x86/hyperv/hv_crash.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * X86 specific Hyper-V root partition kdump/crash support module
 *
 * Copyright (C) 2025, Microsoft, Inc.
 *
 * This module implements hypervisor RAM collection into vmcore for both
 * cases of the hypervisor crash and Linux root crash. Hyper-V implements
 * a disable hypercall with a 32bit protected mode ABI callback. This
 * mechanism must be used to unlock hypervisor RAM. Since the hypervisor RAM
 * is already mapped in Linux, it is automatically collected into Linux vmcore,
 * and can be examined by the crash command (raw RAM dump) or windbg.
 *
 * At a high level:
 *
 *  Hypervisor Crash:
 *    Upon crash, hypervisor goes into an emergency minimal dispatch loop, a
 *    restrictive mode with very limited hypercall and MSR support. Each cpu
 *    then injects NMIs into root vcpus. A shared page is used to check
 *    by Linux in the NMI handler if the hypervisor has crashed. This shared
 *    page is setup in hv_root_crash_init during boot.
 *
 *  Linux Crash:
 *    In case of Linux crash, the callback hv_crash_stop_other_cpus will send
 *    NMIs to all cpus, then proceed to the crash_nmi_callback where it waits
 *    for all cpus to be in NMI.
 *
 *  NMI Handler (upon quorum):
 *    Eventually, in both cases, all cpus will end up in the NMI handler.
 *    Hyper-V requires the disable hypervisor must be done from the BSP. So
 *    the BSP NMI handler saves current context, does some fixups and makes
 *    the hypercall to disable the hypervisor, ie, devirtualize. Hypervisor
 *    at that point will suspend all vcpus (except the BSP), unlock all its
 *    RAM, and return to Linux at the 32bit mode entry RIP.
 *
 *  Linux 32bit entry trampoline will then restore long mode and call C
 *  function here to restore context and continue execution to crash kexec.
 */

#include <linux/delay.h>
#include <linux/kexec.h>
#include <linux/crash_dump.h>
#include <linux/panic.h>
#include <asm/apic.h>
#include <asm/desc.h>
#include <asm/page.h>
#include <asm/pgalloc.h>
#include <asm/mshyperv.h>
#include <asm/nmi.h>
#include <asm/idtentry.h>
#include <asm/reboot.h>
#include <asm/intel_pt.h>

bool hv_crash_enabled;
EXPORT_SYMBOL_GPL(hv_crash_enabled);

struct hv_crash_ctxt {
	ulong rsp;
	ulong cr0;
	ulong cr2;
	ulong cr4;
	ulong cr8;

	u16 cs;
	u16 ss;
	u16 ds;
	u16 es;
	u16 fs;
	u16 gs;

	u16 gdt_fill;
	struct desc_ptr gdtr;
	char idt_fill[6];
	struct desc_ptr idtr;

	u64 gsbase;
	u64 efer;
	u64 pat;
};
static struct hv_crash_ctxt hv_crash_ctxt;

/* Shared hypervisor page that contains crash dump area we peek into.
 * NB: windbg looks for "hv_cda" symbol so don't change it.
 */
static struct hv_crashdump_area *hv_cda;

static u32 trampoline_pa, devirt_arg;
static atomic_t crash_cpus_wait;
static void *hv_crash_ptpgs[4];
static bool hv_has_crashed, lx_has_crashed;

static void __noreturn hv_panic_timeout_reboot(void)
{
	#define PANIC_TIMER_STEP 100

	if (panic_timeout > 0) {
		int i;

		for (i = 0; i < panic_timeout * 1000; i += PANIC_TIMER_STEP)
			mdelay(PANIC_TIMER_STEP);
	}

	if (panic_timeout)
		native_wrmsrq(HV_X64_MSR_RESET, 1);    /* get hyp to reboot */

	for (;;)
		cpu_relax();
}

/* This cannot be inlined as it needs stack */
static noinline __noclone void hv_crash_restore_tss(void)
{
	load_TR_desc();
}

/* This cannot be inlined as it needs stack */
static noinline void hv_crash_clear_kernpt(void)
{
	pgd_t *pgd;
	p4d_t *p4d;

	/* Clear entry so it's not confusing to someone looking at the core */
	pgd = pgd_offset_k(trampoline_pa);
	p4d = p4d_offset(pgd, trampoline_pa);
	native_p4d_clear(p4d);
}

/*
 * This is the C entry point from the asm glue code after the disable hypercall.
 * We enter here in IA32-e long mode, ie, full 64bit mode running on kernel
 * page tables with our below 4G page identity mapped, but using a temporary
 * GDT. ds/fs/gs/es are null. ss is not usable. bp is null. stack is not
 * available. We restore kernel GDT, and rest of the context, and continue
 * to kexec.
 */
static asmlinkage void __noreturn hv_crash_c_entry(void)
{
	struct hv_crash_ctxt *ctxt = &hv_crash_ctxt;

	/* first thing, restore kernel gdt */
	native_load_gdt(&ctxt->gdtr);

	asm volatile("movw %%ax, %%ss" : : "a"(ctxt->ss));
	asm volatile("movq %0, %%rsp" : : "m"(ctxt->rsp));

	asm volatile("movw %%ax, %%ds" : : "a"(ctxt->ds));
	asm volatile("movw %%ax, %%es" : : "a"(ctxt->es));
	asm volatile("movw %%ax, %%fs" : : "a"(ctxt->fs));
	asm volatile("movw %%ax, %%gs" : : "a"(ctxt->gs));

	native_wrmsrq(MSR_IA32_CR_PAT, ctxt->pat);
	asm volatile("movq %0, %%cr0" : : "r"(ctxt->cr0));

	asm volatile("movq %0, %%cr8" : : "r"(ctxt->cr8));
	asm volatile("movq %0, %%cr4" : : "r"(ctxt->cr4));
	asm volatile("movq %0, %%cr2" : : "r"(ctxt->cr4));

	native_load_idt(&ctxt->idtr);
	native_wrmsrq(MSR_GS_BASE, ctxt->gsbase);
	native_wrmsrq(MSR_EFER, ctxt->efer);

	/* restore the original kernel CS now via far return */
	asm volatile("movzwq %0, %%rax\n\t"
		     "pushq %%rax\n\t"
		     "pushq $1f\n\t"
		     "lretq\n\t"
		     "1:nop\n\t" : : "m"(ctxt->cs) : "rax");

	/* We are in asmlinkage without stack frame, hence make C function
	 * calls which will buy stack frames.
	 */
	hv_crash_restore_tss();
	hv_crash_clear_kernpt();

	/* we are now fully in devirtualized normal kernel mode */
	__crash_kexec(NULL);

	hv_panic_timeout_reboot();
}
/* Tell gcc we are using lretq long jump in the above function intentionally */
STACK_FRAME_NON_STANDARD(hv_crash_c_entry);

static void hv_mark_tss_not_busy(void)
{
	struct desc_struct *desc = get_current_gdt_rw();
	tss_desc tss;

	memcpy(&tss, &desc[GDT_ENTRY_TSS], sizeof(tss_desc));
	tss.type = 0x9;        /* available 64-bit TSS. 0xB is busy TSS */
	write_gdt_entry(desc, GDT_ENTRY_TSS, &tss, DESC_TSS);
}

/* Save essential context */
static void hv_hvcrash_ctxt_save(void)
{
	struct hv_crash_ctxt *ctxt = &hv_crash_ctxt;

	asm volatile("movq %%rsp,%0" : "=m"(ctxt->rsp));

	ctxt->cr0 = native_read_cr0();
	ctxt->cr4 = native_read_cr4();

	asm volatile("movq %%cr2, %0" : "=a"(ctxt->cr2));
	asm volatile("movq %%cr8, %0" : "=a"(ctxt->cr8));

	asm volatile("movl %%cs, %%eax" : "=a"(ctxt->cs));
	asm volatile("movl %%ss, %%eax" : "=a"(ctxt->ss));
	asm volatile("movl %%ds, %%eax" : "=a"(ctxt->ds));
	asm volatile("movl %%es, %%eax" : "=a"(ctxt->es));
	asm volatile("movl %%fs, %%eax" : "=a"(ctxt->fs));
	asm volatile("movl %%gs, %%eax" : "=a"(ctxt->gs));

	native_store_gdt(&ctxt->gdtr);
	store_idt(&ctxt->idtr);

	ctxt->gsbase = __rdmsr(MSR_GS_BASE);
	ctxt->efer = __rdmsr(MSR_EFER);
	ctxt->pat = __rdmsr(MSR_IA32_CR_PAT);
}

/* Add trampoline page to the kernel pagetable for transition to kernel PT */
static void hv_crash_fixup_kernpt(void)
{
	pgd_t *pgd;
	p4d_t *p4d;

	pgd = pgd_offset_k(trampoline_pa);
	p4d = p4d_offset(pgd, trampoline_pa);

	/* trampoline_pa is below 4G, so no pre-existing entry to clobber */
	p4d_populate(&init_mm, p4d, (pud_t *)hv_crash_ptpgs[1]);
	p4d->p4d = p4d->p4d & ~(_PAGE_NX);    /* enable execute */
}

/*
 * Notify the hyp that Linux has crashed. This will cause the hyp to quiesce
 * and suspend all guest VPs.
 */
static void hv_notify_prepare_hyp(void)
{
	u64 status;
	struct hv_input_notify_partition_event *input;
	struct hv_partition_event_root_crashdump_input *cda;

	input = *this_cpu_ptr(hyperv_pcpu_input_arg);
	cda = &input->input.crashdump_input;
	memset(input, 0, sizeof(*input));
	input->event = HV_PARTITION_EVENT_ROOT_CRASHDUMP;

	cda->crashdump_action = HV_CRASHDUMP_ENTRY;
	status = hv_do_hypercall(HVCALL_NOTIFY_PARTITION_EVENT, input, NULL);
	if (!hv_result_success(status))
		return;

	cda->crashdump_action = HV_CRASHDUMP_SUSPEND_ALL_VPS;
	hv_do_hypercall(HVCALL_NOTIFY_PARTITION_EVENT, input, NULL);
}

/*
 * Common function for all cpus before devirtualization.
 *
 * Hypervisor crash: all cpus get here in NMI context.
 * Linux crash: the panicing cpu gets here at base level, all others in NMI
 *		context. Note, panicing cpu may not be the BSP.
 *
 * The function is not inlined so it will show on the stack. It is named so
 * because the crash cmd looks for certain well known function names on the
 * stack before looking into the cpu saved note in the elf section, and
 * that work is currently incomplete.
 *
 * Notes:
 *  Hypervisor crash:
 *    - the hypervisor is in a very restrictive mode at this point and any
 *	vmexit it cannot handle would result in reboot. So, no mumbo jumbo,
 *	just get to kexec as quickly as possible.
 *
 *  Devirtualization is supported from the BSP only at present.
 */
static noinline __noclone void crash_nmi_callback(struct pt_regs *regs)
{
	struct hv_input_disable_hyp_ex *input;
	int msecs = 1000, ccpu = smp_processor_id();

	if (ccpu == 0) {
		/* crash_save_cpu() will be done in the kexec path */
		cpu_emergency_stop_pt();	/* disable performance trace */
		atomic_inc(&crash_cpus_wait);
	} else {
		crash_save_cpu(regs, ccpu);
		cpu_emergency_stop_pt();	/* disable performance trace */
		atomic_inc(&crash_cpus_wait);
		for (;;)
			cpu_relax();
	}

	while (atomic_read(&crash_cpus_wait) < num_online_cpus() && msecs--)
		mdelay(1);

	stop_nmi();
	if (!hv_has_crashed)
		hv_notify_prepare_hyp();

	if (crashing_cpu == -1)
		crashing_cpu = ccpu;		/* crash cmd uses this */

	hv_hvcrash_ctxt_save();
	hv_mark_tss_not_busy();
	hv_crash_fixup_kernpt();

	input = *this_cpu_ptr(hyperv_pcpu_input_arg);
	memset(input, 0, sizeof(*input));
	input->rip = trampoline_pa;
	input->arg = devirt_arg;

	(void)hv_do_hypercall(HVCALL_DISABLE_HYP_EX, input, NULL);

	hv_panic_timeout_reboot();
}


static DEFINE_SPINLOCK(hv_crash_reboot_lk);

/*
 * Generic NMI callback handler: could be called without any crash also.
 *   hv crash: hypervisor injects NMI's into all cpus
 *   lx crash: panicing cpu sends NMI to all but self via crash_stop_other_cpus
 */
static int hv_crash_nmi_local(unsigned int cmd, struct pt_regs *regs)
{
	if (!hv_has_crashed && hv_cda && hv_cda->cda_valid)
		hv_has_crashed = true;

	if (!hv_has_crashed && !lx_has_crashed)
		return NMI_DONE;	/* ignore the NMI */

	if (hv_has_crashed && !kexec_crash_loaded()) {
		if (spin_trylock(&hv_crash_reboot_lk))
			hv_panic_timeout_reboot();
		else
			for (;;)
				cpu_relax();
	}

	crash_nmi_callback(regs);

	return NMI_DONE;
}

/*
 * hv_crash_stop_other_cpus() == smp_ops.crash_stop_other_cpus
 *
 * On normal Linux panic, this is called twice: first from panic and then again
 * from native_machine_crash_shutdown.
 *
 * In case of hyperv, 3 ways to get here:
 *  1. hv crash (only BSP will get here):
 *	BSP : NMI callback -> DisableHv -> hv_crash_asm32 -> hv_crash_c_entry
 *		  -> __crash_kexec -> native_machine_crash_shutdown
 *		  -> crash_smp_send_stop -> smp_ops.crash_stop_other_cpus
 *  Linux panic:
 *	2. panic cpu x: panic() -> crash_smp_send_stop
 *				     -> smp_ops.crash_stop_other_cpus
 *	3. BSP: native_machine_crash_shutdown -> crash_smp_send_stop
 *
 * NB: noclone and non standard stack because of call to crash_setup_regs().
 */
static void __noclone hv_crash_stop_other_cpus(void)
{
	static bool crash_stop_done;
	struct pt_regs lregs;
	int ccpu = smp_processor_id();

	if (hv_has_crashed)
		return;		/* all cpus already in NMI handler path */

	if (!kexec_crash_loaded()) {
		hv_notify_prepare_hyp();
		hv_panic_timeout_reboot();	/* no return */
	}

	/* If the hv crashes also, we could come here again before cpus_stopped
	 * is set in crash_smp_send_stop(). So use our own check.
	 */
	if (crash_stop_done)
		return;
	crash_stop_done = true;

	/* Linux has crashed: hv is healthy, we can IPI safely */
	lx_has_crashed = true;
	wmb();			/* NMI handlers look at lx_has_crashed */

	apic->send_IPI_allbutself(NMI_VECTOR);

	if (crashing_cpu == -1)
		crashing_cpu = ccpu;		/* crash cmd uses this */

	/* crash_setup_regs() happens in kexec also, but for the kexec cpu which
	 * is the BSP. We could be here on non-BSP cpu, collect regs if so.
	 */
	if (ccpu)
		crash_setup_regs(&lregs, NULL);

	crash_nmi_callback(&lregs);
}
STACK_FRAME_NON_STANDARD(hv_crash_stop_other_cpus);

/* This GDT is accessed in IA32-e compat mode which uses 32bits addresses */
struct hv_gdtreg_32 {
	u16 fill;
	u16 limit;
	u32 address;
} __packed;

/* We need a CS with L bit to goto IA32-e long mode from 32bit compat mode */
struct hv_crash_tramp_gdt {
	u64 null;	/* index 0, selector 0, null selector */
	u64 cs64;	/* index 1, selector 8, cs64 selector */
} __packed;

/* No stack, so jump via far ptr in memory to load the 64bit CS */
struct hv_cs_jmptgt {
	u32 address;
	u16 csval;
	u16 fill;
} __packed;

/* Linux use only, hypervisor doesn't look at this struct */
struct hv_crash_tramp_data {
	u64 tramp32_cr3;
	u64 kernel_cr3;
	struct hv_gdtreg_32 gdtr32;
	struct hv_crash_tramp_gdt tramp_gdt;
	struct hv_cs_jmptgt cs_jmptgt;
	u64 c_entry_addr;
} __packed;

/*
 * Setup a temporary gdt to allow the asm code to switch to the long mode.
 * Since the asm code is relocated/copied to a below 4G page, it cannot use rip
 * relative addressing, hence we must use trampoline_pa here. Also, save other
 * info like jmp and C entry targets for same reasons.
 *
 * Returns: 0 on success, -1 on error
 */
static int hv_crash_setup_trampdata(u64 trampoline_va)
{
	int size, offs;
	void *dest;
	struct hv_crash_tramp_data *tramp;

	/* These must match exactly the ones in the corresponding asm file */
	BUILD_BUG_ON(offsetof(struct hv_crash_tramp_data, tramp32_cr3) != 0);
	BUILD_BUG_ON(offsetof(struct hv_crash_tramp_data, kernel_cr3) != 8);
	BUILD_BUG_ON(offsetof(struct hv_crash_tramp_data, gdtr32.limit) != 18);
	BUILD_BUG_ON(offsetof(struct hv_crash_tramp_data,
						     cs_jmptgt.address) != 40);
	BUILD_BUG_ON(offsetof(struct hv_crash_tramp_data, c_entry_addr) != 48);

	/* hv_crash_asm_end is beyond last byte by 1 */
	size = &hv_crash_asm_end - &hv_crash_asm32;
	if (size + sizeof(struct hv_crash_tramp_data) > PAGE_SIZE) {
		pr_err("%s: trampoline page overflow\n", __func__);
		return -1;
	}

	dest = (void *)trampoline_va;
	memcpy(dest, &hv_crash_asm32, size);

	dest += size;
	dest = (void *)round_up((ulong)dest, 16);
	tramp = (struct hv_crash_tramp_data *)dest;

	/* see MAX_ASID_AVAILABLE in tlb.c: "PCID 0 is reserved for use by
	 * non-PCID-aware users". Build cr3 with pcid 0
	 */
	tramp->tramp32_cr3 = __sme_pa(hv_crash_ptpgs[0]);

	/* Note, when restoring X86_CR4_PCIDE, cr3[11:0] must be zero */
	tramp->kernel_cr3 = __sme_pa(init_mm.pgd);

	tramp->gdtr32.limit = sizeof(struct hv_crash_tramp_gdt);
	tramp->gdtr32.address = trampoline_pa +
				   (ulong)&tramp->tramp_gdt - trampoline_va;

	 /* base:0 limit:0xfffff type:b dpl:0 P:1 L:1 D:0 avl:0 G:1 */
	tramp->tramp_gdt.cs64 = 0x00af9a000000ffff;

	tramp->cs_jmptgt.csval = 0x8;
	offs = (ulong)&hv_crash_asm64 - (ulong)&hv_crash_asm32;
	tramp->cs_jmptgt.address = trampoline_pa + offs;

	tramp->c_entry_addr = (u64)&hv_crash_c_entry;

	devirt_arg = trampoline_pa + (ulong)dest - trampoline_va;

	return 0;
}

/*
 * Build 32bit trampoline page table for transition from protected mode
 * non-paging to long-mode paging. This transition needs pagetables below 4G.
 */
static void hv_crash_build_tramp_pt(void)
{
	p4d_t *p4d;
	pud_t *pud;
	pmd_t *pmd;
	pte_t *pte;
	u64 pa, addr = trampoline_pa;

	p4d = hv_crash_ptpgs[0] + pgd_index(addr) * sizeof(p4d);
	pa = virt_to_phys(hv_crash_ptpgs[1]);
	set_p4d(p4d, __p4d(_PAGE_TABLE | pa));
	p4d->p4d &= ~(_PAGE_NX);	/* enable execute */

	pud = hv_crash_ptpgs[1] + pud_index(addr) * sizeof(pud);
	pa = virt_to_phys(hv_crash_ptpgs[2]);
	set_pud(pud, __pud(_PAGE_TABLE | pa));

	pmd = hv_crash_ptpgs[2] + pmd_index(addr) * sizeof(pmd);
	pa = virt_to_phys(hv_crash_ptpgs[3]);
	set_pmd(pmd, __pmd(_PAGE_TABLE | pa));

	pte = hv_crash_ptpgs[3] + pte_index(addr) * sizeof(pte);
	set_pte(pte, pfn_pte(addr >> PAGE_SHIFT, PAGE_KERNEL_EXEC));
}

/*
 * Setup trampoline for devirtualization:
 *  - a page below 4G, ie 32bit addr containing asm glue code that hyp jmps to
 *    in protected mode.
 *  - 4 pages for a temporary page table that asm code uses to turn paging on
 *  - a temporary gdt to use in the compat mode.
 *
 *  Returns: 0 on success
 */
static int hv_crash_trampoline_setup(void)
{
	int i, rc, order;
	struct page *page;
	u64 trampoline_va;
	gfp_t flags32 = GFP_KERNEL | GFP_DMA32 | __GFP_ZERO;

	/* page for 32bit trampoline assembly code + hv_crash_tramp_data */
	page = alloc_page(flags32);
	if (page == NULL) {
		pr_err("%s: failed to alloc asm stub page\n", __func__);
		return -1;
	}

	trampoline_va = (u64)page_to_virt(page);
	trampoline_pa = (u32)page_to_phys(page);

	order = 2;	   /* alloc 2^2 pages */
	page = alloc_pages(flags32, order);
	if (page == NULL) {
		pr_err("%s: failed to alloc pt pages\n", __func__);
		free_page(trampoline_va);
		return -1;
	}

	for (i = 0; i < 4; i++, page++)
		hv_crash_ptpgs[i] = page_to_virt(page);

	hv_crash_build_tramp_pt();

	rc = hv_crash_setup_trampdata(trampoline_va);
	if (rc)
		goto errout;

	return 0;

errout:
	free_page(trampoline_va);
	free_pages((ulong)hv_crash_ptpgs[0], order);

	return rc;
}

/* Setup for kdump kexec to collect hypervisor RAM when running as root */
void hv_root_crash_init(void)
{
	int rc;
	struct hv_input_get_system_property *input;
	struct hv_output_get_system_property *output;
	unsigned long flags;
	u64 status;
	union hv_pfn_range cda_info;

	if (pgtable_l5_enabled()) {
		pr_err("Hyper-V: crash dump not yet supported on 5level PTs\n");
		return;
	}

	rc = register_nmi_handler(NMI_LOCAL, hv_crash_nmi_local, NMI_FLAG_FIRST,
				  "hv_crash_nmi");
	if (rc) {
		pr_err("Hyper-V: failed to register crash nmi handler\n");
		return;
	}

	local_irq_save(flags);
	input = *this_cpu_ptr(hyperv_pcpu_input_arg);
	output = *this_cpu_ptr(hyperv_pcpu_output_arg);

	memset(input, 0, sizeof(*input));
	input->property_id = HV_SYSTEM_PROPERTY_CRASHDUMPAREA;

	status = hv_do_hypercall(HVCALL_GET_SYSTEM_PROPERTY, input, output);
	cda_info.as_uint64 = output->hv_cda_info.as_uint64;
	local_irq_restore(flags);

	if (!hv_result_success(status)) {
		pr_err("Hyper-V: %s: property:%d %s\n", __func__,
		       input->property_id, hv_result_to_string(status));
		goto err_out;
	}

	if (cda_info.base_pfn == 0) {
		pr_err("Hyper-V: hypervisor crash dump area pfn is 0\n");
		goto err_out;
	}

	hv_cda = phys_to_virt(cda_info.base_pfn << HV_HYP_PAGE_SHIFT);

	rc = hv_crash_trampoline_setup();
	if (rc)
		goto err_out;

#ifdef CONFIG_SMP
	smp_ops.crash_stop_other_cpus = hv_crash_stop_other_cpus;
#endif

	crash_kexec_post_notifiers = true;
	hv_crash_enabled = true;
	pr_info("Hyper-V: both linux and hypervisor kdump support enabled\n");

	return;

err_out:
	unregister_nmi_handler(NMI_LOCAL, "hv_crash_nmi");
	pr_err("Hyper-V: only linux root kdump support enabled\n");
}