/* * Copyright(c) 2017 Intel Corporation. All rights reserved. * * This program is free software; you can redistribute it and/or modify * it under the terms of version 2 of the GNU General Public License as * published by the Free Software Foundation. * * This program is distributed in the hope that it will be useful, but * WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU * General Public License for more details. * * 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 #undef pr_fmt #define pr_fmt(fmt) "Kernel/User page tables isolation: " fmt /* Backporting helper */ #ifndef __GFP_NOTRACK #define __GFP_NOTRACK 0 #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); } 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")) { 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 (!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 = pti_user_pagetable_walk_p4d(address); pud_t *pud; BUILD_BUG_ON(p4d_large(*p4d) != 0); if (p4d_none(*p4d)) { unsigned long new_pud_page = __get_free_page(gfp); if (!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 (!new_pmd_page) return NULL; set_pud(pud, __pud(_KERNPG_TABLE | __pa(new_pmd_page))); } return pmd_offset(pud, address); } #ifdef CONFIG_X86_VSYSCALL_EMULATION /* * 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 __init pte_t *pti_user_pagetable_walk_pte(unsigned long address) { gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO); pmd_t *pmd = pti_user_pagetable_walk_pmd(address); pte_t *pte; /* 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; } 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 static void pti_clone_pmds(unsigned long start, unsigned long end, pmdval_t clear) { unsigned long addr; /* * Clone the populated PMDs which cover start to end. These PMD areas * can have holes. */ for (addr = start; addr < end; addr += PMD_SIZE) { 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)) continue; pmd = pmd_offset(pud, addr); if (pmd_none(*pmd)) continue; 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 consistentency 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_clear_flags(*pmd, clear); } } #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); kernel_pgd = pgd_offset_k(addr); kernel_p4d = p4d_offset(kernel_pgd, addr); *user_p4d = *kernel_p4d; } /* * Clone the CPU_ENTRY_AREA into the user space visible page table. */ static void __init pti_clone_user_shared(void) { pti_clone_p4d(CPU_ENTRY_AREA_BASE); } #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_pmds(start, end, 0); } #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 and irqentry text and force it RO. */ static void pti_clone_entry_text(void) { pti_clone_pmds((unsigned long) __entry_text_start, (unsigned long) __irqentry_text_end, _PAGE_RW); } /* * 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 litlle 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_GCC_PLUGIN_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 = (unsigned long)__end_rodata_aligned; 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_pmds(start, end, 0); } /* * This is the only user for it and it is not arch-generic like * the other set_memory.h functions. Just extern it. */ extern int set_memory_nonglobal(unsigned long addr, int numpages); 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); if (pti_kernel_image_global_ok()) return; set_memory_nonglobal(start, (end - start) >> PAGE_SHIFT); } /* * Initialize kernel page table isolation */ void __init pti_init(void) { if (!static_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) { /* * We need to clone everything (again) that maps parts of the * kernel image. */ pti_clone_entry_text(); pti_clone_kernel_text(); }