summaryrefslogtreecommitdiffstats
path: root/arch/x86/mm/pti.c
diff options
context:
space:
mode:
Diffstat (limited to '')
-rw-r--r--arch/x86/mm/pti.c666
1 files changed, 666 insertions, 0 deletions
diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
new file mode 100644
index 000000000..ffe3b3a08
--- /dev/null
+++ b/arch/x86/mm/pti.c
@@ -0,0 +1,666 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright(c) 2017 Intel Corporation. All rights reserved.
+ *
+ * This code is based in part on work published here:
+ *
+ * https://github.com/IAIK/KAISER
+ *
+ * The original work was written by and and signed off by for the Linux
+ * kernel by:
+ *
+ * Signed-off-by: Richard Fellner <richard.fellner@student.tugraz.at>
+ * Signed-off-by: Moritz Lipp <moritz.lipp@iaik.tugraz.at>
+ * Signed-off-by: Daniel Gruss <daniel.gruss@iaik.tugraz.at>
+ * Signed-off-by: Michael Schwarz <michael.schwarz@iaik.tugraz.at>
+ *
+ * Major changes to the original code by: Dave Hansen <dave.hansen@intel.com>
+ * Mostly rewritten by Thomas Gleixner <tglx@linutronix.de> and
+ * Andy Lutomirsky <luto@amacapital.net>
+ */
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/string.h>
+#include <linux/types.h>
+#include <linux/bug.h>
+#include <linux/init.h>
+#include <linux/spinlock.h>
+#include <linux/mm.h>
+#include <linux/uaccess.h>
+#include <linux/cpu.h>
+
+#include <asm/cpufeature.h>
+#include <asm/hypervisor.h>
+#include <asm/vsyscall.h>
+#include <asm/cmdline.h>
+#include <asm/pti.h>
+#include <asm/tlbflush.h>
+#include <asm/desc.h>
+#include <asm/sections.h>
+#include <asm/set_memory.h>
+
+#undef pr_fmt
+#define pr_fmt(fmt) "Kernel/User page tables isolation: " fmt
+
+/* Backporting helper */
+#ifndef __GFP_NOTRACK
+#define __GFP_NOTRACK 0
+#endif
+
+/*
+ * Define the page-table levels we clone for user-space on 32
+ * and 64 bit.
+ */
+#ifdef CONFIG_X86_64
+#define PTI_LEVEL_KERNEL_IMAGE PTI_CLONE_PMD
+#else
+#define PTI_LEVEL_KERNEL_IMAGE PTI_CLONE_PTE
+#endif
+
+static void __init pti_print_if_insecure(const char *reason)
+{
+ if (boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
+ pr_info("%s\n", reason);
+}
+
+static void __init pti_print_if_secure(const char *reason)
+{
+ if (!boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
+ pr_info("%s\n", reason);
+}
+
+static enum pti_mode {
+ PTI_AUTO = 0,
+ PTI_FORCE_OFF,
+ PTI_FORCE_ON
+} pti_mode;
+
+void __init pti_check_boottime_disable(void)
+{
+ char arg[5];
+ int ret;
+
+ /* Assume mode is auto unless overridden. */
+ pti_mode = PTI_AUTO;
+
+ if (hypervisor_is_type(X86_HYPER_XEN_PV)) {
+ pti_mode = PTI_FORCE_OFF;
+ pti_print_if_insecure("disabled on XEN PV.");
+ return;
+ }
+
+ ret = cmdline_find_option(boot_command_line, "pti", arg, sizeof(arg));
+ if (ret > 0) {
+ if (ret == 3 && !strncmp(arg, "off", 3)) {
+ pti_mode = PTI_FORCE_OFF;
+ pti_print_if_insecure("disabled on command line.");
+ return;
+ }
+ if (ret == 2 && !strncmp(arg, "on", 2)) {
+ pti_mode = PTI_FORCE_ON;
+ pti_print_if_secure("force enabled on command line.");
+ goto enable;
+ }
+ if (ret == 4 && !strncmp(arg, "auto", 4)) {
+ pti_mode = PTI_AUTO;
+ goto autosel;
+ }
+ }
+
+ if (cmdline_find_option_bool(boot_command_line, "nopti") ||
+ cpu_mitigations_off()) {
+ pti_mode = PTI_FORCE_OFF;
+ pti_print_if_insecure("disabled on command line.");
+ return;
+ }
+
+autosel:
+ if (!boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
+ return;
+enable:
+ setup_force_cpu_cap(X86_FEATURE_PTI);
+}
+
+pgd_t __pti_set_user_pgtbl(pgd_t *pgdp, pgd_t pgd)
+{
+ /*
+ * Changes to the high (kernel) portion of the kernelmode page
+ * tables are not automatically propagated to the usermode tables.
+ *
+ * Users should keep in mind that, unlike the kernelmode tables,
+ * there is no vmalloc_fault equivalent for the usermode tables.
+ * Top-level entries added to init_mm's usermode pgd after boot
+ * will not be automatically propagated to other mms.
+ */
+ if (!pgdp_maps_userspace(pgdp))
+ return pgd;
+
+ /*
+ * The user page tables get the full PGD, accessible from
+ * userspace:
+ */
+ kernel_to_user_pgdp(pgdp)->pgd = pgd.pgd;
+
+ /*
+ * If this is normal user memory, make it NX in the kernel
+ * pagetables so that, if we somehow screw up and return to
+ * usermode with the kernel CR3 loaded, we'll get a page fault
+ * instead of allowing user code to execute with the wrong CR3.
+ *
+ * As exceptions, we don't set NX if:
+ * - _PAGE_USER is not set. This could be an executable
+ * EFI runtime mapping or something similar, and the kernel
+ * may execute from it
+ * - we don't have NX support
+ * - we're clearing the PGD (i.e. the new pgd is not present).
+ */
+ if ((pgd.pgd & (_PAGE_USER|_PAGE_PRESENT)) == (_PAGE_USER|_PAGE_PRESENT) &&
+ (__supported_pte_mask & _PAGE_NX))
+ pgd.pgd |= _PAGE_NX;
+
+ /* return the copy of the PGD we want the kernel to use: */
+ return pgd;
+}
+
+/*
+ * Walk the user copy of the page tables (optionally) trying to allocate
+ * page table pages on the way down.
+ *
+ * Returns a pointer to a P4D on success, or NULL on failure.
+ */
+static p4d_t *pti_user_pagetable_walk_p4d(unsigned long address)
+{
+ pgd_t *pgd = kernel_to_user_pgdp(pgd_offset_k(address));
+ gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
+
+ if (address < PAGE_OFFSET) {
+ WARN_ONCE(1, "attempt to walk user address\n");
+ return NULL;
+ }
+
+ if (pgd_none(*pgd)) {
+ unsigned long new_p4d_page = __get_free_page(gfp);
+ if (WARN_ON_ONCE(!new_p4d_page))
+ return NULL;
+
+ set_pgd(pgd, __pgd(_KERNPG_TABLE | __pa(new_p4d_page)));
+ }
+ BUILD_BUG_ON(pgd_large(*pgd) != 0);
+
+ return p4d_offset(pgd, address);
+}
+
+/*
+ * Walk the user copy of the page tables (optionally) trying to allocate
+ * page table pages on the way down.
+ *
+ * Returns a pointer to a PMD on success, or NULL on failure.
+ */
+static pmd_t *pti_user_pagetable_walk_pmd(unsigned long address)
+{
+ gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
+ p4d_t *p4d;
+ pud_t *pud;
+
+ p4d = pti_user_pagetable_walk_p4d(address);
+ if (!p4d)
+ return NULL;
+
+ BUILD_BUG_ON(p4d_large(*p4d) != 0);
+ if (p4d_none(*p4d)) {
+ unsigned long new_pud_page = __get_free_page(gfp);
+ if (WARN_ON_ONCE(!new_pud_page))
+ return NULL;
+
+ set_p4d(p4d, __p4d(_KERNPG_TABLE | __pa(new_pud_page)));
+ }
+
+ pud = pud_offset(p4d, address);
+ /* The user page tables do not use large mappings: */
+ if (pud_large(*pud)) {
+ WARN_ON(1);
+ return NULL;
+ }
+ if (pud_none(*pud)) {
+ unsigned long new_pmd_page = __get_free_page(gfp);
+ if (WARN_ON_ONCE(!new_pmd_page))
+ return NULL;
+
+ set_pud(pud, __pud(_KERNPG_TABLE | __pa(new_pmd_page)));
+ }
+
+ return pmd_offset(pud, address);
+}
+
+/*
+ * Walk the shadow copy of the page tables (optionally) trying to allocate
+ * page table pages on the way down. Does not support large pages.
+ *
+ * Note: this is only used when mapping *new* kernel data into the
+ * user/shadow page tables. It is never used for userspace data.
+ *
+ * Returns a pointer to a PTE on success, or NULL on failure.
+ */
+static pte_t *pti_user_pagetable_walk_pte(unsigned long address)
+{
+ gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
+ pmd_t *pmd;
+ pte_t *pte;
+
+ pmd = pti_user_pagetable_walk_pmd(address);
+ if (!pmd)
+ return NULL;
+
+ /* We can't do anything sensible if we hit a large mapping. */
+ if (pmd_large(*pmd)) {
+ WARN_ON(1);
+ return NULL;
+ }
+
+ if (pmd_none(*pmd)) {
+ unsigned long new_pte_page = __get_free_page(gfp);
+ if (!new_pte_page)
+ return NULL;
+
+ set_pmd(pmd, __pmd(_KERNPG_TABLE | __pa(new_pte_page)));
+ }
+
+ pte = pte_offset_kernel(pmd, address);
+ if (pte_flags(*pte) & _PAGE_USER) {
+ WARN_ONCE(1, "attempt to walk to user pte\n");
+ return NULL;
+ }
+ return pte;
+}
+
+#ifdef CONFIG_X86_VSYSCALL_EMULATION
+static void __init pti_setup_vsyscall(void)
+{
+ pte_t *pte, *target_pte;
+ unsigned int level;
+
+ pte = lookup_address(VSYSCALL_ADDR, &level);
+ if (!pte || WARN_ON(level != PG_LEVEL_4K) || pte_none(*pte))
+ return;
+
+ target_pte = pti_user_pagetable_walk_pte(VSYSCALL_ADDR);
+ if (WARN_ON(!target_pte))
+ return;
+
+ *target_pte = *pte;
+ set_vsyscall_pgtable_user_bits(kernel_to_user_pgdp(swapper_pg_dir));
+}
+#else
+static void __init pti_setup_vsyscall(void) { }
+#endif
+
+enum pti_clone_level {
+ PTI_CLONE_PMD,
+ PTI_CLONE_PTE,
+};
+
+static void
+pti_clone_pgtable(unsigned long start, unsigned long end,
+ enum pti_clone_level level)
+{
+ unsigned long addr;
+
+ /*
+ * Clone the populated PMDs which cover start to end. These PMD areas
+ * can have holes.
+ */
+ for (addr = start; addr < end;) {
+ pte_t *pte, *target_pte;
+ pmd_t *pmd, *target_pmd;
+ pgd_t *pgd;
+ p4d_t *p4d;
+ pud_t *pud;
+
+ /* Overflow check */
+ if (addr < start)
+ break;
+
+ pgd = pgd_offset_k(addr);
+ if (WARN_ON(pgd_none(*pgd)))
+ return;
+ p4d = p4d_offset(pgd, addr);
+ if (WARN_ON(p4d_none(*p4d)))
+ return;
+
+ pud = pud_offset(p4d, addr);
+ if (pud_none(*pud)) {
+ WARN_ON_ONCE(addr & ~PUD_MASK);
+ addr = round_up(addr + 1, PUD_SIZE);
+ continue;
+ }
+
+ pmd = pmd_offset(pud, addr);
+ if (pmd_none(*pmd)) {
+ WARN_ON_ONCE(addr & ~PMD_MASK);
+ addr = round_up(addr + 1, PMD_SIZE);
+ continue;
+ }
+
+ if (pmd_large(*pmd) || level == PTI_CLONE_PMD) {
+ target_pmd = pti_user_pagetable_walk_pmd(addr);
+ if (WARN_ON(!target_pmd))
+ return;
+
+ /*
+ * Only clone present PMDs. This ensures only setting
+ * _PAGE_GLOBAL on present PMDs. This should only be
+ * called on well-known addresses anyway, so a non-
+ * present PMD would be a surprise.
+ */
+ if (WARN_ON(!(pmd_flags(*pmd) & _PAGE_PRESENT)))
+ return;
+
+ /*
+ * Setting 'target_pmd' below creates a mapping in both
+ * the user and kernel page tables. It is effectively
+ * global, so set it as global in both copies. Note:
+ * the X86_FEATURE_PGE check is not _required_ because
+ * the CPU ignores _PAGE_GLOBAL when PGE is not
+ * supported. The check keeps consistency with
+ * code that only set this bit when supported.
+ */
+ if (boot_cpu_has(X86_FEATURE_PGE))
+ *pmd = pmd_set_flags(*pmd, _PAGE_GLOBAL);
+
+ /*
+ * Copy the PMD. That is, the kernelmode and usermode
+ * tables will share the last-level page tables of this
+ * address range
+ */
+ *target_pmd = *pmd;
+
+ addr += PMD_SIZE;
+
+ } else if (level == PTI_CLONE_PTE) {
+
+ /* Walk the page-table down to the pte level */
+ pte = pte_offset_kernel(pmd, addr);
+ if (pte_none(*pte)) {
+ addr += PAGE_SIZE;
+ continue;
+ }
+
+ /* Only clone present PTEs */
+ if (WARN_ON(!(pte_flags(*pte) & _PAGE_PRESENT)))
+ return;
+
+ /* Allocate PTE in the user page-table */
+ target_pte = pti_user_pagetable_walk_pte(addr);
+ if (WARN_ON(!target_pte))
+ return;
+
+ /* Set GLOBAL bit in both PTEs */
+ if (boot_cpu_has(X86_FEATURE_PGE))
+ *pte = pte_set_flags(*pte, _PAGE_GLOBAL);
+
+ /* Clone the PTE */
+ *target_pte = *pte;
+
+ addr += PAGE_SIZE;
+
+ } else {
+ BUG();
+ }
+ }
+}
+
+#ifdef CONFIG_X86_64
+/*
+ * Clone a single p4d (i.e. a top-level entry on 4-level systems and a
+ * next-level entry on 5-level systems.
+ */
+static void __init pti_clone_p4d(unsigned long addr)
+{
+ p4d_t *kernel_p4d, *user_p4d;
+ pgd_t *kernel_pgd;
+
+ user_p4d = pti_user_pagetable_walk_p4d(addr);
+ if (!user_p4d)
+ return;
+
+ kernel_pgd = pgd_offset_k(addr);
+ kernel_p4d = p4d_offset(kernel_pgd, addr);
+ *user_p4d = *kernel_p4d;
+}
+
+/*
+ * Clone the CPU_ENTRY_AREA and associated data into the user space visible
+ * page table.
+ */
+static void __init pti_clone_user_shared(void)
+{
+ unsigned int cpu;
+
+ pti_clone_p4d(CPU_ENTRY_AREA_BASE);
+
+ for_each_possible_cpu(cpu) {
+ /*
+ * The SYSCALL64 entry code needs one word of scratch space
+ * in which to spill a register. It lives in the sp2 slot
+ * of the CPU's TSS.
+ *
+ * This is done for all possible CPUs during boot to ensure
+ * that it's propagated to all mms.
+ */
+
+ unsigned long va = (unsigned long)&per_cpu(cpu_tss_rw, cpu);
+ phys_addr_t pa = per_cpu_ptr_to_phys((void *)va);
+ pte_t *target_pte;
+
+ target_pte = pti_user_pagetable_walk_pte(va);
+ if (WARN_ON(!target_pte))
+ return;
+
+ *target_pte = pfn_pte(pa >> PAGE_SHIFT, PAGE_KERNEL);
+ }
+}
+
+#else /* CONFIG_X86_64 */
+
+/*
+ * On 32 bit PAE systems with 1GB of Kernel address space there is only
+ * one pgd/p4d for the whole kernel. Cloning that would map the whole
+ * address space into the user page-tables, making PTI useless. So clone
+ * the page-table on the PMD level to prevent that.
+ */
+static void __init pti_clone_user_shared(void)
+{
+ unsigned long start, end;
+
+ start = CPU_ENTRY_AREA_BASE;
+ end = start + (PAGE_SIZE * CPU_ENTRY_AREA_PAGES);
+
+ pti_clone_pgtable(start, end, PTI_CLONE_PMD);
+}
+#endif /* CONFIG_X86_64 */
+
+/*
+ * Clone the ESPFIX P4D into the user space visible page table
+ */
+static void __init pti_setup_espfix64(void)
+{
+#ifdef CONFIG_X86_ESPFIX64
+ pti_clone_p4d(ESPFIX_BASE_ADDR);
+#endif
+}
+
+/*
+ * Clone the populated PMDs of the entry text and force it RO.
+ */
+static void pti_clone_entry_text(void)
+{
+ pti_clone_pgtable((unsigned long) __entry_text_start,
+ (unsigned long) __entry_text_end,
+ PTI_CLONE_PMD);
+}
+
+/*
+ * Global pages and PCIDs are both ways to make kernel TLB entries
+ * live longer, reduce TLB misses and improve kernel performance.
+ * But, leaving all kernel text Global makes it potentially accessible
+ * to Meltdown-style attacks which make it trivial to find gadgets or
+ * defeat KASLR.
+ *
+ * Only use global pages when it is really worth it.
+ */
+static inline bool pti_kernel_image_global_ok(void)
+{
+ /*
+ * Systems with PCIDs get little benefit from global
+ * kernel text and are not worth the downsides.
+ */
+ if (cpu_feature_enabled(X86_FEATURE_PCID))
+ return false;
+
+ /*
+ * Only do global kernel image for pti=auto. Do the most
+ * secure thing (not global) if pti=on specified.
+ */
+ if (pti_mode != PTI_AUTO)
+ return false;
+
+ /*
+ * K8 may not tolerate the cleared _PAGE_RW on the userspace
+ * global kernel image pages. Do the safe thing (disable
+ * global kernel image). This is unlikely to ever be
+ * noticed because PTI is disabled by default on AMD CPUs.
+ */
+ if (boot_cpu_has(X86_FEATURE_K8))
+ return false;
+
+ /*
+ * RANDSTRUCT derives its hardening benefits from the
+ * attacker's lack of knowledge about the layout of kernel
+ * data structures. Keep the kernel image non-global in
+ * cases where RANDSTRUCT is in use to help keep the layout a
+ * secret.
+ */
+ if (IS_ENABLED(CONFIG_RANDSTRUCT))
+ return false;
+
+ return true;
+}
+
+/*
+ * For some configurations, map all of kernel text into the user page
+ * tables. This reduces TLB misses, especially on non-PCID systems.
+ */
+static void pti_clone_kernel_text(void)
+{
+ /*
+ * rodata is part of the kernel image and is normally
+ * readable on the filesystem or on the web. But, do not
+ * clone the areas past rodata, they might contain secrets.
+ */
+ unsigned long start = PFN_ALIGN(_text);
+ unsigned long end_clone = (unsigned long)__end_rodata_aligned;
+ unsigned long end_global = PFN_ALIGN((unsigned long)_etext);
+
+ if (!pti_kernel_image_global_ok())
+ return;
+
+ pr_debug("mapping partial kernel image into user address space\n");
+
+ /*
+ * Note that this will undo _some_ of the work that
+ * pti_set_kernel_image_nonglobal() did to clear the
+ * global bit.
+ */
+ pti_clone_pgtable(start, end_clone, PTI_LEVEL_KERNEL_IMAGE);
+
+ /*
+ * pti_clone_pgtable() will set the global bit in any PMDs
+ * that it clones, but we also need to get any PTEs in
+ * the last level for areas that are not huge-page-aligned.
+ */
+
+ /* Set the global bit for normal non-__init kernel text: */
+ set_memory_global(start, (end_global - start) >> PAGE_SHIFT);
+}
+
+static void pti_set_kernel_image_nonglobal(void)
+{
+ /*
+ * The identity map is created with PMDs, regardless of the
+ * actual length of the kernel. We need to clear
+ * _PAGE_GLOBAL up to a PMD boundary, not just to the end
+ * of the image.
+ */
+ unsigned long start = PFN_ALIGN(_text);
+ unsigned long end = ALIGN((unsigned long)_end, PMD_PAGE_SIZE);
+
+ /*
+ * This clears _PAGE_GLOBAL from the entire kernel image.
+ * pti_clone_kernel_text() map put _PAGE_GLOBAL back for
+ * areas that are mapped to userspace.
+ */
+ set_memory_nonglobal(start, (end - start) >> PAGE_SHIFT);
+}
+
+/*
+ * Initialize kernel page table isolation
+ */
+void __init pti_init(void)
+{
+ if (!boot_cpu_has(X86_FEATURE_PTI))
+ return;
+
+ pr_info("enabled\n");
+
+#ifdef CONFIG_X86_32
+ /*
+ * We check for X86_FEATURE_PCID here. But the init-code will
+ * clear the feature flag on 32 bit because the feature is not
+ * supported on 32 bit anyway. To print the warning we need to
+ * check with cpuid directly again.
+ */
+ if (cpuid_ecx(0x1) & BIT(17)) {
+ /* Use printk to work around pr_fmt() */
+ printk(KERN_WARNING "\n");
+ printk(KERN_WARNING "************************************************************\n");
+ printk(KERN_WARNING "** WARNING! WARNING! WARNING! WARNING! WARNING! WARNING! **\n");
+ printk(KERN_WARNING "** **\n");
+ printk(KERN_WARNING "** You are using 32-bit PTI on a 64-bit PCID-capable CPU. **\n");
+ printk(KERN_WARNING "** Your performance will increase dramatically if you **\n");
+ printk(KERN_WARNING "** switch to a 64-bit kernel! **\n");
+ printk(KERN_WARNING "** **\n");
+ printk(KERN_WARNING "** WARNING! WARNING! WARNING! WARNING! WARNING! WARNING! **\n");
+ printk(KERN_WARNING "************************************************************\n");
+ }
+#endif
+
+ pti_clone_user_shared();
+
+ /* Undo all global bits from the init pagetables in head_64.S: */
+ pti_set_kernel_image_nonglobal();
+ /* Replace some of the global bits just for shared entry text: */
+ pti_clone_entry_text();
+ pti_setup_espfix64();
+ pti_setup_vsyscall();
+}
+
+/*
+ * Finalize the kernel mappings in the userspace page-table. Some of the
+ * mappings for the kernel image might have changed since pti_init()
+ * cloned them. This is because parts of the kernel image have been
+ * mapped RO and/or NX. These changes need to be cloned again to the
+ * userspace page-table.
+ */
+void pti_finalize(void)
+{
+ if (!boot_cpu_has(X86_FEATURE_PTI))
+ return;
+ /*
+ * We need to clone everything (again) that maps parts of the
+ * kernel image.
+ */
+ pti_clone_entry_text();
+ pti_clone_kernel_text();
+
+ debug_checkwx_user();
+}
generated by cgit v1.2.3 (git 2.39.1) at 2024-12-12 15:03:45 +0000