kasan: remove use after scope bugs detection.

Use after scope bugs detector seems to be almost entirely useless for
the linux kernel. It exists over two years, but I've seen only one
valid bug so far [1]. And the bug was fixed before it has been
reported. There were some other use-after-scope reports, but they were
false-positives due to different reasons like incompatibility with
structleak plugin.

This feature significantly increases stack usage, especially with GCC <
9 version, and causes a 32K stack overflow. It probably adds
performance penalty too.

Given all that, let's remove use-after-scope detector entirely.

While preparing this patch I've noticed that we mistakenly enable
use-after-scope detection for clang compiler regardless of
CONFIG_KASAN_EXTRA setting. This is also fixed now.

[1] http://lkml.kernel.org/r/<[e-mail]>

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Andrey Ryabinin <[e-mail]>
Acked-by: Will Deacon <[e-mail]> [arm64]
Cc: Qian Cai <[e-mail]>
Cc: Alexander Potapenko <[e-mail]>
Cc: Dmitry Vyukov <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/arm64/include/asm/memory.h

···
  */
 #ifdef CONFIG_KASAN
 #define KASAN_SHADOW_SIZE	(UL(1) << (VA_BITS - KASAN_SHADOW_SCALE_SHIFT))
-#ifdef CONFIG_KASAN_EXTRA
-#define KASAN_THREAD_SHIFT	2
-#else
 #define KASAN_THREAD_SHIFT	1
-#endif /* CONFIG_KASAN_EXTRA */
 #else
 #define KASAN_SHADOW_SIZE	(0)
 #define KASAN_THREAD_SHIFT	0

lib/Kconfig.debug

···
 config FRAME_WARN
 	int "Warn for stack frames larger than (needs gcc 4.4)"
 	range 0 8192
-	default 3072 if KASAN_EXTRA
 	default 2048 if GCC_PLUGIN_LATENT_ENTROPY
 	default 1280 if (!64BIT && PARISC)
 	default 1024 if (!64BIT && !PARISC)

lib/Kconfig.kasan

···
 
 endchoice
 
-config KASAN_EXTRA
-	bool "KASAN: extra checks"
-	depends on KASAN_GENERIC && DEBUG_KERNEL && !COMPILE_TEST
-	help
-	  This enables further checks in generic KASAN, for now it only
-	  includes the address-use-after-scope check that can lead to
-	  excessive kernel stack usage, frame size warnings and longer
-	  compile time.
-	  See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81715
-
 choice
 	prompt "Instrumentation type"
 	depends on KASAN

lib/test_kasan.c

···
 	kfree(kmem);
 }
 
-static noinline void __init use_after_scope_test(void)
-{
-	volatile char *volatile p;
-
-	pr_info("use-after-scope on int\n");
-	{
-		int local = 0;
-
-		p = (char *)&local;
-	}
-	p[0] = 1;
-	p[3] = 1;
-
-	pr_info("use-after-scope on array\n");
-	{
-		char local[1024] = {0};
-
-		p = local;
-	}
-	p[0] = 1;
-	p[1023] = 1;
-}
-
 static noinline void __init kasan_alloca_oob_left(void)
 {
 	volatile int i = 10;
···
 	kasan_alloca_oob_right();
 	ksize_unpoisons_memory();
 	copy_user_test();
-	use_after_scope_test();
 	kmem_cache_double_free();
 	kmem_cache_invalid_free();
 	kasan_memchr();

mm/kasan/generic.c

···
 void __asan_handle_no_return(void) {}
 EXPORT_SYMBOL(__asan_handle_no_return);
 
-/* Emitted by compiler to poison large objects when they go out of scope. */
-void __asan_poison_stack_memory(const void *addr, size_t size)
-{
-	/*
-	 * Addr is KASAN_SHADOW_SCALE_SIZE-aligned and the object is surrounded
-	 * by redzones, so we simply round up size to simplify logic.
-	 */
-	kasan_poison_shadow(addr, round_up(size, KASAN_SHADOW_SCALE_SIZE),
-			    KASAN_USE_AFTER_SCOPE);
-}
-EXPORT_SYMBOL(__asan_poison_stack_memory);
-
-/* Emitted by compiler to unpoison large objects when they go into scope. */
-void __asan_unpoison_stack_memory(const void *addr, size_t size)
-{
-	kasan_unpoison_shadow(addr, size);
-}
-EXPORT_SYMBOL(__asan_unpoison_stack_memory);
-
 /* Emitted by compiler to poison alloca()ed objects. */
 void __asan_alloca_poison(unsigned long addr, size_t size)
 {

mm/kasan/generic_report.c

···
 	case KASAN_KMALLOC_FREE:
 		bug_type = "use-after-free";
 		break;
-	case KASAN_USE_AFTER_SCOPE:
-		bug_type = "use-after-scope";
-		break;
 	case KASAN_ALLOCA_LEFT:
 	case KASAN_ALLOCA_RIGHT:
 		bug_type = "alloca-out-of-bounds";

mm/kasan/kasan.h

···
 #define KASAN_STACK_MID         0xF2
 #define KASAN_STACK_RIGHT       0xF3
 #define KASAN_STACK_PARTIAL     0xF4
-#define KASAN_USE_AFTER_SCOPE   0xF8
 
 /*
  * alloca redzone shadow values
···
 void __asan_loadN(unsigned long addr, size_t size);
 void __asan_storeN(unsigned long addr, size_t size);
 void __asan_handle_no_return(void);
-void __asan_poison_stack_memory(const void *addr, size_t size);
-void __asan_unpoison_stack_memory(const void *addr, size_t size);
 void __asan_alloca_poison(unsigned long addr, size_t size);
 void __asan_allocas_unpoison(const void *stack_top, const void *stack_bottom);
 

scripts/Makefile.kasan

···
 	 $(call cc-param,asan-globals=1) \
 	 $(call cc-param,asan-instrumentation-with-call-threshold=$(call_threshold)) \
 	 $(call cc-param,asan-stack=$(CONFIG_KASAN_STACK)) \
-	 $(call cc-param,asan-use-after-scope=1) \
 	 $(call cc-param,asan-instrument-allocas=1)
-endif
-
-ifdef CONFIG_KASAN_EXTRA
-CFLAGS_KASAN += $(call cc-option, -fsanitize-address-use-after-scope)
 endif
 
 endif # CONFIG_KASAN_GENERIC

scripts/gcc-plugins/Kconfig

···
 
 config GCC_PLUGIN_STRUCTLEAK
 	bool "Force initialization of variables containing userspace addresses"
-	# Currently STRUCTLEAK inserts initialization out of live scope of
-	# variables from KASAN point of view. This leads to KASAN false
-	# positive reports. Prohibit this combination for now.
-	depends on !KASAN_EXTRA
 	help
 	  This plugin zero-initializes any structures containing a
 	  __user attribute. This can prevent some classes of information