Re: [PATCH v2 21/37] kasan: introduce CONFIG_KASAN_HW_TAGS

[Marco Elver <elver@google.com>]

On Tue, Sep 15, 2020 at 11:16PM +0200, Andrey Konovalov wrote:
> This patch adds a configuration option for a new KASAN mode called
> hardware tag-based KASAN. This mode uses the memory tagging approach
> like the software tag-based mode, but relies on arm64 Memory Tagging
> Extension feature for tag management and access checking.
> 
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
> ---
> Change-Id: I246c2def9fffa6563278db1bddfbe742ca7bdefe
> ---
>  lib/Kconfig.kasan | 56 +++++++++++++++++++++++++++++++++--------------
>  1 file changed, 39 insertions(+), 17 deletions(-)
> 
> diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
> index b4cf6c519d71..17c9ecfaecb9 100644
> --- a/lib/Kconfig.kasan
> +++ b/lib/Kconfig.kasan
> @@ -6,7 +6,10 @@ config HAVE_ARCH_KASAN
>  config HAVE_ARCH_KASAN_SW_TAGS
>  	bool
>  
> -config	HAVE_ARCH_KASAN_VMALLOC
> +config HAVE_ARCH_KASAN_HW_TAGS
> +	bool
> +
> +config HAVE_ARCH_KASAN_VMALLOC
>  	bool
>  
>  config CC_HAS_KASAN_GENERIC
> @@ -20,10 +23,11 @@ config CC_HAS_WORKING_NOSANITIZE_ADDRESS
>  
>  menuconfig KASAN
>  	bool "KASAN: runtime memory debugger"
> -	depends on (HAVE_ARCH_KASAN && CC_HAS_KASAN_GENERIC) || \
> -		   (HAVE_ARCH_KASAN_SW_TAGS && CC_HAS_KASAN_SW_TAGS)
> +	depends on (((HAVE_ARCH_KASAN && CC_HAS_KASAN_GENERIC) || \
> +		     (HAVE_ARCH_KASAN_SW_TAGS && CC_HAS_KASAN_SW_TAGS)) && \
> +		    CC_HAS_WORKING_NOSANITIZE_ADDRESS) || \
> +		   HAVE_ARCH_KASAN_HW_TAGS
>  	depends on (SLUB && SYSFS) || (SLAB && !DEBUG_SLAB)
> -	depends on CC_HAS_WORKING_NOSANITIZE_ADDRESS
>  	select SLUB_DEBUG if SLUB

Is SLUB_DEBUG necessary with HW_TAGS?

>  	select CONSTRUCTORS
>  	select STACKDEPOT
> @@ -38,13 +42,18 @@ choice
>  	prompt "KASAN mode"
>  	default KASAN_GENERIC
>  	help
> -	  KASAN has two modes: generic KASAN (similar to userspace ASan,
> -	  x86_64/arm64/xtensa, enabled with CONFIG_KASAN_GENERIC) and
> -	  software tag-based KASAN (a version based on software memory
> -	  tagging, arm64 only, similar to userspace HWASan, enabled with
> -	  CONFIG_KASAN_SW_TAGS).
> +	  KASAN has three modes:
> +	  1. generic KASAN (similar to userspace ASan,
> +	     x86_64/arm64/xtensa, enabled with CONFIG_KASAN_GENERIC),
> +	  2. software tag-based KASAN (arm64 only, based on software
> +	     memory tagging (similar to userspace HWASan), enabled with
> +	     CONFIG_KASAN_SW_TAGS), and
> +	  3. hardware tag-based KASAN (arm64 only, based on hardware
> +	     memory tagging, enabled with CONFIG_KASAN_HW_TAGS).
>  
> -	  Both generic and tag-based KASAN are strictly debugging features.
> +	  All KASAN modes are strictly debugging features.
> +
> +	  For better error detection enable CONFIG_STACKTRACE.

I don't think CONFIG_STACKTRACE improves error detection, right? It only
makes the reports more readable

>  
>  config KASAN_GENERIC
>  	bool "Generic mode"
> @@ -61,8 +70,6 @@ config KASAN_GENERIC
>  	  and introduces an overhead of ~x1.5 for the rest of the allocations.
>  	  The performance slowdown is ~x3.
>  
> -	  For better error detection enable CONFIG_STACKTRACE.
> -
>  	  Currently CONFIG_KASAN_GENERIC doesn't work with CONFIG_DEBUG_SLAB
>  	  (the resulting kernel does not boot).
>  
> @@ -72,9 +79,11 @@ config KASAN_SW_TAGS
>  	help
>  	  Enables software tag-based KASAN mode.
>  
> -	  This mode requires Top Byte Ignore support by the CPU and therefore
> -	  is only supported for arm64. This mode requires Clang version 7.0.0
> -	  or later.
> +	  This mode require software memory tagging support in the form of
> +	  HWASan-like compiler instrumentation.
> +
> +	  Currently this mode is only implemented for arm64 CPUs and relies on
> +	  Top Byte Ignore. This mode requires Clang version 7.0.0 or later.
>  
>  	  This mode consumes about 1/16th of available memory at kernel start
>  	  and introduces an overhead of ~20% for the rest of the allocations.
> @@ -82,15 +91,27 @@ config KASAN_SW_TAGS
>  	  casting and comparison, as it embeds tags into the top byte of each
>  	  pointer.
>  
> -	  For better error detection enable CONFIG_STACKTRACE.
> -
>  	  Currently CONFIG_KASAN_SW_TAGS doesn't work with CONFIG_DEBUG_SLAB
>  	  (the resulting kernel does not boot).
>  
> +config KASAN_HW_TAGS
> +	bool "Hardware tag-based mode"
> +	depends on HAVE_ARCH_KASAN_HW_TAGS
> +	depends on SLUB
> +	help
> +	  Enables hardware tag-based KASAN mode.
> +
> +	  This mode requires hardware memory tagging support, and can be used
> +	  by any architecture that provides it.
> +
> +	  Currently this mode is only implemented for arm64 CPUs starting from
> +	  ARMv8.5 and relies on Memory Tagging Extension and Top Byte Ignore.
> +
>  endchoice
>  
>  choice
>  	prompt "Instrumentation type"
> +	depends on KASAN_GENERIC || KASAN_SW_TAGS
>  	default KASAN_OUTLINE
>  
>  config KASAN_OUTLINE
> @@ -114,6 +135,7 @@ endchoice
>  
>  config KASAN_STACK_ENABLE
>  	bool "Enable stack instrumentation (unsafe)" if CC_IS_CLANG && !COMPILE_TEST
> +	depends on KASAN_GENERIC || KASAN_SW_TAGS
>  	help
>  	  The LLVM stack address sanitizer has a know problem that
>  	  causes excessive stack usage in a lot of functions, see

How about something like the below change (introduce KASAN_INSTRUMENTED
Kconfig var) to avoid the repeated "KASAN_GENERIC || KASAN_SW_TAGS".
This could then also be used in the various .c/.h files (and make some
of the code more readable hopefully).

+config KASAN_INSTRUMENTED
+	def_bool KASAN_GENERIC || KASAN_SW_TAGS
+
 choice
 	prompt "Instrumentation type"
-	depends on KASAN_GENERIC || KASAN_SW_TAGS
+	depends on KASAN_INSTRUMENTED
 	default KASAN_OUTLINE
 
 config KASAN_OUTLINE
@@ -135,7 +141,7 @@ endchoice
 
 config KASAN_STACK_ENABLE
 	bool "Enable stack instrumentation (unsafe)" if CC_IS_CLANG && !COMPILE_TEST
-	depends on KASAN_GENERIC || KASAN_SW_TAGS
+	depends on KASAN_INSTRUMENTED
 	help
 	  The LLVM stack address sanitizer has a know problem that
 	  causes excessive stack usage in a lot of functions, see

Thanks,
-- Marco