[PATCH 05/23] x86, kaiser: unmap kernel from userspace page tables (core patch)

From: Dave Hansen
Date: Wed Nov 22 2017 - 19:45:25 EST

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From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

These patches are based on work from a team at Graz University of
Technology: https://github.com/IAIK/KAISER . This work would not have
been possible without their work as a starting point.

KAISER is a countermeasure against side channel attacks against kernel
virtual memory. It leaves the existing page tables largely alone and
refers to them as the "kernel page tables. It adds a "shadow" pgd for
every process which is intended for use when running userspace. The
shadow pgd maps all the same user memory as the "kernel" copy, but
only maps a minimal set of kernel memory.

Whenever entering the kernel (syscalls, interrupts, exceptions), the
pgd is switched to the "kernel" copy. When switching back to user
mode, the shadow pgd is used.

The minimalistic kernel page tables try to map only what is needed to
enter/exit the kernel such as the entry/exit functions themselves and
the interrupt descriptors (IDT).

=== Page Table Poisoning ===

KAISER has two copies of the page tables: one for the kernel and
one for when running in userspace. There is also a kernel
portion of each of the page tables: the part that *maps* the
kernel.

The kernel portion is relatively static and uses pre-populated
PGDs. Nobody ever calls set_pgd() on the kernel portion during
normal operation.

The userspace portion of the page tables is updated frequently as
userspace pages are mapped and page table pages are allocated.
These updates of the userspace *portion* of the tables need to be
reflected into both the kernel and user/shadow copies.

The original KAISER patches did this by effectively looking at the
address that is being updated. If it is <PAGE_OFFSET, it is
considered to be doing an update for the userspace portion of the page
tables and must make an entry in the shadow.

However, this has a wrinkle: there are a few places where low
addresses are used in supervisor (kernel) mode. When EFI calls
are made, they use what are traditionally user addresses in
supervisor mode and trip over these checks. The trampoline code
that used for booting secondary CPUs has a similar issue.

Remember, there are two things that KAISER needs performed on a
userspace PGD:

1. Populate the shadow itself
2. Poison the kernel PGD so it can not be used by userspace.

Only perform these actions when dealing with a user address *and* the
PGD has _PAGE_USER set. That way, in-kernel users of low addresses
typically used by userspace are not accidentally poisoned.

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Moritz Lipp <moritz.lipp@xxxxxxxxxxxxxx>
Cc: Daniel Gruss <daniel.gruss@xxxxxxxxxxxxxx>
Cc: Michael Schwarz <michael.schwarz@xxxxxxxxxxxxxx>
Cc: Richard Fellner <richard.fellner@xxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Changes from original KAISER patch:
* Gobs of coding style cleanups
* The original patch tried to allocate an order-2 page, then
8k-align the result. That's silly since order-2 is already
guaranteed to be 16k-aligned. Removed that gunk and just
allocate an order-1 page.
* Handle (or at least detect and warn on) allocation failures
* Use _KERNPG_TABLE, not _PAGE_TABLE when creating mappings for
the kernel in the shadow (user) page tables.
* BUG_ON() for !pte_none() case was totally insane: it checked
the physical address of the 'struct page' against the physical
address of the page being mapped.
* Added 5-level page table support
* Never free kaiser page tables. We don't have the locking to
keep them from getting referenced during the freeing process.
* Use a totally different scheme in the entry code. The
original code just fell apart in horrific ways in debug faults,
NMIs, or when iret faults. Big thanks to Andy Lutomirski for
reducing the number of places that needed to be patched. He
made the code a ton simpler.
* Use new entry trampoline instead of mapping process stacks.

Note: The original KAISER authors signed-off on their patch. Some of
their code has been broken out into other patches in this series, but
their SoB was only retained here.

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Richard Fellner <richard.fellner@xxxxxxxxxxxxxxxxx>
Cc: Moritz Lipp <moritz.lipp@xxxxxxxxxxxxxx>
Cc: Daniel Gruss <daniel.gruss@xxxxxxxxxxxxxx>
Cc: Michael Schwarz <michael.schwarz@xxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx

---

b/Documentation/x86/kaiser.txt | 162 +++++++++++++
b/arch/x86/entry/calling.h | 1
b/arch/x86/include/asm/kaiser.h | 57 ++++
b/arch/x86/include/asm/pgtable.h | 5
b/arch/x86/include/asm/pgtable_64.h | 132 ++++++++++
b/arch/x86/kernel/espfix_64.c | 17 +
b/arch/x86/kernel/head_64.S | 14 -
b/arch/x86/mm/Makefile | 1
b/arch/x86/mm/kaiser.c | 441 ++++++++++++++++++++++++++++++++++++
b/arch/x86/mm/pageattr.c | 2
b/arch/x86/mm/pgtable.c | 16 +
b/include/linux/kaiser.h | 29 ++
b/init/main.c | 3
b/kernel/fork.c | 1
14 files changed, 875 insertions(+), 6 deletions(-)

diff -puN arch/x86/entry/calling.h~kaiser-base arch/x86/entry/calling.h
--- a/arch/x86/entry/calling.h~kaiser-base 2017-11-22 15:45:46.527619745 -0800
+++ b/arch/x86/entry/calling.h 2017-11-22 15:45:46.547619745 -0800
@@ -2,6 +2,7 @@
#include <linux/jump_label.h>
#include <asm/unwind_hints.h>
#include <asm/cpufeatures.h>
+#include <asm/page_types.h>

/*

diff -puN /dev/null arch/x86/include/asm/kaiser.h
--- /dev/null 2017-11-06 07:51:38.702108459 -0800
+++ b/arch/x86/include/asm/kaiser.h 2017-11-22 15:45:46.548619745 -0800
@@ -0,0 +1,57 @@
+#ifndef _ASM_X86_KAISER_H
+#define _ASM_X86_KAISER_H
+/*
+ * 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.
+ *
+ * Based on work published here: https://github.com/IAIK/KAISER
+ * Modified by Dave Hansen <dave.hansen@xxxxxxxxx to actually work.
+ */
+#ifndef __ASSEMBLY__
+
+#ifdef CONFIG_KAISER
+/**
+ * kaiser_add_mapping - map a kernel range into the user page tables
+ * @addr: the start address of the range
+ * @size: the size of the range
+ * @flags: The mapping flags of the pages
+ *
+ * Use this on all data and code that need to be mapped into both
+ * copies of the page tables. This includes the code that switches
+ * to/from userspace and all of the hardware structures that are
+ * virtually-addressed and needed in userspace like the interrupt
+ * table.
+ */
+extern int kaiser_add_mapping(unsigned long addr, unsigned long size,
+ unsigned long flags);
+
+/**
+ * kaiser_remove_mapping - remove a kernel mapping from the userpage tables
+ * @addr: the start address of the range
+ * @size: the size of the range
+ */
+extern void kaiser_remove_mapping(unsigned long start, unsigned long size);
+
+/**
+ * kaiser_init - Initialize the shadow mapping
+ *
+ * Most parts of the shadow mapping can be mapped upon boot
+ * time. Only per-process things like the thread stacks
+ * or a new LDT have to be mapped at runtime. These boot-
+ * time mappings are permanent and never unmapped.
+ */
+extern void kaiser_init(void);
+
+#endif
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* _ASM_X86_KAISER_H */
diff -puN arch/x86/include/asm/pgtable_64.h~kaiser-base arch/x86/include/asm/pgtable_64.h
--- a/arch/x86/include/asm/pgtable_64.h~kaiser-base 2017-11-22 15:45:46.529619745 -0800
+++ b/arch/x86/include/asm/pgtable_64.h 2017-11-22 15:45:46.548619745 -0800
@@ -131,9 +131,137 @@ static inline pud_t native_pudp_get_and_
#endif
}

+#ifdef CONFIG_KAISER
+/*
+ * All top-level KAISER page tables are order-1 pages (8k-aligned
+ * and 8k in size). The kernel one is at the beginning 4k and
+ * the user (shadow) one is in the last 4k. To switch between
+ * them, you just need to flip the 12th bit in their addresses.
+ */
+#define KAISER_PGTABLE_SWITCH_BIT PAGE_SHIFT
+
+/*
+ * This generates better code than the inline assembly in
+ * __set_bit().
+ */
+static inline void *ptr_set_bit(void *ptr, int bit)
+{
+ unsigned long __ptr = (unsigned long)ptr;
+
+ __ptr |= (1<<bit);
+ return (void *)__ptr;
+}
+static inline void *ptr_clear_bit(void *ptr, int bit)
+{
+ unsigned long __ptr = (unsigned long)ptr;
+
+ __ptr &= ~(1<<bit);
+ return (void *)__ptr;
+}
+
+static inline pgd_t *kernel_to_shadow_pgdp(pgd_t *pgdp)
+{
+ return ptr_set_bit(pgdp, KAISER_PGTABLE_SWITCH_BIT);
+}
+static inline pgd_t *shadow_to_kernel_pgdp(pgd_t *pgdp)
+{
+ return ptr_clear_bit(pgdp, KAISER_PGTABLE_SWITCH_BIT);
+}
+static inline p4d_t *kernel_to_shadow_p4dp(p4d_t *p4dp)
+{
+ return ptr_set_bit(p4dp, KAISER_PGTABLE_SWITCH_BIT);
+}
+static inline p4d_t *shadow_to_kernel_p4dp(p4d_t *p4dp)
+{
+ return ptr_clear_bit(p4dp, KAISER_PGTABLE_SWITCH_BIT);
+}
+#endif /* CONFIG_KAISER */
+
+/*
+ * Page table pages are page-aligned. The lower half of the top
+ * level is used for userspace and the top half for the kernel.
+ *
+ * Returns true for parts of the PGD that map userspace and
+ * false for the parts that map the kernel.
+ */
+static inline bool pgdp_maps_userspace(void *__ptr)
+{
+ unsigned long ptr = (unsigned long)__ptr;
+
+ return (ptr & ~PAGE_MASK) < (PAGE_SIZE / 2);
+}
+
+/*
+ * Does this PGD allow access from userspace?
+ */
+static inline bool pgd_userspace_access(pgd_t pgd)
+{
+ return pgd.pgd & _PAGE_USER;
+}
+
+/*
+ * Take a PGD location (pgdp) and a pgd value that needs
+ * to be set there. Populates the shadow and returns
+ * the resulting PGD that must be set in the kernel copy
+ * of the page tables.
+ */
+static inline pgd_t kaiser_set_shadow_pgd(pgd_t *pgdp, pgd_t pgd)
+{
+#ifdef CONFIG_KAISER
+ if (pgd_userspace_access(pgd)) {
+ if (pgdp_maps_userspace(pgdp)) {
+ /*
+ * The user/shadow page tables get the full
+ * PGD, accessible from userspace:
+ */
+ kernel_to_shadow_pgdp(pgdp)->pgd = pgd.pgd;
+ /*
+ * For the copy of the pgd that the kernel
+ * uses, make it unusable to userspace. This
+ * ensures if we get out to userspace with the
+ * wrong CR3 value, userspace will crash
+ * instead of running.
+ */
+ pgd.pgd |= _PAGE_NX;
+ }
+ } else if (pgd_userspace_access(*pgdp)) {
+ /*
+ * We are clearing a _PAGE_USER PGD for which we
+ * presumably populated the shadow. We must now
+ * clear the shadow PGD entry.
+ */
+ if (pgdp_maps_userspace(pgdp)) {
+ kernel_to_shadow_pgdp(pgdp)->pgd = pgd.pgd;
+ } else {
+ /*
+ * Attempted to clear a _PAGE_USER PGD which
+ * is in the kernel porttion of the address
+ * space. PGDs are pre-populated and we
+ * never clear them.
+ */
+ WARN_ON_ONCE(1);
+ }
+ } else {
+ /*
+ * _PAGE_USER was not set in either the PGD being set
+ * or cleared. All kernel PGDs should be
+ * pre-populated so this should never happen after
+ * boot.
+ */
+ }
+#endif
+ /* return the copy of the PGD we want the kernel to use: */
+ return pgd;
+}
+
+
static inline void native_set_p4d(p4d_t *p4dp, p4d_t p4d)
{
+#if defined(CONFIG_KAISER) && !defined(CONFIG_X86_5LEVEL)
+ p4dp->pgd = kaiser_set_shadow_pgd(&p4dp->pgd, p4d.pgd);
+#else /* CONFIG_KAISER */
*p4dp = p4d;
+#endif
}

static inline void native_p4d_clear(p4d_t *p4d)
@@ -147,7 +275,11 @@ static inline void native_p4d_clear(p4d_

static inline void native_set_pgd(pgd_t *pgdp, pgd_t pgd)
{
+#ifdef CONFIG_KAISER
+ *pgdp = kaiser_set_shadow_pgd(pgdp, pgd);
+#else /* CONFIG_KAISER */
*pgdp = pgd;
+#endif
}

static inline void native_pgd_clear(pgd_t *pgd)
diff -puN arch/x86/include/asm/pgtable.h~kaiser-base arch/x86/include/asm/pgtable.h
--- a/arch/x86/include/asm/pgtable.h~kaiser-base 2017-11-22 15:45:46.531619745 -0800
+++ b/arch/x86/include/asm/pgtable.h 2017-11-22 15:45:46.549619745 -0800
@@ -1106,6 +1106,11 @@ static inline void pmdp_set_wrprotect(st
static inline void clone_pgd_range(pgd_t *dst, pgd_t *src, int count)
{
memcpy(dst, src, count * sizeof(pgd_t));
+#ifdef CONFIG_KAISER
+ /* Clone the shadow pgd part as well */
+ memcpy(kernel_to_shadow_pgdp(dst), kernel_to_shadow_pgdp(src),
+ count * sizeof(pgd_t));
+#endif
}

#define PTE_SHIFT ilog2(PTRS_PER_PTE)
diff -puN arch/x86/kernel/espfix_64.c~kaiser-base arch/x86/kernel/espfix_64.c
--- a/arch/x86/kernel/espfix_64.c~kaiser-base 2017-11-22 15:45:46.533619745 -0800
+++ b/arch/x86/kernel/espfix_64.c 2017-11-22 15:45:46.549619745 -0800
@@ -41,6 +41,7 @@
#include <asm/pgalloc.h>
#include <asm/setup.h>
#include <asm/espfix.h>
+#include <asm/kaiser.h>

/*
* Note: we only need 6*8 = 48 bytes for the espfix stack, but round
@@ -128,6 +129,22 @@ void __init init_espfix_bsp(void)
pgd = &init_top_pgt[pgd_index(ESPFIX_BASE_ADDR)];
p4d = p4d_alloc(&init_mm, pgd, ESPFIX_BASE_ADDR);
p4d_populate(&init_mm, p4d, espfix_pud_page);
+ /*
+ * Just copy the top-level PGD that is mapping the espfix
+ * area to ensure it is mapped into the shadow user page
+ * tables.
+ *
+ * For 5-level paging, the espfix pgd was populated when
+ * kaiser_init() pre-populated all the pgd entries. The above
+ * p4d_alloc() would never do anything and the p4d_populate()
+ * would be done to a p4d already mapped in the userspace pgd.
+ */
+#ifdef CONFIG_KAISER
+ if (CONFIG_PGTABLE_LEVELS <= 4) {
+ set_pgd(kernel_to_shadow_pgdp(pgd),
+ __pgd(_KERNPG_TABLE | (p4d_pfn(*p4d) << PAGE_SHIFT)));
+ }
+#endif

/* Randomize the locations */
init_espfix_random();
diff -puN arch/x86/kernel/head_64.S~kaiser-base arch/x86/kernel/head_64.S
--- a/arch/x86/kernel/head_64.S~kaiser-base 2017-11-22 15:45:46.534619745 -0800
+++ b/arch/x86/kernel/head_64.S 2017-11-22 15:45:46.549619745 -0800
@@ -341,6 +341,14 @@ GLOBAL(early_recursion_flag)
.balign PAGE_SIZE; \
GLOBAL(name)

+#ifdef CONFIG_KAISER
+#define NEXT_PGD_PAGE(name) \
+ .balign 2 * PAGE_SIZE; \
+GLOBAL(name)
+#else
+#define NEXT_PGD_PAGE(name) NEXT_PAGE(name)
+#endif
+
/* Automate the creation of 1 to 1 mapping pmd entries */
#define PMDS(START, PERM, COUNT) \
i = 0 ; \
@@ -350,7 +358,7 @@ GLOBAL(name)
.endr

__INITDATA
-NEXT_PAGE(early_top_pgt)
+NEXT_PGD_PAGE(early_top_pgt)
.fill 511,8,0
#ifdef CONFIG_X86_5LEVEL
.quad level4_kernel_pgt - __START_KERNEL_map + _PAGE_TABLE_NOENC
@@ -364,7 +372,7 @@ NEXT_PAGE(early_dynamic_pgts)
.data

#if defined(CONFIG_XEN_PV) || defined(CONFIG_XEN_PVH)
-NEXT_PAGE(init_top_pgt)
+NEXT_PGD_PAGE(init_top_pgt)
.quad level3_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE_NOENC
.org init_top_pgt + PGD_PAGE_OFFSET*8, 0
.quad level3_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE_NOENC
@@ -381,7 +389,7 @@ NEXT_PAGE(level2_ident_pgt)
*/
PMDS(0, __PAGE_KERNEL_IDENT_LARGE_EXEC, PTRS_PER_PMD)
#else
-NEXT_PAGE(init_top_pgt)
+NEXT_PGD_PAGE(init_top_pgt)
.fill 512,8,0
#endif

diff -puN /dev/null arch/x86/mm/kaiser.c
--- /dev/null 2017-11-06 07:51:38.702108459 -0800
+++ b/arch/x86/mm/kaiser.c 2017-11-22 15:45:46.550619745 -0800
@@ -0,0 +1,441 @@
+/*
+ * 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 <richard.fellner@xxxxxxxxxxxxxxxxx>
+ * Signed-off-by: Moritz Lipp <moritz.lipp@xxxxxxxxxxxxxx>
+ * Signed-off-by: Daniel Gruss <daniel.gruss@xxxxxxxxxxxxxx>
+ * Signed-off-by: Michael Schwarz <michael.schwarz@xxxxxxxxxxxxxx>
+ *
+ * Major changes to the original code by: Dave Hansen <dave.hansen@xxxxxxxxx>
+ */
+#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 <asm/kaiser.h>
+#include <asm/pgtable.h>
+#include <asm/pgalloc.h>
+#include <asm/tlbflush.h>
+#include <asm/desc.h>
+
+#define KAISER_WALK_ATOMIC 0x1
+
+/*
+ * At runtime, the only things we map are some things for CPU
+ * hotplug, and stacks for new processes. No two CPUs will ever
+ * be populating the same addresses, so we only need to ensure
+ * that we protect between two CPUs trying to allocate and
+ * populate the same page table page.
+ *
+ * Only take this lock when doing a set_p[4um]d(), but it is not
+ * needed for doing a set_pte(). We assume that only the *owner*
+ * of a given allocation will be doing this for _their_
+ * allocation.
+ *
+ * This ensures that once a system has been running for a while
+ * and there have been stacks all over and these page tables
+ * are fully populated, there will be no further acquisitions of
+ * this lock.
+ */
+static DEFINE_SPINLOCK(shadow_table_allocation_lock);
+
+/*
+ * This is only for walking kernel addresses. We use it to help
+ * recreate the "shadow" page tables which are used while we are in
+ * userspace.
+ *
+ * This can be called on any kernel memory addresses and will work
+ * with any page sizes and any types: normal linear map memory,
+ * vmalloc(), even kmap().
+ *
+ * Note: this is only used when mapping new *kernel* entries into
+ * the user/shadow page tables. It is never used for userspace
+ * addresses.
+ *
+ * Returns -1 on error.
+ */
+static inline unsigned long get_pa_from_kernel_map(unsigned long vaddr)
+{
+ pgd_t *pgd;
+ p4d_t *p4d;
+ pud_t *pud;
+ pmd_t *pmd;
+ pte_t *pte;
+
+ /* We should only be asked to walk kernel addresses */
+ if (vaddr < PAGE_OFFSET) {
+ WARN_ON_ONCE(1);
+ return -1;
+ }
+
+ pgd = pgd_offset_k(vaddr);
+ /*
+ * We made all the kernel PGDs present in kaiser_init().
+ * We expect them to stay that way.
+ */
+ if (pgd_none(*pgd)) {
+ WARN_ON_ONCE(1);
+ return -1;
+ }
+ /*
+ * PGDs are either 512GB or 128TB on all x86_64
+ * configurations. We don't handle these.
+ */
+ BUILD_BUG_ON(pgd_large(*pgd) != 0);
+
+ p4d = p4d_offset(pgd, vaddr);
+ if (p4d_none(*p4d)) {
+ WARN_ON_ONCE(1);
+ return -1;
+ }
+
+ pud = pud_offset(p4d, vaddr);
+ if (pud_none(*pud)) {
+ WARN_ON_ONCE(1);
+ return -1;
+ }
+
+ if (pud_large(*pud))
+ return (pud_pfn(*pud) << PAGE_SHIFT) | (vaddr & ~PUD_PAGE_MASK);
+
+ pmd = pmd_offset(pud, vaddr);
+ if (pmd_none(*pmd)) {
+ WARN_ON_ONCE(1);
+ return -1;
+ }
+
+ if (pmd_large(*pmd))
+ return (pmd_pfn(*pmd) << PAGE_SHIFT) | (vaddr & ~PMD_PAGE_MASK);
+
+ pte = pte_offset_kernel(pmd, vaddr);
+ if (pte_none(*pte)) {
+ WARN_ON_ONCE(1);
+ return -1;
+ }
+
+ return (pte_pfn(*pte) << PAGE_SHIFT) | (vaddr & ~PAGE_MASK);
+}
+
+/*
+ * Walk the shadow copy of the page tables (optionally) trying to
+ * allocate page table pages on the way down. Does not support
+ * large pages since the data we are mapping is (generally) not
+ * large enough or aligned to 2MB.
+ *
+ * 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 *kaiser_shadow_pagetable_walk(unsigned long address,
+ unsigned long flags)
+{
+ pte_t *pte;
+ pmd_t *pmd;
+ pud_t *pud;
+ p4d_t *p4d;
+ pgd_t *pgd = kernel_to_shadow_pgdp(pgd_offset_k(address));
+ gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
+
+ if (flags & KAISER_WALK_ATOMIC) {
+ gfp &= ~GFP_KERNEL;
+ gfp |= __GFP_HIGH | __GFP_ATOMIC;
+ }
+
+ if (address < PAGE_OFFSET) {
+ WARN_ONCE(1, "attempt to walk user address\n");
+ return NULL;
+ }
+
+ if (pgd_none(*pgd)) {
+ WARN_ONCE(1, "All shadow pgds should have been populated\n");
+ return NULL;
+ }
+ BUILD_BUG_ON(pgd_large(*pgd) != 0);
+
+ p4d = p4d_offset(pgd, address);
+ 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;
+
+ spin_lock(&shadow_table_allocation_lock);
+ if (p4d_none(*p4d))
+ set_p4d(p4d, __p4d(_KERNPG_TABLE | __pa(new_pud_page)));
+ else
+ free_page(new_pud_page);
+ spin_unlock(&shadow_table_allocation_lock);
+ }
+
+ pud = pud_offset(p4d, address);
+ /* The shadow 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;
+
+ spin_lock(&shadow_table_allocation_lock);
+ if (pud_none(*pud))
+ set_pud(pud, __pud(_KERNPG_TABLE | __pa(new_pmd_page)));
+ else
+ free_page(new_pmd_page);
+ spin_unlock(&shadow_table_allocation_lock);
+ }
+
+ pmd = pmd_offset(pud, address);
+ /* The shadow page tables do not use large mappings: */
+ 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;
+
+ spin_lock(&shadow_table_allocation_lock);
+ if (pmd_none(*pmd))
+ set_pmd(pmd, __pmd(_KERNPG_TABLE | __pa(new_pte_page)));
+ else
+ free_page(new_pte_page);
+ spin_unlock(&shadow_table_allocation_lock);
+ }
+
+ 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;
+}
+
+/*
+ * Given a kernel address, @__start_addr, copy that mapping into
+ * the user (shadow) page tables. This may need to allocate page
+ * table pages.
+ */
+int kaiser_add_user_map(const void *__start_addr, unsigned long size,
+ unsigned long flags)
+{
+ pte_t *pte;
+ unsigned long start_addr = (unsigned long)__start_addr;
+ unsigned long address = start_addr & PAGE_MASK;
+ unsigned long end_addr = PAGE_ALIGN(start_addr + size);
+ unsigned long target_address;
+
+ for (; address < end_addr; address += PAGE_SIZE) {
+ target_address = get_pa_from_kernel_map(address);
+ if (target_address == -1)
+ return -EIO;
+
+ pte = kaiser_shadow_pagetable_walk(address, false);
+ /*
+ * Errors come from either -ENOMEM for a page
+ * table page, or something screwy that did a
+ * WARN_ON(). Just return -ENOMEM.
+ */
+ if (!pte)
+ return -ENOMEM;
+ if (pte_none(*pte)) {
+ set_pte(pte, __pte(flags | target_address));
+ } else {
+ pte_t tmp;
+ /*
+ * Make a fake, temporary PTE that mimics the
+ * one we would have created.
+ */
+ set_pte(&tmp, __pte(flags | target_address));
+ /*
+ * Warn if the pte that would have been
+ * created is different from the one that
+ * was there previously. In other words,
+ * we allow the same PTE value to be set,
+ * but not changed.
+ */
+ WARN_ON_ONCE(!pte_same(*pte, tmp));
+ }
+ }
+ return 0;
+}
+
+int kaiser_add_user_map_ptrs(const void *__start_addr,
+ const void *__end_addr,
+ unsigned long flags)
+{
+ return kaiser_add_user_map(__start_addr,
+ __end_addr - __start_addr,
+ flags);
+}
+
+/*
+ * Ensure that the top level of the (shadow) page tables are
+ * entirely populated. This ensures that all processes that get
+ * forked have the same entries. This way, we do not have to
+ * ever go set up new entries in older processes.
+ *
+ * Note: we never free these, so there are no updates to them
+ * after this.
+ */
+static void __init kaiser_init_all_pgds(void)
+{
+ pgd_t *pgd;
+ int i;
+
+ pgd = kernel_to_shadow_pgdp(pgd_offset_k(0UL));
+ for (i = PTRS_PER_PGD / 2; i < PTRS_PER_PGD; i++) {
+ /*
+ * Each PGD entry moves up PGDIR_SIZE bytes through
+ * the address space, so get the first virtual
+ * address mapped by PGD #i:
+ */
+ unsigned long addr = i * PGDIR_SIZE;
+#if CONFIG_PGTABLE_LEVELS > 4
+ p4d_t *p4d = p4d_alloc_one(&init_mm, addr);
+ if (!p4d) {
+ WARN_ON(1);
+ break;
+ }
+ set_pgd(pgd + i, __pgd(_KERNPG_TABLE | __pa(p4d)));
+#else /* CONFIG_PGTABLE_LEVELS <= 4 */
+ pud_t *pud = pud_alloc_one(&init_mm, addr);
+ if (!pud) {
+ WARN_ON(1);
+ break;
+ }
+ set_pgd(pgd + i, __pgd(_KERNPG_TABLE | __pa(pud)));
+#endif /* CONFIG_PGTABLE_LEVELS */
+ }
+}
+
+/*
+ * Page table allocations called by kaiser_add_user_map() can
+ * theoretically fail, but are very unlikely to fail in early boot.
+ * This would at least output a warning before crashing.
+ *
+ * Do the checking and warning in a macro to make it more readable and
+ * preserve line numbers in the warning message that you would not get
+ * with an inline.
+ */
+#define kaiser_add_user_map_early(start, size, flags) do { \
+ int __ret = kaiser_add_user_map(start, size, flags); \
+ WARN_ON(__ret); \
+} while (0)
+
+#define kaiser_add_user_map_ptrs_early(start, end, flags) do { \
+ int __ret = kaiser_add_user_map_ptrs(start, end, flags); \
+ WARN_ON(__ret); \
+} while (0)
+
+extern char __per_cpu_user_mapped_start[], __per_cpu_user_mapped_end[];
+/*
+ * If anything in here fails, we will likely die on one of the
+ * first kernel->user transitions and init will die. But, we
+ * will have most of the kernel up by then and should be able to
+ * get a clean warning out of it. If we BUG_ON() here, we run
+ * the risk of being before we have good console output.
+ *
+ * When KAISER is enabled, we remove _PAGE_GLOBAL from all of the
+ * kernel PTE permissions. This ensures that the TLB entries for
+ * the kernel are not available when in userspace. However, for
+ * the pages that are available to userspace *anyway*, we might as
+ * well continue to map them _PAGE_GLOBAL and enjoy the potential
+ * performance advantages.
+ */
+void __init kaiser_init(void)
+{
+ int cpu;
+
+ kaiser_init_all_pgds();
+
+ for_each_possible_cpu(cpu) {
+ void *percpu_vaddr = __per_cpu_user_mapped_start +
+ per_cpu_offset(cpu);
+ unsigned long percpu_sz = __per_cpu_user_mapped_end -
+ __per_cpu_user_mapped_start;
+ kaiser_add_user_map_early(percpu_vaddr, percpu_sz,
+ __PAGE_KERNEL | _PAGE_GLOBAL);
+ }
+
+ kaiser_add_user_map_ptrs_early(__entry_text_start, __entry_text_end,
+ __PAGE_KERNEL_RX | _PAGE_GLOBAL);
+
+ /* the fixed map address of the idt_table */
+ kaiser_add_user_map_early((void *)idt_descr.address,
+ sizeof(gate_desc) * NR_VECTORS,
+ __PAGE_KERNEL_RO | _PAGE_GLOBAL);
+}
+
+int kaiser_add_mapping(unsigned long addr, unsigned long size,
+ unsigned long flags)
+{
+ return kaiser_add_user_map((const void *)addr, size, flags);
+}
+
+void kaiser_remove_mapping(unsigned long start, unsigned long size)
+{
+ unsigned long addr;
+
+ /* The shadow page tables always use small pages: */
+ for (addr = start; addr < start + size; addr += PAGE_SIZE) {
+ /*
+ * Do an "atomic" walk in case this got called from an atomic
+ * context. This should not do any allocations because we
+ * should only be walking things that are known to be mapped.
+ */
+ pte_t *pte = kaiser_shadow_pagetable_walk(addr, KAISER_WALK_ATOMIC);
+
+ /*
+ * We are removing a mapping that should
+ * exist. WARN if it was not there:
+ */
+ if (!pte) {
+ WARN_ON_ONCE(1);
+ continue;
+ }
+
+ pte_clear(&init_mm, addr, pte);
+ }
+ /*
+ * This ensures that the TLB entries used to map this data are
+ * no longer usable on *this* CPU. We theoretically want to
+ * flush the entries on all CPUs here, but that's too
+ * expensive right now: this is called to unmap process
+ * stacks in the exit() path.
+ *
+ * This can change if we get to the point where this is not
+ * in a remotely hot path, like only called via write_ldt().
+ *
+ * Note: we could probably also just invalidate the individual
+ * addresses to take care of *this* PCID and then do a
+ * tlb_flush_shared_nonglobals() to ensure that all other
+ * PCIDs get flushed before being used again.
+ */
+ __native_flush_tlb_global();
+}
diff -puN arch/x86/mm/Makefile~kaiser-base arch/x86/mm/Makefile
--- a/arch/x86/mm/Makefile~kaiser-base 2017-11-22 15:45:46.536619745 -0800
+++ b/arch/x86/mm/Makefile 2017-11-22 15:45:46.550619745 -0800
@@ -46,6 +46,7 @@ obj-$(CONFIG_NUMA_EMU) += numa_emulatio
obj-$(CONFIG_X86_INTEL_MPX) += mpx.o
obj-$(CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS) += pkeys.o
obj-$(CONFIG_RANDOMIZE_MEMORY) += kaslr.o
+obj-$(CONFIG_KAISER) += kaiser.o

obj-$(CONFIG_AMD_MEM_ENCRYPT) += mem_encrypt.o
obj-$(CONFIG_AMD_MEM_ENCRYPT) += mem_encrypt_boot.o
diff -puN arch/x86/mm/pageattr.c~kaiser-base arch/x86/mm/pageattr.c
--- a/arch/x86/mm/pageattr.c~kaiser-base 2017-11-22 15:45:46.538619745 -0800
+++ b/arch/x86/mm/pageattr.c 2017-11-22 15:45:46.551619745 -0800
@@ -859,7 +859,7 @@ static void unmap_pmd_range(pud_t *pud,
pud_clear(pud);
}

-static void unmap_pud_range(p4d_t *p4d, unsigned long start, unsigned long end)
+void unmap_pud_range(p4d_t *p4d, unsigned long start, unsigned long end)
{
pud_t *pud = pud_offset(p4d, start);

diff -puN arch/x86/mm/pgtable.c~kaiser-base arch/x86/mm/pgtable.c
--- a/arch/x86/mm/pgtable.c~kaiser-base 2017-11-22 15:45:46.540619745 -0800
+++ b/arch/x86/mm/pgtable.c 2017-11-22 15:45:46.551619745 -0800
@@ -355,14 +355,26 @@ static inline void _pgd_free(pgd_t *pgd)
kmem_cache_free(pgd_cache, pgd);
}
#else
+
+#ifdef CONFIG_KAISER
+/*
+ * Instead of one pgd, we aquire two pgds. Being order-1, it is
+ * both 8k in size and 8k-aligned. That lets us just flip bit 12
+ * in a pointer to swap between the two 4k halves.
+ */
+#define PGD_ALLOCATION_ORDER 1
+#else
+#define PGD_ALLOCATION_ORDER 0
+#endif
+
static inline pgd_t *_pgd_alloc(void)
{
- return (pgd_t *)__get_free_page(PGALLOC_GFP);
+ return (pgd_t *)__get_free_pages(PGALLOC_GFP, PGD_ALLOCATION_ORDER);
}

static inline void _pgd_free(pgd_t *pgd)
{
- free_page((unsigned long)pgd);
+ free_pages((unsigned long)pgd, PGD_ALLOCATION_ORDER);
}
#endif /* CONFIG_X86_PAE */

diff -puN /dev/null Documentation/x86/kaiser.txt
--- /dev/null 2017-11-06 07:51:38.702108459 -0800
+++ b/Documentation/x86/kaiser.txt 2017-11-22 15:45:46.552619745 -0800
@@ -0,0 +1,162 @@
+Overview
+========
+
+KAISER is a countermeasure against attacks on kernel address
+information. There are at least three existing, published,
+approaches using the shared user/kernel mapping and hardware features
+to defeat KASLR. One approach referenced in the paper locates the
+kernel by observing differences in page fault timing between
+present-but-inaccessable kernel pages and non-present pages.
+
+When the kernel is entered via syscalls, interrupts or exceptions,
+page tables are switched to the full "kernel" copy. When the
+system switches back to user mode, the user/shadow copy is used.
+
+The minimalistic kernel portion of the user page tables try to
+map only what is needed to enter/exit the kernel such as the
+entry/exit functions themselves and the interrupt descriptor
+table (IDT). There are a few unnecessary things that get mapped
+such as the first C function when entering an interrupt (see
+comments in kaiser.c).
+
+This helps to ensure that side-channel attacks that leverage the
+paging structures do not function when KAISER is enabled. It can be
+enabled by setting CONFIG_KAISER=y
+
+Page Table Management
+=====================
+
+When KAISER is enabled, the kernel manages two sets of page
+tables. The first copy is very similar to what would be present
+for a kernel without KAISER. This includes a complete mapping of
+userspace that the kernel can use for things like copy_to_user().
+
+The second (shadow) is used when running userspace and mirrors the
+mapping of userspace present in the kernel copy. It maps a only
+the kernel data needed to enter and exit the kernel.
+
+The shadow is populated by the kaiser_add_*() functions. Only
+kernel data which has been explicity mapped will appear in the
+shadow copy. These calls are rare at runtime.
+
+For a new userspace mapping, the kernel makes the entries in its
+page tables like normal. The only difference is when the kernel
+makes entries in the top (PGD) level. In addition to setting the
+entry in the main kernel PGD, a copy if the entry is made in the
+shadow PGD.
+
+For user space mappings the kernel creates an entry in the kernel
+PGD and the same entry in the shadow PGD, so the underlying page
+table to which the PGD entry points is shared down to the PTE
+level. This leaves a single, shared set of userspace page tables
+to manage. One PTE to lock, one set set of accessed bits, dirty
+bits, etc...
+
+Overhead
+========
+
+Protection against side-channel attacks is important. But,
+this protection comes at a cost:
+
+1. Increased Memory Use
+ a. Each process now needs an order-1 PGD instead of order-0.
+ (Consumes 4k per process).
+ b. The pre-allocated second-level (p4d or pud) kernel page
+ table pages cost ~1MB of additional memory at boot. This
+ is not totally wasted because some of these pages would
+ have been needed eventually for normal kernel page tables
+ and things in the vmalloc() area like vmemmap[].
+ c. Statically-allocated structures and entry/exit text must
+ be padded out to 4k (or 8k for PGDs) so they can be mapped
+ into the user page tables. This bloats the kernel image
+ by ~20-30k.
+ d. The shadow page tables eventually grow to map all of used
+ vmalloc() space. They can have roughly the same memory
+ consumption as the vmalloc() page tables.
+
+2. Runtime Cost
+ a. CR3 manipulation to switch between the page table copies
+ must be done at interrupt, syscall, and exception entry
+ and exit (it can be skipped when the kernel is interrupted,
+ though.) Moves to CR3 are on the order of a hundred
+ cycles, and are required every at entry and every at exit.
+ b. Task stacks must be mapped/unmapped. We need to walk
+ and modify the shadow page tables at fork() and exit().
+ c. Global pages are disabled. This feature of the MMU
+ allows different processes to share TLB entries mapping
+ the kernel. Losing the feature means potentially more
+ TLB misses after a context switch.
+ d. Process Context IDentifiers (PCID) is a CPU feature that
+ allows us to skip flushing the entire TLB when switching
+ page tables. This makes switching the page tables (at
+ context switch, or kernel entry/exit) cheaper. But, on
+ systems with PCID support, the context switch code must flush
+ both the user and kernel entries out of the TLB, with an
+ INVPCID in addition to the CR3 write. This INVPCID is
+ generally slower than a CR3 write, but still on the order of
+ a hundred cycles.
+ e. The shadow page tables must be populated for each new
+ process. Even without KAISER, the shared kernel mappings
+ are created by copying top-level (PGD) entries into each
+ new process. But, with KAISER, there are now *two* kernel
+ mappings: one in the kernel page tables that maps everything
+ and one in the user/shadow page tables mapping the "minimal"
+ kernel. At fork(), a copy of the portion of the shadow PGD
+ that maps the minimal kernel structures is needed in
+ addition to the normal kernel PGD.
+ f. In addition to the fork()-time copying, there must also
+ be an update to the shadow PGD any time a set_pgd() is done
+ on a PGD used to map userspace. This ensures that the kernel
+ and user/shadow copies always map the same userspace
+ memory.
+ g. On systems without PCID support, each CR3 write flushes
+ the entire TLB. That means that each syscall, interrupt
+ or exception flushes the TLB.
+
+Possible Future Work:
+1. We can be more careful about not actually writing to CR3
+ unless its value is actually changed.
+2. Compress the user/shadow-mapped data to be mapped together
+ underneath a single PGD entry.
+3. Re-enable global pages, but use them for mappings in the
+ user/shadow page tables. This would allow the kernel to
+ take advantage of TLB entries that were established from
+ the user page tables. This might speed up the entry/exit
+ code or userspace since it will not have to reload all of
+ its TLB entries. However, its upside is limited by PCID
+ being used.
+4. Allow KAISER to enabled/disabled at runtime so folks can
+ run a single kernel image.
+
+Debugging:
+
+Bugs in KAISER cause a few different signatures of crashes
+that are worth noting here.
+
+ * Crashes in early boot, especially around CPU bringup. Bugs
+ in the trampoline code or mappings cause these.
+ * Crashes at the first interrupt. Caused by bugs in entry_64.S,
+ like screwing up a page table switch. Also caused by
+ incorrectly mapping the IRQ handler entry code.
+ * Crashes at the first NMI. The NMI code is separate from main
+ interrupt handlers and can have bugs that do not affect
+ normal interrupts. Also caused by incorrectly mapping NMI
+ code. NMIs that interrupt the entry code must be very
+ careful and can be the cause of crashes that show up when
+ running perf.
+ * Kernel crashes at the first exit to userspace. entry_64.S
+ bugs, or failing to map some of the exit code.
+ * Crashes at first interrupt that interrupts userspace. The paths
+ in entry_64.S that return to userspace are sometimes separate
+ from the ones that return to the kernel.
+ * Double faults: overflowing the kernel stack because of page
+ faults upon page faults. Caused by touching non-kaiser-mapped
+ data in the entry code, or forgetting to switch to kernel
+ CR3 before calling into C functions which are not kaiser-mapped.
+ * Failures of the selftests/x86 code. Usually a bug in one of the
+ more obscure corners of entry_64.S
+ * Userspace segfaults early in boot, sometimes manifesting
+ as mount(8) failing to mount the rootfs. These have
+ tended to be TLB invalidation issues. Usually invalidating
+ the wrong PCID, or otherwise missing an invalidation.
+
diff -puN /dev/null include/linux/kaiser.h
--- /dev/null 2017-11-06 07:51:38.702108459 -0800
+++ b/include/linux/kaiser.h 2017-11-22 15:45:46.552619745 -0800
@@ -0,0 +1,29 @@
+#ifndef _INCLUDE_KAISER_H
+#define _INCLUDE_KAISER_H
+
+#ifdef CONFIG_KAISER
+#include <asm/kaiser.h>
+#else
+
+/*
+ * These stubs are used whenever CONFIG_KAISER is off, which
+ * includes architectures that support KAISER, but have it
+ * disabled.
+ */
+
+static inline void kaiser_init(void)
+{
+}
+
+static inline void kaiser_remove_mapping(unsigned long start, unsigned long size)
+{
+}
+
+static inline int kaiser_add_mapping(unsigned long addr, unsigned long size,
+ unsigned long flags)
+{
+ return 0;
+}
+
+#endif /* !CONFIG_KAISER */
+#endif /* _INCLUDE_KAISER_H */
diff -puN init/main.c~kaiser-base init/main.c
--- a/init/main.c~kaiser-base 2017-11-22 15:45:46.542619745 -0800
+++ b/init/main.c 2017-11-22 15:45:46.552619745 -0800
@@ -76,6 +76,7 @@
#include <linux/slab.h>
#include <linux/perf_event.h>
#include <linux/ptrace.h>
+#include <linux/kaiser.h>
#include <linux/blkdev.h>
#include <linux/elevator.h>
#include <linux/sched_clock.h>
@@ -505,6 +506,8 @@ static void __init mm_init(void)
pgtable_init();
vmalloc_init();
ioremap_huge_init();
+ /* This just needs to be done before we first run userspace: */
+ kaiser_init();
}

asmlinkage __visible void __init start_kernel(void)
diff -puN kernel/fork.c~kaiser-base kernel/fork.c
--- a/kernel/fork.c~kaiser-base 2017-11-22 15:45:46.544619745 -0800
+++ b/kernel/fork.c 2017-11-22 15:45:46.553619745 -0800
@@ -70,6 +70,7 @@
#include <linux/tsacct_kern.h>
#include <linux/cn_proc.h>
#include <linux/freezer.h>
+#include <linux/kaiser.h>
#include <linux/delayacct.h>
#include <linux/taskstats_kern.h>
#include <linux/random.h>
_

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