linux/arch/x86/mm/cpu_entry_area.c
Michal Koutný a3f547addc x86/mm: Do not shuffle CPU entry areas without KASLR
The commit 97e3d26b5e ("x86/mm: Randomize per-cpu entry area") fixed
an omission of KASLR on CPU entry areas. It doesn't take into account
KASLR switches though, which may result in unintended non-determinism
when a user wants to avoid it (e.g. debugging, benchmarking).

Generate only a single combination of CPU entry areas offsets -- the
linear array that existed prior randomization when KASLR is turned off.

Since we have 3f148f3318 ("x86/kasan: Map shadow for percpu pages on
demand") and followups, we can use the more relaxed guard
kasrl_enabled() (in contrast to kaslr_memory_enabled()).

Fixes: 97e3d26b5e ("x86/mm: Randomize per-cpu entry area")
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/all/20230306193144.24605-1-mkoutny%40suse.com
2023-03-22 10:42:47 -07:00

280 lines
7.8 KiB
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/pgtable.h>
#include <asm/cpu_entry_area.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;
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;
/* O(sodding terrible) */
for_each_possible_cpu(i) {
unsigned int cea;
again:
cea = get_random_u32_below(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 */
DECLARE_PER_CPU_PAGE_ALIGNED(struct doublefault_stack, doublefault_stack);
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)
{
struct cpu_entry_area *cea = get_cpu_entry_area(cpu);
cea_map_percpu_pages(&cea->doublefault_stack,
&per_cpu(doublefault_stack, cpu), 1, PAGE_KERNEL);
}
#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 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.
*/
pgprot_t gdt_prot = 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();
}