kernel/arch/x86/mm/cpu_entry_area.c

// SPDX-License-Identifier: GPL-2.0

#include <linux/spinlock.h>
#include <linux/percpu.h>
#include <linux/kallsyms.h>
#include <linux/kcore.h>
#include <linux/prandom.h>

#include <asm/cpu_entry_area.h>
#include <asm/pgtable.h>
#include <asm/fixmap.h>
#include <asm/desc.h>
#include <asm/kasan.h>
#include <asm/setup.h>

static DEFINE_PER_CPU_PAGE_ALIGNED(struct entry_stack_page, entry_stack_storage);

#ifdef CONFIG_X86_64
static DEFINE_PER_CPU_PAGE_ALIGNED(struct exception_stacks, exception_stacks);
DEFINE_PER_CPU(struct cea_exception_stacks*, cea_exception_stacks);

static DEFINE_PER_CPU_READ_MOSTLY(unsigned long, _cea_offset);

static __always_inline unsigned int cea_offset(unsigned int cpu)
{
	return per_cpu(_cea_offset, cpu);
}

static __init void init_cea_offsets(void)
{
	unsigned int max_cea;
	unsigned int i, j;
	struct rnd_state cea_rnd;

	if (!kaslr_enabled()) {
		for_each_possible_cpu(i)
			per_cpu(_cea_offset, i) = i;
		return;
	}

	max_cea = (CPU_ENTRY_AREA_MAP_SIZE - PAGE_SIZE) / CPU_ENTRY_AREA_SIZE;

	prandom_seed_state(&cea_rnd, kaslr_get_random_long("init_cea_offsets"));
	prandom_warmup(&cea_rnd);

	/* O(sodding terrible) */
	for_each_possible_cpu(i) {
		unsigned int cea;

again:
		cea = prandom_u32_state(&cea_rnd) % max_cea;

		for_each_possible_cpu(j) {
			if (cea_offset(j) == cea)
				goto again;

			if (i == j)
				break;
		}

		per_cpu(_cea_offset, i) = cea;
	}
}
#else /* !X86_64 */

static __always_inline unsigned int cea_offset(unsigned int cpu)
{
	return cpu;
}
static inline void init_cea_offsets(void) { }
#endif

/* Is called from entry code, so must be noinstr */
noinstr struct cpu_entry_area *get_cpu_entry_area(int cpu)
{
	unsigned long va = CPU_ENTRY_AREA_PER_CPU + cea_offset(cpu) * CPU_ENTRY_AREA_SIZE;
	BUILD_BUG_ON(sizeof(struct cpu_entry_area) % PAGE_SIZE != 0);

	return (struct cpu_entry_area *) va;
}
EXPORT_SYMBOL(get_cpu_entry_area);

void cea_set_pte(void *cea_vaddr, phys_addr_t pa, pgprot_t flags)
{
	unsigned long va = (unsigned long) cea_vaddr;
	pte_t pte = pfn_pte(pa >> PAGE_SHIFT, flags);

	/*
	 * The cpu_entry_area is shared between the user and kernel
	 * page tables.  All of its ptes can safely be global.
	 * _PAGE_GLOBAL gets reused to help indicate PROT_NONE for
	 * non-present PTEs, so be careful not to set it in that
	 * case to avoid confusion.
	 */
	if (boot_cpu_has(X86_FEATURE_PGE) &&
	    (pgprot_val(flags) & _PAGE_PRESENT))
		pte = pte_set_flags(pte, _PAGE_GLOBAL);

	set_pte_vaddr(va, pte);
}

static void __init
cea_map_percpu_pages(void *cea_vaddr, void *ptr, int pages, pgprot_t prot)
{
	for ( ; pages; pages--, cea_vaddr+= PAGE_SIZE, ptr += PAGE_SIZE)
		cea_set_pte(cea_vaddr, per_cpu_ptr_to_phys(ptr), prot);
}

static void __init percpu_setup_debug_store(unsigned int cpu)
{
#ifdef CONFIG_CPU_SUP_INTEL
	unsigned int npages;
	void *cea;

	if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL)
		return;

	cea = &get_cpu_entry_area(cpu)->cpu_debug_store;
	npages = sizeof(struct debug_store) / PAGE_SIZE;
	BUILD_BUG_ON(sizeof(struct debug_store) % PAGE_SIZE != 0);
	cea_map_percpu_pages(cea, &per_cpu(cpu_debug_store, cpu), npages,
			     PAGE_KERNEL);

	cea = &get_cpu_entry_area(cpu)->cpu_debug_buffers;
	/*
	 * Force the population of PMDs for not yet allocated per cpu
	 * memory like debug store buffers.
	 */
	npages = sizeof(struct debug_store_buffers) / PAGE_SIZE;
	for (; npages; npages--, cea += PAGE_SIZE)
		cea_set_pte(cea, 0, PAGE_NONE);
#endif
}

#ifdef CONFIG_X86_64

#define cea_map_stack(name) do {					\
	npages = sizeof(estacks->name## _stack) / PAGE_SIZE;		\
	cea_map_percpu_pages(cea->estacks.name## _stack,		\
			estacks->name## _stack, npages, PAGE_KERNEL);	\
	} while (0)

static void __init percpu_setup_exception_stacks(unsigned int cpu)
{
	struct exception_stacks *estacks = per_cpu_ptr(&exception_stacks, cpu);
	struct cpu_entry_area *cea = get_cpu_entry_area(cpu);
	unsigned int npages;

	BUILD_BUG_ON(sizeof(exception_stacks) % PAGE_SIZE != 0);

	per_cpu(cea_exception_stacks, cpu) = &cea->estacks;

	/*
	 * The exceptions stack mappings in the per cpu area are protected
	 * by guard pages so each stack must be mapped separately. DB2 is
	 * not mapped; it just exists to catch triple nesting of #DB.
	 */
	cea_map_stack(DF);
	cea_map_stack(NMI);
	cea_map_stack(DB);
	cea_map_stack(MCE);

	if (IS_ENABLED(CONFIG_AMD_MEM_ENCRYPT)) {
		if (cc_platform_has(CC_ATTR_GUEST_STATE_ENCRYPT)) {
			cea_map_stack(VC);
			cea_map_stack(VC2);
		}
	}
}
#else
static inline void percpu_setup_exception_stacks(unsigned int cpu) {}
#endif

/* Setup the fixmap mappings only once per-processor */
static void __init setup_cpu_entry_area(unsigned int cpu)
{
	struct cpu_entry_area *cea = get_cpu_entry_area(cpu);
#ifdef CONFIG_X86_64
	/* On 64-bit systems, we use a read-only fixmap GDT and TSS. */
	pgprot_t gdt_prot = PAGE_KERNEL_RO;
	pgprot_t tss_prot = PAGE_KERNEL_RO;
#else
	/*
	 * On native 32-bit systems, the GDT cannot be read-only because
	 * our double fault handler uses a task gate, and entering through
	 * a task gate needs to change an available TSS to busy.  If the
	 * GDT is read-only, that will triple fault.  The TSS cannot be
	 * read-only because the CPU writes to it on task switches.
	 *
	 * On Xen PV, the GDT must be read-only because the hypervisor
	 * requires it.
	 */
	pgprot_t gdt_prot = boot_cpu_has(X86_FEATURE_XENPV) ?
		PAGE_KERNEL_RO : PAGE_KERNEL;
	pgprot_t tss_prot = PAGE_KERNEL;
#endif

	kasan_populate_shadow_for_vaddr(cea, CPU_ENTRY_AREA_SIZE,
					early_cpu_to_node(cpu));

	cea_set_pte(&cea->gdt, get_cpu_gdt_paddr(cpu), gdt_prot);

	cea_map_percpu_pages(&cea->entry_stack_page,
			     per_cpu_ptr(&entry_stack_storage, cpu), 1,
			     PAGE_KERNEL);

	/*
	 * The Intel SDM says (Volume 3, 7.2.1):
	 *
	 *  Avoid placing a page boundary in the part of the TSS that the
	 *  processor reads during a task switch (the first 104 bytes). The
	 *  processor may not correctly perform address translations if a
	 *  boundary occurs in this area. During a task switch, the processor
	 *  reads and writes into the first 104 bytes of each TSS (using
	 *  contiguous physical addresses beginning with the physical address
	 *  of the first byte of the TSS). So, after TSS access begins, if
	 *  part of the 104 bytes is not physically contiguous, the processor
	 *  will access incorrect information without generating a page-fault
	 *  exception.
	 *
	 * There are also a lot of errata involving the TSS spanning a page
	 * boundary.  Assert that we're not doing that.
	 */
	BUILD_BUG_ON((offsetof(struct tss_struct, x86_tss) ^
		      offsetofend(struct tss_struct, x86_tss)) & PAGE_MASK);
	BUILD_BUG_ON(sizeof(struct tss_struct) % PAGE_SIZE != 0);
	/*
	 * VMX changes the host TR limit to 0x67 after a VM exit. This is
	 * okay, since 0x67 covers the size of struct x86_hw_tss. Make sure
	 * that this is correct.
	 */
	BUILD_BUG_ON(offsetof(struct tss_struct, x86_tss) != 0);
	BUILD_BUG_ON(sizeof(struct x86_hw_tss) != 0x68);

	cea_map_percpu_pages(&cea->tss, &per_cpu(cpu_tss_rw, cpu),
			     sizeof(struct tss_struct) / PAGE_SIZE, tss_prot);

#ifdef CONFIG_X86_32
	per_cpu(cpu_entry_area, cpu) = cea;
#endif

	percpu_setup_exception_stacks(cpu);

	percpu_setup_debug_store(cpu);
}

static __init void setup_cpu_entry_area_ptes(void)
{
#ifdef CONFIG_X86_32
	unsigned long start, end;

	/* The +1 is for the readonly IDT: */
	BUILD_BUG_ON((CPU_ENTRY_AREA_PAGES+1)*PAGE_SIZE != CPU_ENTRY_AREA_MAP_SIZE);
	BUG_ON(CPU_ENTRY_AREA_BASE & ~PMD_MASK);

	start = CPU_ENTRY_AREA_BASE;
	end = start + CPU_ENTRY_AREA_MAP_SIZE;

	/* Careful here: start + PMD_SIZE might wrap around */
	for (; start < end && start >= CPU_ENTRY_AREA_BASE; start += PMD_SIZE)
		populate_extra_pte(start);
#endif
}

void __init setup_cpu_entry_areas(void)
{
	unsigned int cpu;

	init_cea_offsets();

	setup_cpu_entry_area_ptes();

	for_each_possible_cpu(cpu)
		setup_cpu_entry_area(cpu);

	/*
	 * This is the last essential update to swapper_pgdir which needs
	 * to be synchronized to initial_page_table on 32bit.
	 */
	sync_initial_page_table();
}