Re: [PATCH v15 1/4] kasan: allow an architecture to disable inline instrumentation

[Andrey Konovalov <andreyknvl@gmail.com>]

On Thu, Jun 17, 2021 at 12:30 PM Daniel Axtens <dja@axtens.net> 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@google.com>
> Signed-off-by: Daniel Axtens <dja@axtens.net>
> ---
>  lib/Kconfig.kasan | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
> index cffc2ebbf185..cb5e02d09e11 100644
> --- a/lib/Kconfig.kasan
> +++ b/lib/Kconfig.kasan
> @@ -12,6 +12,15 @@ config HAVE_ARCH_KASAN_HW_TAGS
>  config HAVE_ARCH_KASAN_VMALLOC
>         bool
>
> +config ARCH_DISABLE_KASAN_INLINE
> +       bool
> +       help
> +         Sometimes an architecture might not be able to support inline
> +         instrumentation but might be able to support outline instrumentation.
> +         This option allows an architecture to prevent inline and stack
> +         instrumentation from being enabled.

This seems too wordy.

How about: "An architecture might not support inline instrumentation.
When this option is selected, inline and stack instrumentation are
disabled."

> +
> +

Drop the extra empty line.

>  config CC_HAS_KASAN_GENERIC
>         def_bool $(cc-option, -fsanitize=kernel-address)
>
> @@ -130,6 +139,7 @@ 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 +151,7 @@ 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 +165,9 @@ 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, this is

this => stack instrumentation



> +         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
>