Re: [PATCH v16 1/4] kasan: allow an architecture to disable inline instrumentation

From: Andrey Konovalov
Date: Fri Jun 25 2021 - 09:45:29 EST


On Thu, Jun 24, 2021 at 6:41 AM Daniel Axtens <dja@xxxxxxxxxx> wrote:
>
> For annoying architectural reasons, it's very difficult to support inline
> instrumentation on powerpc64.*
>
> Add a Kconfig flag to allow an arch to disable inline. (It's a bit
> annoying to be 'backwards', but I'm not aware of any way to have
> an arch force a symbol to be 'n', rather than 'y'.)
>
> We also disable stack instrumentation in this case as it does things that
> are functionally equivalent to inline instrumentation, namely adding
> code that touches the shadow directly without going through a C helper.
>
> * on ppc64 atm, the shadow lives in virtual memory and isn't accessible in
> real mode. However, before we turn on virtual memory, we parse the device
> tree to determine which platform and MMU we're running under. That calls
> generic DT code, which is instrumented. Inline instrumentation in DT would
> unconditionally attempt to touch the shadow region, which we won't have
> set up yet, and would crash. We can make outline mode wait for the arch to
> be ready, but we can't change what the compiler inserts for inline mode.
>
> Reviewed-by: Marco Elver <elver@xxxxxxxxxx>
> Signed-off-by: Daniel Axtens <dja@xxxxxxxxxx>
> ---
> lib/Kconfig.kasan | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
> diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
> index cffc2ebbf185..c3b228828a80 100644
> --- a/lib/Kconfig.kasan
> +++ b/lib/Kconfig.kasan
> @@ -12,6 +12,13 @@ config HAVE_ARCH_KASAN_HW_TAGS
> config HAVE_ARCH_KASAN_VMALLOC
> bool
>
> +config ARCH_DISABLE_KASAN_INLINE
> + bool
> + help
> + An architecture might not support inline instrumentation.
> + When this option is selected, inline and stack instrumentation are
> + disabled.
> +
> config CC_HAS_KASAN_GENERIC
> def_bool $(cc-option, -fsanitize=kernel-address)
>
> @@ -130,6 +137,7 @@ config KASAN_OUTLINE
>
> config KASAN_INLINE
> bool "Inline instrumentation"
> + depends on !ARCH_DISABLE_KASAN_INLINE
> help
> Compiler directly inserts code checking shadow memory before
> memory accesses. This is faster than outline (in some workloads
> @@ -141,6 +149,7 @@ endchoice
> config KASAN_STACK
> bool "Enable stack instrumentation (unsafe)" if CC_IS_CLANG && !COMPILE_TEST
> depends on KASAN_GENERIC || KASAN_SW_TAGS
> + depends on !ARCH_DISABLE_KASAN_INLINE
> default y if CC_IS_GCC
> help
> The LLVM stack address sanitizer has a know problem that
> @@ -154,6 +163,9 @@ config KASAN_STACK
> but clang users can still enable it for builds without
> CONFIG_COMPILE_TEST. On gcc it is assumed to always be safe
> to use and enabled by default.
> + If the architecture disables inline instrumentation, stack
> + instrumentation is also disabled as it adds inline-style
> + instrumentation that is run unconditionally.
>
> config KASAN_SW_TAGS_IDENTIFY
> bool "Enable memory corruption identification"
> --
> 2.30.2
>

Reviewed-by: Andrey Konovalov <andreyknvl@xxxxxxxxx>

Thanks, Daniel!