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Subject[patch V2 5/5] x86/kaiser: Add boottime disable switch
KAISER comes with overhead. The most expensive part is the CR3 switching in
the entry code.

Add a command line parameter which allows to disable KAISER at boot time.

Most code pathes simply check a variable, but the entry code uses a static
branch. The other code pathes cannot use a static branch because they are
used before jump label patching is possible. Not an issue as the code
pathes are not so performance sensitive as the entry/exit code.

This makes KAISER depend on JUMP_LABEL and on a GCC which supports
it, but that's a resonable requirement.

The PGD allocation is still 8k when CONFIG_KAISER is enabled. This can be
addressed on top of this.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
arch/x86/entry/calling.h | 7 +++++++
arch/x86/include/asm/kaiser.h | 10 ++++++++++
arch/x86/include/asm/pgtable_64.h | 6 ++++++
arch/x86/mm/dump_pagetables.c | 5 ++++-
arch/x86/mm/init.c | 7 ++++---
arch/x86/mm/kaiser.c | 30 ++++++++++++++++++++++++++++++
security/Kconfig | 2 +-
7 files changed, 62 insertions(+), 5 deletions(-)

--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -210,18 +210,23 @@ For 32-bit we have the following convent
.endm

.macro SWITCH_TO_KERNEL_CR3 scratch_reg:req
+ STATIC_JUMP_IF_FALSE .Lend_\@, kaiser_enabled_key, def=1
mov %cr3, \scratch_reg
ADJUST_KERNEL_CR3 \scratch_reg
mov \scratch_reg, %cr3
+.Lend_\@:
.endm

.macro SWITCH_TO_USER_CR3 scratch_reg:req
+ STATIC_JUMP_IF_FALSE .Lend_\@, kaiser_enabled_key, def=1
mov %cr3, \scratch_reg
ADJUST_USER_CR3 \scratch_reg
mov \scratch_reg, %cr3
+.Lend_\@:
.endm

.macro SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg:req save_reg:req
+ STATIC_JUMP_IF_FALSE .Ldone_\@, kaiser_enabled_key, def=1
movq %cr3, %r\scratch_reg
movq %r\scratch_reg, \save_reg
/*
@@ -244,11 +249,13 @@ For 32-bit we have the following convent
.endm

.macro RESTORE_CR3 save_reg:req
+ STATIC_JUMP_IF_FALSE .Lend_\@, kaiser_enabled_key, def=1
/*
* The CR3 write could be avoided when not changing its value,
* but would require a CR3 read *and* a scratch register.
*/
movq \save_reg, %cr3
+.Lend_\@:
.endm

#else /* CONFIG_KAISER=n: */
--- a/arch/x86/include/asm/kaiser.h
+++ b/arch/x86/include/asm/kaiser.h
@@ -56,6 +56,16 @@ extern void kaiser_remove_mapping(unsign
*/
extern void kaiser_init(void);

+/* True if kaiser is enabled at boot time */
+extern struct static_key_true kaiser_enabled_key;
+extern bool kaiser_enabled;
+extern void kaiser_check_cmdline(void);
+
+#else /* CONFIG_KAISER */
+
+#define kaiser_enabled (false)
+static inline void kaiser_check_cmdline(void) { }
+
#endif

#endif /* __ASSEMBLY__ */
--- a/arch/x86/include/asm/pgtable_64.h
+++ b/arch/x86/include/asm/pgtable_64.h
@@ -175,6 +175,9 @@ static inline p4d_t *shadow_to_kernel_p4
{
return ptr_clear_bit(p4dp, KAISER_PGTABLE_SWITCH_BIT);
}
+
+extern bool kaiser_enabled;
+
#endif /* CONFIG_KAISER */

/*
@@ -208,6 +211,9 @@ static inline bool pgd_userspace_access(
static inline pgd_t kaiser_set_shadow_pgd(pgd_t *pgdp, pgd_t pgd)
{
#ifdef CONFIG_KAISER
+ if (!kaiser_enabled)
+ return pgd;
+
if (pgd_userspace_access(pgd)) {
if (pgdp_maps_userspace(pgdp)) {
/*
--- a/arch/x86/mm/dump_pagetables.c
+++ b/arch/x86/mm/dump_pagetables.c
@@ -20,6 +20,7 @@
#include <linux/seq_file.h>

#include <asm/pgtable.h>
+#include <asm/kaiser.h>

/*
* The dumper groups pagetable entries of the same type into one, and for
@@ -503,7 +504,7 @@ void ptdump_walk_pgd_level(struct seq_fi

void ptdump_walk_pgd_level_debugfs(struct seq_file *m, pgd_t *pgd, bool shadow)
{
- if (shadow)
+ if (shadow && kaiser_enabled)
pgd += PTRS_PER_PGD;
ptdump_walk_pgd_level_core(m, pgd, false, false);
}
@@ -514,6 +515,8 @@ void ptdump_walk_shadow_pgd_level_checkw
#ifdef CONFIG_KAISER
pgd_t *pgd = (pgd_t *) &init_top_pgt;

+ if (!kaiser_enabled)
+ return;
pr_info("x86/mm: Checking shadow page tables\n");
pgd += PTRS_PER_PGD;
ptdump_walk_pgd_level_core(NULL, pgd, true, false);
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -20,6 +20,7 @@
#include <asm/kaslr.h>
#include <asm/hypervisor.h>
#include <asm/cpufeature.h>
+#include <asm/kaiser.h>

/*
* We need to define the tracepoints somewhere, and tlb.c
@@ -163,9 +164,8 @@ static int page_size_mask;

static void enable_global_pages(void)
{
-#ifndef CONFIG_KAISER
- __supported_pte_mask |= _PAGE_GLOBAL;
-#endif
+ if (!kaiser_enabled)
+ __supported_pte_mask |= _PAGE_GLOBAL;
}

static void __init probe_page_size_mask(void)
@@ -656,6 +656,7 @@ void __init init_mem_mapping(void)
{
unsigned long end;

+ kaiser_check_cmdline();
probe_page_size_mask();
setup_pcid();

--- a/arch/x86/mm/kaiser.c
+++ b/arch/x86/mm/kaiser.c
@@ -34,6 +34,7 @@
#include <linux/mm.h>
#include <linux/uaccess.h>

+#include <asm/cmdline.h>
#include <asm/kaiser.h>
#include <asm/pgtable.h>
#include <asm/pgalloc.h>
@@ -44,6 +45,16 @@

static pteval_t kaiser_pte_mask __ro_after_init = ~(_PAGE_NX | _PAGE_GLOBAL);

+/* Global flag for boot time kaiser enable/disable */
+bool kaiser_enabled __ro_after_init = true;
+DEFINE_STATIC_KEY_TRUE(kaiser_enabled_key);
+
+void __init kaiser_check_cmdline(void)
+{
+ if (cmdline_find_option_bool(boot_command_line, "nokaiser"))
+ kaiser_enabled = false;
+}
+
/*
* At runtime, the only things we map are some things for CPU
* hotplug, and stacks for new processes. No two CPUs will ever
@@ -252,6 +263,9 @@ int kaiser_add_user_map(const void *__st
unsigned long target_address;
pte_t *pte;

+ if (!kaiser_enabled)
+ return 0;
+
/* Clear not supported bits */
flags &= kaiser_pte_mask;

@@ -402,6 +416,9 @@ void __init kaiser_init(void)
{
int cpu;

+ if (!kaiser_enabled)
+ return;
+
kaiser_init_all_pgds();

for_each_possible_cpu(cpu) {
@@ -436,6 +453,16 @@ void __init kaiser_init(void)
kaiser_add_mapping_cpu_entry(0);
}

+static int __init kaiser_boottime_control(void)
+{
+ if (!kaiser_enabled) {
+ static_branch_disable(&kaiser_enabled_key);
+ pr_info("kaiser: Disabled on command line\n");
+ }
+ return 0;
+}
+subsys_initcall(kaiser_boottime_control);
+
int kaiser_add_mapping(unsigned long addr, unsigned long size,
unsigned long flags)
{
@@ -446,6 +473,9 @@ void kaiser_remove_mapping(unsigned long
{
unsigned long addr;

+ if (!kaiser_enabled)
+ return;
+
/* The shadow page tables always use small pages: */
for (addr = start; addr < start + size; addr += PAGE_SIZE) {
/*
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -56,7 +56,7 @@ config SECURITY_NETWORK

config KAISER
bool "Remove the kernel mapping in user mode"
- depends on X86_64 && SMP && !PARAVIRT
+ depends on X86_64 && SMP && !PARAVIRT && JUMP_LABEL
help
This feature reduces the number of hardware side channels by
ensuring that the majority of kernel addresses are not mapped

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