[PATCH v5 2/6] kasan: Add KASAN mode kernel parameter

[Vincenzo Frascino]

Architectures supported by KASAN_HW_TAGS can provide a sync or async mode
of execution. On an MTE enabled arm64 hw for example this can be identified
with the synchronous or asynchronous tagging mode of execution.
In synchronous mode, an exception is triggered if a tag check fault occurs.
In asynchronous mode, if a tag check fault occurs, the TFSR_EL1 register is
updated asynchronously. The kernel checks the corresponding bits
periodically.

KASAN requires a specific kernel command line parameter to make use of this
hw features.

Add KASAN HW execution mode kernel command line parameter.

Note: This patch adds the kasan.mode kernel parameter and the
sync/async kernel command line options to enable the described features.

Cc: Dmitry Vyukov <dvyukov at google.com>
Cc: Andrey Ryabinin <aryabinin at virtuozzo.com>
Cc: Alexander Potapenko <glider at google.com>
Cc: Andrey Konovalov <andreyknvl at google.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino at arm.com>
---
 Documentation/dev-tools/kasan.rst |  7 +++++++
 lib/test_kasan.c                  |  2 +-
 mm/kasan/hw_tags.c                | 27 ++++++++++++++++++++++++++-
 mm/kasan/kasan.h                  |  6 ++++--
 4 files changed, 38 insertions(+), 4 deletions(-)

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index e022b7506e37..7e4a6e0c9f57 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -161,6 +161,13 @@ particular KASAN features.
 
 - ``kasan=off`` or ``=on`` controls whether KASAN is enabled (default: ``on``).
 
+- ``kasan.mode=sync`` or ``=async`` controls whether KASAN is configured in
+  synchronous or asynchronous mode of execution (default: ``sync``).
+  ``synchronous mode``: an exception is triggered if a tag check fault occurs.
+  ``asynchronous mode``: if a tag check fault occurs, the information is stored
+  asynchronously in hardware (e.g. in the TFSR_EL1 register for arm64). The kernel
+  checks the hardware location and reports an error if the fault is detected.
+
 - ``kasan.stacktrace=off`` or ``=on`` disables or enables alloc and free stack
   traces collection (default: ``on`` for ``CONFIG_DEBUG_KERNEL=y``, otherwise
   ``off``).
diff --git a/lib/test_kasan.c b/lib/test_kasan.c
index d16ec9e66806..7285dcf9fcc1 100644
--- a/lib/test_kasan.c
+++ b/lib/test_kasan.c
@@ -97,7 +97,7 @@ static void kasan_test_exit(struct kunit *test)
 			READ_ONCE(fail_data.report_found));	\
 	if (IS_ENABLED(CONFIG_KASAN_HW_TAGS)) {			\
 		if (READ_ONCE(fail_data.report_found))		\
-			hw_enable_tagging();			\
+			hw_enable_tagging_sync();		\
 		migrate_enable();				\
 	}							\
 } while (0)
diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
index e529428e7a11..224a2187839c 100644
--- a/mm/kasan/hw_tags.c
+++ b/mm/kasan/hw_tags.c
@@ -25,6 +25,11 @@ enum kasan_arg {
 	KASAN_ARG_ON,
 };
 
+enum kasan_arg_mode {
+	KASAN_ARG_MODE_SYNC,
+	KASAN_ARG_MODE_ASYNC,
+};
+
 enum kasan_arg_stacktrace {
 	KASAN_ARG_STACKTRACE_DEFAULT,
 	KASAN_ARG_STACKTRACE_OFF,
@@ -38,6 +43,7 @@ enum kasan_arg_fault {
 };
 
 static enum kasan_arg kasan_arg __ro_after_init;
+static enum kasan_arg_mode kasan_arg_mode __ro_after_init;
 static enum kasan_arg_stacktrace kasan_arg_stacktrace __ro_after_init;
 static enum kasan_arg_fault kasan_arg_fault __ro_after_init;
 
@@ -68,6 +74,21 @@ static int __init early_kasan_flag(char *arg)
 }
 early_param("kasan", early_kasan_flag);
 
+/* kasan.mode=sync/async */
+static int __init early_kasan_mode(char *arg)
+{
+	/* If arg is not set the default mode is sync */
+	if ((!arg) || !strcmp(arg, "sync"))
+		kasan_arg_mode = KASAN_ARG_MODE_SYNC;
+	else if (!strcmp(arg, "async"))
+		kasan_arg_mode = KASAN_ARG_MODE_ASYNC;
+	else
+		return -EINVAL;
+
+	return 0;
+}
+early_param("kasan.mode", early_kasan_mode);
+
 /* kasan.stacktrace=off/on */
 static int __init early_kasan_flag_stacktrace(char *arg)
 {
@@ -115,7 +136,11 @@ void kasan_init_hw_tags_cpu(void)
 		return;
 
 	hw_init_tags(KASAN_TAG_MAX);
-	hw_enable_tagging();
+
+	if (kasan_arg_mode == KASAN_ARG_MODE_ASYNC)
+		hw_enable_tagging_async();
+	else
+		hw_enable_tagging_sync();
 }
 
 /* kasan_init_hw_tags() is called once on boot CPU. */
diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index 07ef7fc742ad..3923d9744105 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h
@@ -294,7 +294,8 @@ static inline const void *arch_kasan_set_tag(const void *addr, u8 tag)
 #define arch_set_mem_tag_range(addr, size, tag) ((void *)(addr))
 #endif
 
-#define hw_enable_tagging()			arch_enable_tagging()
+#define hw_enable_tagging_sync()		arch_enable_tagging_sync()
+#define hw_enable_tagging_async()		arch_enable_tagging_async()
 #define hw_init_tags(max_tag)			arch_init_tags(max_tag)
 #define hw_set_tagging_report_once(state)	arch_set_tagging_report_once(state)
 #define hw_get_random_tag()			arch_get_random_tag()
@@ -303,7 +304,8 @@ static inline const void *arch_kasan_set_tag(const void *addr, u8 tag)
 
 #else /* CONFIG_KASAN_HW_TAGS */
 
-#define hw_enable_tagging()
+#define hw_enable_tagging_sync()
+#define hw_enable_tagging_async()
 #define hw_set_tagging_report_once(state)
 
 #endif /* CONFIG_KASAN_HW_TAGS */
-- 
2.30.0