[PATCH RFC v2 04/29] mm: asi: Add infrastructure for boot-time enablement

Brendan Jackman
2025-01-10T18:40:57Z

Add a boot time parameter to control the newly added X86_FEATURE_ASI.
"asi=on" or "asi=off" can be used in the kernel command line to enable
or disable ASI at boot time. If not specified, ASI enablement depends
on CONFIG_ADDRESS_SPACE_ISOLATION_DEFAULT_ON, which is off by default.
asi_check_boottime_disable() is modeled after
pti_check_boottime_disable().

The boot parameter is currently ignored until ASI is fully functional.

Once we have a set of ASI features checked in that we have actually
tested, we will stop ignoring the flag. But for now let's just add the
infrastructure so we can implement the usage code.

Ignoring checkpatch.pl CONFIG_DESCRIPTION because the _DEFAULT_ON
Kconfig is trivial to explain.

Checkpatch-args: --ignore CONFIG_DESCRIPTION
Co-developed-by: Junaid Shahid <junaids@google.com>
Signed-off-by: Junaid Shahid <junaids@google.com>
Co-developed-by: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Brendan Jackman <jackmanb@google.com>
---
arch/x86/Kconfig | 9 +++++
arch/x86/include/asm/asi.h | 19 ++++++++--
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/include/asm/disabled-features.h | 8 ++++-
arch/x86/mm/asi.c | 61 +++++++++++++++++++++++++++-----
arch/x86/mm/init.c | 4 ++-
include/asm-generic/asi.h | 4 +++
7 files changed, 92 insertions(+), 14 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 5a50582eb210e9d1309856a737d32b76fa1bfc85..1fcb52cb8cd5084ac3cef04af61b7d1653162bdb 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2533,6 +2533,15 @@ config MITIGATION_ADDRESS_SPACE_ISOLATION
there are likely to be unhandled cases, in particular concerning TLB
flushes.

+
+config ADDRESS_SPACE_ISOLATION_DEFAULT_ON
+ bool "Enable address space isolation by default"
+ default n
+ depends on MITIGATION_ADDRESS_SPACE_ISOLATION
+ help
+ If selected, ASI is enabled by default at boot if the asi=on or
+ asi=off are not specified.
+
config MITIGATION_RETPOLINE
bool "Avoid speculative indirect branches in kernel"
select OBJTOOL if HAVE_OBJTOOL
diff --git a/arch/x86/include/asm/asi.h b/arch/x86/include/asm/asi.h
index 7cc635b6653a3970ba9dbfdc9c828a470e27bd44..b9671ef2dd3278adceed18507fd260e21954d574 100644
--- a/arch/x86/include/asm/asi.h
+++ b/arch/x86/include/asm/asi.h
@@ -8,6 +8,7 @@

#include <asm/pgtable_types.h>
#include <asm/percpu.h>
+#include <asm/cpufeature.h>
#include <asm/processor.h>

#ifdef CONFIG_MITIGATION_ADDRESS_SPACE_ISOLATION
@@ -66,6 +67,8 @@
* the N ASI classes.
*/

+#define static_asi_enabled() cpu_feature_enabled(X86_FEATURE_ASI)
+
/*
* ASI uses a per-CPU tainting model to track what mitigation actions are
* required on domain transitions. Taints exist along two dimensions:
@@ -131,6 +134,8 @@ struct asi {

DECLARE_PER_CPU_ALIGNED(struct asi *, curr_asi);

+void asi_check_boottime_disable(void);
+
void asi_init_mm_state(struct mm_struct *mm);

int asi_init_class(enum asi_class_id class_id, struct asi_taint_policy *taint_policy);
@@ -155,7 +160,9 @@ void asi_exit(void);
/* The target is the domain we'll enter when returning to process context. */
static __always_inline struct asi *asi_get_target(struct task_struct *p)
{
- return p->thread.asi_state.target;
+ return static_asi_enabled()
+ ? p->thread.asi_state.target
+ : NULL;
}

static __always_inline void asi_set_target(struct task_struct *p,
@@ -166,7 +173,9 @@ static __always_inline void asi_set_target(struct task_struct *p,

static __always_inline struct asi *asi_get_current(void)
{
- return this_cpu_read(curr_asi);
+ return static_asi_enabled()
+ ? this_cpu_read(curr_asi)
+ : NULL;
}

/* Are we currently in a restricted address space? */
@@ -175,7 +184,11 @@ static __always_inline bool asi_is_restricted(void)
return (bool)asi_get_current();
}

-/* If we exit/have exited, can we stay that way until the next asi_enter? */
+/*
+ * If we exit/have exited, can we stay that way until the next asi_enter?
+ *
+ * When ASI is disabled, this returns true.
+ */
static __always_inline bool asi_is_relaxed(void)
{
return !asi_get_target(current);
diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 913fd3a7bac6506141de65f33b9ee61c615c7d7d..d6a808d10c3b8900d190ea01c66fc248863f05e2 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -474,6 +474,7 @@
#define X86_FEATURE_CLEAR_BHB_HW (21*32+ 3) /* BHI_DIS_S HW control enabled */
#define X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT (21*32+ 4) /* Clear branch history at vmexit using SW loop */
#define X86_FEATURE_FAST_CPPC (21*32 + 5) /* AMD Fast CPPC */
+#define X86_FEATURE_ASI (21*32+6) /* Kernel Address Space Isolation */

/*
* BUG word(s)
diff --git a/arch/x86/include/asm/disabled-features.h b/arch/x86/include/asm/disabled-features.h
index c492bdc97b0595ec77f89dc9b0cefe5e3e64be41..c7964ed4fef8b9441e1c0453da587787d8008d9d 100644
--- a/arch/x86/include/asm/disabled-features.h
+++ b/arch/x86/include/asm/disabled-features.h
@@ -50,6 +50,12 @@
# define DISABLE_PTI (1 << (X86_FEATURE_PTI & 31))
#endif

+#ifdef CONFIG_MITIGATION_ADDRESS_SPACE_ISOLATION
+# define DISABLE_ASI 0
+#else
+# define DISABLE_ASI (1 << (X86_FEATURE_ASI & 31))
+#endif
+
#ifdef CONFIG_MITIGATION_RETPOLINE
# define DISABLE_RETPOLINE 0
#else
@@ -154,7 +160,7 @@
#define DISABLED_MASK17 0
#define DISABLED_MASK18 (DISABLE_IBT)
#define DISABLED_MASK19 (DISABLE_SEV_SNP)
-#define DISABLED_MASK20 0
+#define DISABLED_MASK20 (DISABLE_ASI)
#define DISABLED_MASK21 0
#define DISABLED_MASK_CHECK BUILD_BUG_ON_ZERO(NCAPINTS != 22)

diff --git a/arch/x86/mm/asi.c b/arch/x86/mm/asi.c
index 105cd8b43eaf5c20acc80d4916b761559fb95d74..5baf563a078f5b3a6cd4b9f5e92baaf81b0774c4 100644
--- a/arch/x86/mm/asi.c
+++ b/arch/x86/mm/asi.c
@@ -4,6 +4,7 @@
#include <linux/percpu.h>
#include <linux/spinlock.h>

+#include <linux/init.h>
#include <asm/asi.h>
#include <asm/cmdline.h>
#include <asm/cpufeature.h>
@@ -29,6 +30,9 @@ static inline bool asi_class_id_valid(enum asi_class_id class_id)

static inline bool asi_class_initialized(enum asi_class_id class_id)
{
+ if (!boot_cpu_has(X86_FEATURE_ASI))
+ return 0;
+
if (WARN_ON(!asi_class_id_valid(class_id)))
return false;

@@ -51,6 +55,9 @@ EXPORT_SYMBOL_GPL(asi_init_class);

void asi_uninit_class(enum asi_class_id class_id)
{
+ if (!boot_cpu_has(X86_FEATURE_ASI))
+ return;
+
if (!asi_class_initialized(class_id))
return;

@@ -66,10 +73,36 @@ const char *asi_class_name(enum asi_class_id class_id)
return asi_class_names[class_id];
}

+void __init asi_check_boottime_disable(void)
+{
+ bool enabled = IS_ENABLED(CONFIG_MITIGATION_ADDRESS_SPACE_ISOLATION_DEFAULT_ON);
+ char arg[4];
+ int ret;
+
+ ret = cmdline_find_option(boot_command_line, "asi", arg, sizeof(arg));
+ if (ret == 3 && !strncmp(arg, "off", 3)) {
+ enabled = false;
+ pr_info("ASI disabled through kernel command line.\n");
+ } else if (ret == 2 && !strncmp(arg, "on", 2)) {
+ enabled = true;
+ pr_info("Ignoring asi=on param while ASI implementation is incomplete.\n");
+ } else {
+ pr_info("ASI %s by default.\n",
+ enabled ? "enabled" : "disabled");
+ }
+
+ if (enabled)
+ pr_info("ASI enablement ignored due to incomplete implementation.\n");
+}
+
static void __asi_destroy(struct asi *asi)
{
- lockdep_assert_held(&asi->mm->asi_init_lock);
+ WARN_ON_ONCE(asi->ref_count <= 0);
+ if (--(asi->ref_count) > 0)
+ return;

+ free_pages((ulong)asi->pgd, PGD_ALLOCATION_ORDER);
+ memset(asi, 0, sizeof(struct asi));
}

int asi_init(struct mm_struct *mm, enum asi_class_id class_id, struct asi **out_asi)
@@ -79,6 +112,9 @@ int asi_init(struct mm_struct *mm, enum asi_class_id class_id, struct asi **out_

*out_asi = NULL;

+ if (!boot_cpu_has(X86_FEATURE_ASI))
+ return 0;
+
if (WARN_ON(!asi_class_initialized(class_id)))
return -EINVAL;

@@ -122,7 +158,7 @@ void asi_destroy(struct asi *asi)
{
struct mm_struct *mm;

- if (!asi)
+ if (!boot_cpu_has(X86_FEATURE_ASI) || !asi)
return;

if (WARN_ON(!asi_class_initialized(asi->class_id)))
@@ -134,11 +170,7 @@ void asi_destroy(struct asi *asi)
* to block concurrent asi_init calls.
*/
mutex_lock(&mm->asi_init_lock);
- WARN_ON_ONCE(asi->ref_count <= 0);
- if (--(asi->ref_count) == 0) {
- free_pages((ulong)asi->pgd, PGD_ALLOCATION_ORDER);
- memset(asi, 0, sizeof(struct asi));
- }
+ __asi_destroy(asi);
mutex_unlock(&mm->asi_init_lock);
}
EXPORT_SYMBOL_GPL(asi_destroy);
@@ -255,6 +287,9 @@ static noinstr void __asi_enter(void)

noinstr void asi_enter(struct asi *asi)
{
+ if (!static_asi_enabled())
+ return;
+
VM_WARN_ON_ONCE(!asi);

/* Should not have an asi_enter() without a prior asi_relax(). */
@@ -269,8 +304,10 @@ EXPORT_SYMBOL_GPL(asi_enter);

noinstr void asi_relax(void)
{
- barrier();
- asi_set_target(current, NULL);
+ if (static_asi_enabled()) {
+ barrier();
+ asi_set_target(current, NULL);
+ }
}
EXPORT_SYMBOL_GPL(asi_relax);

@@ -279,6 +316,9 @@ noinstr void asi_exit(void)
u64 unrestricted_cr3;
struct asi *asi;

+ if (!static_asi_enabled())
+ return;
+
preempt_disable_notrace();

VM_BUG_ON(this_cpu_read(cpu_tlbstate.loaded_mm) ==
@@ -310,6 +350,9 @@ EXPORT_SYMBOL_GPL(asi_exit);

void asi_init_mm_state(struct mm_struct *mm)
{
+ if (!boot_cpu_has(X86_FEATURE_ASI))
+ return;
+
memset(mm->asi, 0, sizeof(mm->asi));
mutex_init(&mm->asi_init_lock);
}
diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index de4227ed5169ff84d0ce80b677caffc475198fa6..ded3a47f2a9c1f554824d4ad19f3b48bce271274 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -28,6 +28,7 @@
#include <asm/text-patching.h>
#include <asm/memtype.h>
#include <asm/paravirt.h>
+#include <asm/asi.h>

/*
* We need to define the tracepoints somewhere, and tlb.c
@@ -251,7 +252,7 @@ static void __init probe_page_size_mask(void)
__default_kernel_pte_mask = __supported_pte_mask;
/* Except when with PTI where the kernel is mostly non-Global: */
if (cpu_feature_enabled(X86_FEATURE_PTI) ||
- IS_ENABLED(CONFIG_MITIGATION_ADDRESS_SPACE_ISOLATION))
+ cpu_feature_enabled(X86_FEATURE_ASI))
__default_kernel_pte_mask &= ~_PAGE_GLOBAL;

/* Enable 1 GB linear kernel mappings if available: */
@@ -754,6 +755,7 @@ void __init init_mem_mapping(void)
unsigned long end;

pti_check_boottime_disable();
+ asi_check_boottime_disable();
probe_page_size_mask();
setup_pcid();

diff --git a/include/asm-generic/asi.h b/include/asm-generic/asi.h
index 6b84202837605fa57e4a910318c8353b3f816f06..eedc961ee916a9e1da631ca489ea4a7bc9e6089f 100644
--- a/include/asm-generic/asi.h
+++ b/include/asm-generic/asi.h
@@ -65,6 +65,10 @@ static inline pgd_t *asi_pgd(struct asi *asi) { return NULL; }

static inline void asi_handle_switch_mm(void) { }

+#define static_asi_enabled() false
+
+static inline void asi_check_boottime_disable(void) { }
+
#endif /* !CONFIG_MITIGATION_ADDRESS_SPACE_ISOLATION */

#endif /* !_ASSEMBLY_ */

--
2.47.1.613.gc27f4b7a9f-goog