// 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 * Signed-off-by: Moritz Lipp * Signed-off-by: Daniel Gruss * Signed-off-by: Michael Schwarz * * Major changes to the original code by: Dave Hansen * Mostly rewritten by Thomas Gleixner and * Andy Lutomirsky */ #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #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_leaf(*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, bool late_text) { 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; /* Large PMD mapping found */ if (pmd_large(*pmd)) { /* Clear the PMD if we hit a large mapping from the first round */ if (late_text) { set_pmd(pmd, __pmd(0)); } else { WARN_ON_ONCE(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, false); 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, bool late_text) { 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 = round_up(addr + 1, 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 = round_up(addr + 1, 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, late_text); 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 = round_up(addr + 1, 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, false); 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, false); } #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(bool late) { pti_clone_pgtable((unsigned long) __entry_text_start, (unsigned long) __entry_text_end, PTI_LEVEL_KERNEL_IMAGE, late); } /* * 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, false); /* * 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_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: */ /* * This is very early in boot. Device and Late initcalls can do * modprobe before free_initmem() and mark_readonly(). This * pti_clone_entry_text() allows those user-mode-helpers to function, * but notably the text is still RW. */ pti_clone_entry_text(false); 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; /* * This is after free_initmem() (all initcalls are done) and we've done * mark_readonly(). Text is now NX which might've split some PMDs * relative to the early clone. */ pti_clone_entry_text(true); pti_clone_kernel_text(); debug_checkwx_user(); }