text: // 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/pgtable.h>
  #include <asm/pgalloc.h>
  #include <asm/tlbflush.h>
  #include <asm/desc.h>
  #include <asm/sections.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 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;
  
  			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 to be able to find the
  		 * thread stack and needs one word of scratch space in which
  		 * to spill a register.  All of this lives in the TSS, in
  		 * the sp1 and sp2 slots.
  		 *
  		 * This is done for all possible CPUs during boot to ensure
  		 * that it's propagated to all mms.  If we were to add one of
  		 * these mappings during CPU hotplug, we would need to take
  		 * some measure to make sure that every mm that subsequently
  		 * ran on that CPU would have the relevant PGD entry in its
  		 * pagetables.  The usual vmalloc_fault() mechanism would not
  		 * work for page faults taken in entry_SYSCALL_64 before RSP
  		 * is set up.
  		 */
  
  		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 and irqentry text and force it RO.
   */
  static void pti_clone_entry_text(void)
  {
  	pti_clone_pgtable((unsigned long) __entry_text_start,
  			  (unsigned long) __irqentry_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 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;
  }
  
  /*
   * This is the only user for these and it is not arch-generic
   * like the other set_memory.h functions.  Just extern them.
   */
  extern int set_memory_nonglobal(unsigned long addr, int numpages);
  extern int set_memory_global(unsigned long addr, int numpages);
  
  /*
   * 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();
  }
  