kasan: dynamically allocate stack ring entries

Instead of using a large static array, allocate the stack ring dynamically
via memblock_alloc().

The size of the stack ring is controlled by a new kasan.stack_ring_size
command-line parameter.  When kasan.stack_ring_size is not provided, the
default value of 32 << 10 is used.

When the stack trace collection is disabled via kasan.stacktrace=off, the
stack ring is not allocated.

Link: https://lkml.kernel.org/r/03b82ab60db53427e9818e0b0c1971baa10c3cbc.1662411800.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Acked-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Peter Collingbourne <pcc@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/dev-tools/kasan.rst

@@ -112,10 +112,12 @@ parameter can be used to control panic and reporting behaviour:
   if ``kasan_multi_shot`` is enabled.
 
 Software and Hardware Tag-Based KASAN modes (see the section about various
-modes below) support disabling stack trace collection:
+modes below) support altering stack trace collection behavior:
 
 - ``kasan.stacktrace=off`` or ``=on`` disables or enables alloc and free stack
   traces collection (default: ``on``).
+- ``kasan.stack_ring_size=<number of entries>`` specifies the number of entries
+  in the stack ring (default: ``32768``).
 
 Hardware Tag-Based KASAN mode is intended for use in production as a security
 mitigation. Therefore, it supports additional boot parameters that allow

mm/kasan/kasan.h

@@ -252,12 +252,11 @@ struct kasan_stack_ring_entry {
 	bool is_free;
 };
 
-#define KASAN_STACK_RING_SIZE (32 << 10)
-
 struct kasan_stack_ring {
 	rwlock_t lock;
+	size_t size;
 	atomic64_t pos;
-	struct kasan_stack_ring_entry entries[KASAN_STACK_RING_SIZE];
+	struct kasan_stack_ring_entry *entries;
 };
 
 #endif /* CONFIG_KASAN_SW_TAGS || CONFIG_KASAN_HW_TAGS */

mm/kasan/report_tags.c

@@ -56,11 +56,11 @@ void kasan_complete_mode_report_info(struct kasan_report_info *info)
 	 * entries relevant to the buggy object can be overwritten.
 	 */
 
-	for (u64 i = pos - 1; i != pos - 1 - KASAN_STACK_RING_SIZE; i--) {
+	for (u64 i = pos - 1; i != pos - 1 - stack_ring.size; i--) {
 		if (alloc_found && free_found)
 			break;
 
-		entry = &stack_ring.entries[i % KASAN_STACK_RING_SIZE];
+		entry = &stack_ring.entries[i % stack_ring.size];
 
 		/* Paired with smp_store_release() in save_stack_info(). */
 		ptr = (void *)smp_load_acquire(&entry->ptr);

mm/kasan/tags.c

@@ -10,6 +10,7 @@
 #include <linux/init.h>
 #include <linux/kasan.h>
 #include <linux/kernel.h>
+#include <linux/memblock.h>
 #include <linux/memory.h>
 #include <linux/mm.h>
 #include <linux/static_key.h>
@@ -19,6 +20,8 @@
 #include "kasan.h"
 #include "../slab.h"
 
+#define KASAN_STACK_RING_SIZE_DEFAULT (32 << 10)
+
 enum kasan_arg_stacktrace {
 	KASAN_ARG_STACKTRACE_DEFAULT,
 	KASAN_ARG_STACKTRACE_OFF,
@@ -54,6 +57,16 @@ static int __init early_kasan_flag_stacktrace(char *arg)
 }
 early_param("kasan.stacktrace", early_kasan_flag_stacktrace);
 
+/* kasan.stack_ring_size=<number of entries> */
+static int __init early_kasan_flag_stack_ring_size(char *arg)
+{
+	if (!arg)
+		return -EINVAL;
+
+	return kstrtoul(arg, 0, &stack_ring.size);
+}
+early_param("kasan.stack_ring_size", early_kasan_flag_stack_ring_size);
+
 void __init kasan_init_tags(void)
 {
 	switch (kasan_arg_stacktrace) {
@@ -67,6 +80,16 @@ void __init kasan_init_tags(void)
 		static_branch_enable(&kasan_flag_stacktrace);
 		break;
 	}
+
+	if (kasan_stack_collection_enabled()) {
+		if (!stack_ring.size)
+			stack_ring.size = KASAN_STACK_RING_SIZE_DEFAULT;
+		stack_ring.entries = memblock_alloc(
+			sizeof(stack_ring.entries[0]) * stack_ring.size,
+			SMP_CACHE_BYTES);
+		if (WARN_ON(!stack_ring.entries))
+			static_branch_disable(&kasan_flag_stacktrace);
+	}
 }
 
 static void save_stack_info(struct kmem_cache *cache, void *object,
@@ -88,7 +111,7 @@ static void save_stack_info(struct kmem_cache *cache, void *object,
 
 next:
 	pos = atomic64_fetch_add(1, &stack_ring.pos);
-	entry = &stack_ring.entries[pos % KASAN_STACK_RING_SIZE];
+	entry = &stack_ring.entries[pos % stack_ring.size];
 
 	/* Detect stack ring entry slots that are being written to. */
 	old_ptr = READ_ONCE(entry->ptr);