x86/speculation/mds: Clear CPU buffers on exit to user

[mirror_ubuntu-bionic-kernel.git] / arch / x86 / kernel / nmi.c

/*
 *  Copyright (C) 1991, 1992  Linus Torvalds
 *  Copyright (C) 2000, 2001, 2002 Andi Kleen, SuSE Labs
 *  Copyright (C) 2011  Don Zickus Red Hat, Inc.
 *
 *  Pentium III FXSR, SSE support
 *      Gareth Hughes <gareth@valinux.com>, May 2000
 */

/*
 * Handle hardware traps and faults.
 */
#include <linux/spinlock.h>
#include <linux/kprobes.h>
#include <linux/kdebug.h>
#include <linux/sched/debug.h>
#include <linux/nmi.h>
#include <linux/debugfs.h>
#include <linux/delay.h>
#include <linux/hardirq.h>
#include <linux/ratelimit.h>
#include <linux/slab.h>
#include <linux/export.h>
#include <linux/sched/clock.h>

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

#include <linux/atomic.h>
#include <asm/traps.h>
#include <asm/mach_traps.h>
#include <asm/nmi.h>
#include <asm/x86_init.h>
#include <asm/reboot.h>
#include <asm/cache.h>
#include <asm/nospec-branch.h>

#define CREATE_TRACE_POINTS
#include <trace/events/nmi.h>

struct nmi_desc {
        raw_spinlock_t lock;
        struct list_head head;
};

static struct nmi_desc nmi_desc[NMI_MAX] = 
{
        {
                .lock = __RAW_SPIN_LOCK_UNLOCKED(&nmi_desc[0].lock),
                .head = LIST_HEAD_INIT(nmi_desc[0].head),
        },
        {
                .lock = __RAW_SPIN_LOCK_UNLOCKED(&nmi_desc[1].lock),
                .head = LIST_HEAD_INIT(nmi_desc[1].head),
        },
        {
                .lock = __RAW_SPIN_LOCK_UNLOCKED(&nmi_desc[2].lock),
                .head = LIST_HEAD_INIT(nmi_desc[2].head),
        },
        {
                .lock = __RAW_SPIN_LOCK_UNLOCKED(&nmi_desc[3].lock),
                .head = LIST_HEAD_INIT(nmi_desc[3].head),
        },

};

struct nmi_stats {
        unsigned int normal;
        unsigned int unknown;
        unsigned int external;
        unsigned int swallow;
};

static DEFINE_PER_CPU(struct nmi_stats, nmi_stats);

static int ignore_nmis __read_mostly;

int unknown_nmi_panic;
/*
 * Prevent NMI reason port (0x61) being accessed simultaneously, can
 * only be used in NMI handler.
 */
static DEFINE_RAW_SPINLOCK(nmi_reason_lock);

static int __init setup_unknown_nmi_panic(char *str)
{
        unknown_nmi_panic = 1;
        return 1;
}
__setup("unknown_nmi_panic", setup_unknown_nmi_panic);

#define nmi_to_desc(type) (&nmi_desc[type])

static u64 nmi_longest_ns = 1 * NSEC_PER_MSEC;

static int __init nmi_warning_debugfs(void)
{
        debugfs_create_u64("nmi_longest_ns", 0644,
                        arch_debugfs_dir, &nmi_longest_ns);
        return 0;
}
fs_initcall(nmi_warning_debugfs);

static void nmi_max_handler(struct irq_work *w)
{
        struct nmiaction *a = container_of(w, struct nmiaction, irq_work);
        int remainder_ns, decimal_msecs;
        u64 whole_msecs = READ_ONCE(a->max_duration);

        remainder_ns = do_div(whole_msecs, (1000 * 1000));
        decimal_msecs = remainder_ns / 1000;

        printk_ratelimited(KERN_INFO
                "INFO: NMI handler (%ps) took too long to run: %lld.%03d msecs\n",
                a->handler, whole_msecs, decimal_msecs);
}

static int nmi_handle(unsigned int type, struct pt_regs *regs)
{
        struct nmi_desc *desc = nmi_to_desc(type);
        struct nmiaction *a;
        int handled=0;

        rcu_read_lock();

        /*
         * NMIs are edge-triggered, which means if you have enough
         * of them concurrently, you can lose some because only one
         * can be latched at any given time.  Walk the whole list
         * to handle those situations.
         */
        list_for_each_entry_rcu(a, &desc->head, list) {
                int thishandled;
                u64 delta;

                delta = sched_clock();
                thishandled = a->handler(type, regs);
                handled += thishandled;
                delta = sched_clock() - delta;
                trace_nmi_handler(a->handler, (int)delta, thishandled);

                if (delta < nmi_longest_ns || delta < a->max_duration)
                        continue;

                a->max_duration = delta;
                irq_work_queue(&a->irq_work);
        }

        rcu_read_unlock();

        /* return total number of NMI events handled */
        return handled;
}
NOKPROBE_SYMBOL(nmi_handle);

int __register_nmi_handler(unsigned int type, struct nmiaction *action)
{
        struct nmi_desc *desc = nmi_to_desc(type);
        unsigned long flags;

        if (!action->handler)
                return -EINVAL;

        init_irq_work(&action->irq_work, nmi_max_handler);

        raw_spin_lock_irqsave(&desc->lock, flags);

        /*
         * Indicate if there are multiple registrations on the
         * internal NMI handler call chains (SERR and IO_CHECK).
         */
        WARN_ON_ONCE(type == NMI_SERR && !list_empty(&desc->head));
        WARN_ON_ONCE(type == NMI_IO_CHECK && !list_empty(&desc->head));

        /*
         * some handlers need to be executed first otherwise a fake
         * event confuses some handlers (kdump uses this flag)
         */
        if (action->flags & NMI_FLAG_FIRST)
                list_add_rcu(&action->list, &desc->head);
        else
                list_add_tail_rcu(&action->list, &desc->head);
        
        raw_spin_unlock_irqrestore(&desc->lock, flags);
        return 0;
}
EXPORT_SYMBOL(__register_nmi_handler);

void unregister_nmi_handler(unsigned int type, const char *name)
{
        struct nmi_desc *desc = nmi_to_desc(type);
        struct nmiaction *n;
        unsigned long flags;

        raw_spin_lock_irqsave(&desc->lock, flags);

        list_for_each_entry_rcu(n, &desc->head, list) {
                /*
                 * the name passed in to describe the nmi handler
                 * is used as the lookup key
                 */
                if (!strcmp(n->name, name)) {
                        WARN(in_nmi(),
                                "Trying to free NMI (%s) from NMI context!\n", n->name);
                        list_del_rcu(&n->list);
                        break;
                }
        }

        raw_spin_unlock_irqrestore(&desc->lock, flags);
        synchronize_rcu();
}
EXPORT_SYMBOL_GPL(unregister_nmi_handler);

static void
pci_serr_error(unsigned char reason, struct pt_regs *regs)
{
        /* check to see if anyone registered against these types of errors */
        if (nmi_handle(NMI_SERR, regs))
                return;

        pr_emerg("NMI: PCI system error (SERR) for reason %02x on CPU %d.\n",
                 reason, smp_processor_id());

        if (panic_on_unrecovered_nmi)
                nmi_panic(regs, "NMI: Not continuing");

        pr_emerg("Dazed and confused, but trying to continue\n");

        /* Clear and disable the PCI SERR error line. */
        reason = (reason & NMI_REASON_CLEAR_MASK) | NMI_REASON_CLEAR_SERR;
        outb(reason, NMI_REASON_PORT);
}
NOKPROBE_SYMBOL(pci_serr_error);

static void
io_check_error(unsigned char reason, struct pt_regs *regs)
{
        unsigned long i;

        /* check to see if anyone registered against these types of errors */
        if (nmi_handle(NMI_IO_CHECK, regs))
                return;

        pr_emerg(
        "NMI: IOCK error (debug interrupt?) for reason %02x on CPU %d.\n",
                 reason, smp_processor_id());
        show_regs(regs);

        if (panic_on_io_nmi) {
                nmi_panic(regs, "NMI IOCK error: Not continuing");

                /*
                 * If we end up here, it means we have received an NMI while
                 * processing panic(). Simply return without delaying and
                 * re-enabling NMIs.
                 */
                return;
        }

        /* Re-enable the IOCK line, wait for a few seconds */
        reason = (reason & NMI_REASON_CLEAR_MASK) | NMI_REASON_CLEAR_IOCHK;
        outb(reason, NMI_REASON_PORT);

        i = 20000;
        while (--i) {
                touch_nmi_watchdog();
                udelay(100);
        }

        reason &= ~NMI_REASON_CLEAR_IOCHK;
        outb(reason, NMI_REASON_PORT);
}
NOKPROBE_SYMBOL(io_check_error);

static void
unknown_nmi_error(unsigned char reason, struct pt_regs *regs)
{
        int handled;

        /*
         * Use 'false' as back-to-back NMIs are dealt with one level up.
         * Of course this makes having multiple 'unknown' handlers useless
         * as only the first one is ever run (unless it can actually determine
         * if it caused the NMI)
         */
        handled = nmi_handle(NMI_UNKNOWN, regs);
        if (handled) {
                __this_cpu_add(nmi_stats.unknown, handled);
                return;
        }

        __this_cpu_add(nmi_stats.unknown, 1);

        pr_emerg("Uhhuh. NMI received for unknown reason %02x on CPU %d.\n",
                 reason, smp_processor_id());

        pr_emerg("Do you have a strange power saving mode enabled?\n");
        if (unknown_nmi_panic || panic_on_unrecovered_nmi)
                nmi_panic(regs, "NMI: Not continuing");

        pr_emerg("Dazed and confused, but trying to continue\n");
}
NOKPROBE_SYMBOL(unknown_nmi_error);

static DEFINE_PER_CPU(bool, swallow_nmi);
static DEFINE_PER_CPU(unsigned long, last_nmi_rip);

static void default_do_nmi(struct pt_regs *regs)
{
        unsigned char reason = 0;
        int handled;
        bool b2b = false;

        /*
         * CPU-specific NMI must be processed before non-CPU-specific
         * NMI, otherwise we may lose it, because the CPU-specific
         * NMI can not be detected/processed on other CPUs.
         */

        /*
         * Back-to-back NMIs are interesting because they can either
         * be two NMI or more than two NMIs (any thing over two is dropped
         * due to NMI being edge-triggered).  If this is the second half
         * of the back-to-back NMI, assume we dropped things and process
         * more handlers.  Otherwise reset the 'swallow' NMI behaviour
         */
        if (regs->ip == __this_cpu_read(last_nmi_rip))
                b2b = true;
        else
                __this_cpu_write(swallow_nmi, false);

        __this_cpu_write(last_nmi_rip, regs->ip);

        handled = nmi_handle(NMI_LOCAL, regs);
        __this_cpu_add(nmi_stats.normal, handled);
        if (handled) {
                /*
                 * There are cases when a NMI handler handles multiple
                 * events in the current NMI.  One of these events may
                 * be queued for in the next NMI.  Because the event is
                 * already handled, the next NMI will result in an unknown
                 * NMI.  Instead lets flag this for a potential NMI to
                 * swallow.
                 */
                if (handled > 1)
                        __this_cpu_write(swallow_nmi, true);
                return;
        }

        /*
         * Non-CPU-specific NMI: NMI sources can be processed on any CPU.
         *
         * Another CPU may be processing panic routines while holding
         * nmi_reason_lock. Check if the CPU issued the IPI for crash dumping,
         * and if so, call its callback directly.  If there is no CPU preparing
         * crash dump, we simply loop here.
         */
        while (!raw_spin_trylock(&nmi_reason_lock)) {
                run_crash_ipi_callback(regs);
                cpu_relax();
        }

        reason = x86_platform.get_nmi_reason();

        if (reason & NMI_REASON_MASK) {
                if (reason & NMI_REASON_SERR)
                        pci_serr_error(reason, regs);
                else if (reason & NMI_REASON_IOCHK)
                        io_check_error(reason, regs);
#ifdef CONFIG_X86_32
                /*
                 * Reassert NMI in case it became active
                 * meanwhile as it's edge-triggered:
                 */
                reassert_nmi();
#endif
                __this_cpu_add(nmi_stats.external, 1);
                raw_spin_unlock(&nmi_reason_lock);
                return;
        }
        raw_spin_unlock(&nmi_reason_lock);

        /*
         * Only one NMI can be latched at a time.  To handle
         * this we may process multiple nmi handlers at once to
         * cover the case where an NMI is dropped.  The downside
         * to this approach is we may process an NMI prematurely,
         * while its real NMI is sitting latched.  This will cause
         * an unknown NMI on the next run of the NMI processing.
         *
         * We tried to flag that condition above, by setting the
         * swallow_nmi flag when we process more than one event.
         * This condition is also only present on the second half
         * of a back-to-back NMI, so we flag that condition too.
         *
         * If both are true, we assume we already processed this
         * NMI previously and we swallow it.  Otherwise we reset
         * the logic.
         *
         * There are scenarios where we may accidentally swallow
         * a 'real' unknown NMI.  For example, while processing
         * a perf NMI another perf NMI comes in along with a
         * 'real' unknown NMI.  These two NMIs get combined into
         * one (as descibed above).  When the next NMI gets
         * processed, it will be flagged by perf as handled, but
         * noone will know that there was a 'real' unknown NMI sent
         * also.  As a result it gets swallowed.  Or if the first
         * perf NMI returns two events handled then the second
         * NMI will get eaten by the logic below, again losing a
         * 'real' unknown NMI.  But this is the best we can do
         * for now.
         */
        if (b2b && __this_cpu_read(swallow_nmi))
                __this_cpu_add(nmi_stats.swallow, 1);
        else
                unknown_nmi_error(reason, regs);
}
NOKPROBE_SYMBOL(default_do_nmi);

/*
 * NMIs can page fault or hit breakpoints which will cause it to lose
 * its NMI context with the CPU when the breakpoint or page fault does an IRET.
 *
 * As a result, NMIs can nest if NMIs get unmasked due an IRET during
 * NMI processing.  On x86_64, the asm glue protects us from nested NMIs
 * if the outer NMI came from kernel mode, but we can still nest if the
 * outer NMI came from user mode.
 *
 * To handle these nested NMIs, we have three states:
 *
 *  1) not running
 *  2) executing
 *  3) latched
 *
 * When no NMI is in progress, it is in the "not running" state.
 * When an NMI comes in, it goes into the "executing" state.
 * Normally, if another NMI is triggered, it does not interrupt
 * the running NMI and the HW will simply latch it so that when
 * the first NMI finishes, it will restart the second NMI.
 * (Note, the latch is binary, thus multiple NMIs triggering,
 *  when one is running, are ignored. Only one NMI is restarted.)
 *
 * If an NMI executes an iret, another NMI can preempt it. We do not
 * want to allow this new NMI to run, but we want to execute it when the
 * first one finishes.  We set the state to "latched", and the exit of
 * the first NMI will perform a dec_return, if the result is zero
 * (NOT_RUNNING), then it will simply exit the NMI handler. If not, the
 * dec_return would have set the state to NMI_EXECUTING (what we want it
 * to be when we are running). In this case, we simply jump back to
 * rerun the NMI handler again, and restart the 'latched' NMI.
 *
 * No trap (breakpoint or page fault) should be hit before nmi_restart,
 * thus there is no race between the first check of state for NOT_RUNNING
 * and setting it to NMI_EXECUTING. The HW will prevent nested NMIs
 * at this point.
 *
 * In case the NMI takes a page fault, we need to save off the CR2
 * because the NMI could have preempted another page fault and corrupt
 * the CR2 that is about to be read. As nested NMIs must be restarted
 * and they can not take breakpoints or page faults, the update of the
 * CR2 must be done before converting the nmi state back to NOT_RUNNING.
 * Otherwise, there would be a race of another nested NMI coming in
 * after setting state to NOT_RUNNING but before updating the nmi_cr2.
 */
enum nmi_states {
        NMI_NOT_RUNNING = 0,
        NMI_EXECUTING,
        NMI_LATCHED,
};
static DEFINE_PER_CPU(enum nmi_states, nmi_state);
static DEFINE_PER_CPU(unsigned long, nmi_cr2);

#ifdef CONFIG_X86_64
/*
 * In x86_64, we need to handle breakpoint -> NMI -> breakpoint.  Without
 * some care, the inner breakpoint will clobber the outer breakpoint's
 * stack.
 *
 * If a breakpoint is being processed, and the debug stack is being
 * used, if an NMI comes in and also hits a breakpoint, the stack
 * pointer will be set to the same fixed address as the breakpoint that
 * was interrupted, causing that stack to be corrupted. To handle this
 * case, check if the stack that was interrupted is the debug stack, and
 * if so, change the IDT so that new breakpoints will use the current
 * stack and not switch to the fixed address. On return of the NMI,
 * switch back to the original IDT.
 */
static DEFINE_PER_CPU(int, update_debug_stack);
#endif

dotraplinkage notrace void
do_nmi(struct pt_regs *regs, long error_code)
{
        if (this_cpu_read(nmi_state) != NMI_NOT_RUNNING) {
                this_cpu_write(nmi_state, NMI_LATCHED);
                return;
        }
        this_cpu_write(nmi_state, NMI_EXECUTING);
        this_cpu_write(nmi_cr2, read_cr2());
nmi_restart:

#ifdef CONFIG_X86_64
        /*
         * If we interrupted a breakpoint, it is possible that
         * the nmi handler will have breakpoints too. We need to
         * change the IDT such that breakpoints that happen here
         * continue to use the NMI stack.
         */
        if (unlikely(is_debug_stack(regs->sp))) {
                debug_stack_set_zero();
                this_cpu_write(update_debug_stack, 1);
        }
#endif

        nmi_enter();

        inc_irq_stat(__nmi_count);

        if (!ignore_nmis)
                default_do_nmi(regs);

        nmi_exit();

#ifdef CONFIG_X86_64
        if (unlikely(this_cpu_read(update_debug_stack))) {
                debug_stack_reset();
                this_cpu_write(update_debug_stack, 0);
        }
#endif

        if (unlikely(this_cpu_read(nmi_cr2) != read_cr2()))
                write_cr2(this_cpu_read(nmi_cr2));
        if (this_cpu_dec_return(nmi_state))
                goto nmi_restart;

        if (user_mode(regs))
                mds_user_clear_cpu_buffers();
}
NOKPROBE_SYMBOL(do_nmi);

void stop_nmi(void)
{
        ignore_nmis++;
}

void restart_nmi(void)
{
        ignore_nmis--;
}

/* reset the back-to-back NMI logic */
void local_touch_nmi(void)
{
        __this_cpu_write(last_nmi_rip, 0);
}
EXPORT_SYMBOL_GPL(local_touch_nmi);