = local_db_save();

        /*
         * 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.
         */
        get_debugreg(*dr6, 6);
        set_debugreg(0, 6);
        /* Filter out all the reserved bits which are preset to 1 */
        *dr6 &= ~DR6_RESERVED;
}

static __always_inline void debug_exit(unsigned long dr7)
{
        local_db_restore(dr7);
}

/*
 * 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.
 */
static void handle_debug(struct pt_regs *regs, unsigned long dr6, bool user)
{
        struct task_struct *tsk = current;
        bool user_icebp;
        int si_code;

        /*
         * 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_thread_flag(TIF_BLOCKSTEP);

        /*
         * If DR6 is zero, no point in trying to handle it. The kernel is
         * not using INT1.
         */
        if (!user && !dr6)
                return;

        /*
         * 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.
         */
        user_icebp = user && !dr6;

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

#ifdef CONFIG_KPROBES
        if (kprobe_debug_handler(regs)) {
                return;
        }
#endif

        if (notify_die(DIE_DEBUG, "debug", regs, (long)&dr6, 0,
                       SIGTRAP) == NOTIFY_STOP) {
                return;
        }

        /* 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, 0,
                                 X86_TRAP_DB);
                goto out;
        }

        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(regs, 0, si_code);

out:
        cond_local_irq_disable(regs);
}

static __always_inline void exc_debug_kernel(struct pt_regs *regs,
                                             unsigned long dr6)
{
        nmi_enter();
        instrumentation_begin();
        trace_hardirqs_off_finish();

        /*
         * Catch SYSENTER with TF set and clear DR_STEP. If this hit a
         * watchpoint at the same time then that will still be handled.
         */
        if ((dr6 & DR_STEP) && is_sysenter_singlestep(regs))
                dr6 &= ~DR_STEP;

        handle_debug(regs, dr6, false);

        if (regs->flags & X86_EFLAGS_IF)
                trace_hardirqs_on_prepare();
        instrumentation_end();
        nmi_exit();
}

static __always_inline void exc_debug_user(struct pt_regs *regs,
                                           unsigned long dr6)
{
        idtentry_enter_user(regs);
        instrumentation_begin();

        handle_debug(regs, dr6, true);
        instrumentation_end();
        idtentry_exit_user(regs);
}

#ifdef CONFIG_X86_64
/* IST stack entry */
DEFINE_IDTENTRY_DEBUG(exc_debug)
{
        unsigned long dr6, dr7;

        debug_enter(&dr6, &dr7);
        exc_debug_kernel(regs, dr6);
        debug_exit(dr7);
}

/* User entry, runs on regular task stack */
DEFINE_IDTENTRY_DEBUG_USER(exc_debug)
{
        unsigned long dr6, dr7;

        debug_enter(&dr6, &dr7);
        exc_debug_user(regs, dr6);
        debug_exit(dr7);
}
#else
/* 32 bit does not have separate entry points. */
DEFINE_IDTENTRY_DEBUG(exc_debug)
{
        unsigned long dr6, dr7;

        debug_enter(&dr6, &dr7);

        if (user_mode(regs))
                exc_debug_user(regs, dr6);
        else
                exc_debug_kernel(regs, dr6);

        debug_exit(dr7);
}
#endif

/*
 * 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 trapnr)
{
        struct task_struct *task = current;
        struct fpu *fpu = &task->thread.fpu;
        int si_code;
        char *str = (trapnr == X86_TRAP_MF) ? "fpu exception" :
                                                "simd exception";

        cond_local_irq_enable(regs);

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

                task->thread.error_code = 0;
                task->thread.trap_nr = trapnr;

                if (notify_die(DIE_TRAP, str, regs, 0, trapnr,
                               SIGFPE) != NOTIFY_STOP)
                        die(str, regs, 0);
                goto exit;
        }

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

        task->thread.trap_nr    = trapnr;
        task->thread.error_code = 0;

        si_code = fpu__exception_code(fpu, trapnr);
        /* Retry when we get spurious exceptions: */
        if (!si_code)
                goto exit;

        force_sig_fault(SIGFPE, si_code,
                        (void __user *)uprobe_get_trap_addr(regs));
exit:
        cond_local_irq_disable(regs);
}

DEFINE_IDTENTRY(exc_coprocessor_error)
{
        math_error(regs, X86_TRAP_MF);
}

DEFINE_IDTENTRY(exc_simd_coprocessor_error)
{
        if (IS_ENABLED(CONFIG_X86_INVD_BUG)) {
                /* AMD 486 bug: INVD in CPL 0 raises #XF instead of #GP */
                if (!static_cpu_has(X86_FEATURE_XMM)) {
                        __exc_general_protection(regs, 0);
                        return;
                }
        }
        math_error(regs, X86_TRAP_XF);
}

DEFINE_IDTENTRY(exc_spurious_interrupt_bug)
{
        /*
         * This addresses a Pentium Pro Erratum:
         *
         * PROBLEM: If the APIC subsystem is configured in mixed mode with
         * Virtual Wire mode implemented through the local APIC, an
         * interrupt vector of 0Fh (Intel reserved encoding) may be
         * generated by the local APIC (Int 15).  This vector may be
         * generated upon receipt of a spurious interrupt (an interrupt
         * which is removed before the system receives the INTA sequence)
         * instead of the programmed 8259 spurious interrupt vector.
         *
         * IMPLICATION: The spurious interrupt vector programmed in the
         * 8259 is normally handled by an operating system's spurious
         * interrupt handler. However, a vector of 0Fh is unknown to some
         * operating systems, which would crash if this erratum occurred.
         *
         * In theory this could be limited to 32bit, but the handler is not
         * hurting and who knows which other CPUs suffer from this.
         */
}

DEFINE_IDTENTRY(exc_device_not_available)
{
        unsigned long cr0 = read_cr0();

#ifdef CONFIG_MATH_EMULATION
        if (!boot_cpu_has(X86_FEATURE_FPU) && (cr0 & X86_CR0_EM)) {
                struct math_emu_info info = { };

                cond_local_irq_enable(regs);

                info.regs = regs;
                math_emulate(&info);

                cond_local_irq_disable(regs);
                return;
        }
#endif

        /* This should not happen. */
        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, 0);
        }
}

#ifdef CONFIG_X86_32
DEFINE_IDTENTRY_SW(iret_error)
{
        local_irq_enable();
        if (notify_die(DIE_TRAP, "iret exception", regs, 0,
                        X86_TRAP_IRET, SIGILL) != NOTIFY_STOP) {
                do_trap(X86_TRAP_IRET, SIGILL, "iret exception", regs, 0,
                        ILL_BADSTK, (void __user *)NULL);
        }
        local_irq_disable();
}
#endif

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

        idt_setup_traps();

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

        idt_setup_ist_traps();
}