Re: [PATCH mm v4 35/39] kasan: add kasan.vmalloc command line flag

[Alexander Potapenko <glider@google.com>]

On Mon, Dec 20, 2021 at 11:02 PM <andrey.konovalov@linux.dev> wrote:
>
> From: Andrey Konovalov <andreyknvl@google.com>
>
> Allow disabling vmalloc() tagging for HW_TAGS KASAN via a kasan.vmalloc
> command line switch.
>
> This is a fail-safe switch intended for production systems that enable
> HW_TAGS KASAN. In case vmalloc() tagging ends up having an issue not
> detected during testing but that manifests in production, kasan.vmalloc
> allows to turn vmalloc() tagging off while leaving page_alloc/slab
> tagging on.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
>
> ---
>
> Changes v1->v2:
> - Mark kasan_arg_stacktrace as __initdata instead of __ro_after_init.
> - Combine KASAN_ARG_VMALLOC_DEFAULT and KASAN_ARG_VMALLOC_ON switch
>   cases.
> ---
>  mm/kasan/hw_tags.c | 45 ++++++++++++++++++++++++++++++++++++++++++++-
>  mm/kasan/kasan.h   |  6 ++++++
>  2 files changed, 50 insertions(+), 1 deletion(-)
>
> diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
> index 99230e666c1b..657b23cebe28 100644
> --- a/mm/kasan/hw_tags.c
> +++ b/mm/kasan/hw_tags.c
> @@ -32,6 +32,12 @@ enum kasan_arg_mode {
>         KASAN_ARG_MODE_ASYMM,
>  };
>
> +enum kasan_arg_vmalloc {
> +       KASAN_ARG_VMALLOC_DEFAULT,
> +       KASAN_ARG_VMALLOC_OFF,
> +       KASAN_ARG_VMALLOC_ON,
> +};
> +
>  enum kasan_arg_stacktrace {
>         KASAN_ARG_STACKTRACE_DEFAULT,
>         KASAN_ARG_STACKTRACE_OFF,
> @@ -40,6 +46,7 @@ enum kasan_arg_stacktrace {
>
>  static enum kasan_arg kasan_arg __ro_after_init;
>  static enum kasan_arg_mode kasan_arg_mode __ro_after_init;
> +static enum kasan_arg_vmalloc kasan_arg_vmalloc __initdata;
>  static enum kasan_arg_stacktrace kasan_arg_stacktrace __initdata;
>
>  /* Whether KASAN is enabled at all. */
> @@ -50,6 +57,9 @@ EXPORT_SYMBOL(kasan_flag_enabled);
>  enum kasan_mode kasan_mode __ro_after_init;
>  EXPORT_SYMBOL_GPL(kasan_mode);
>
> +/* Whether to enable vmalloc tagging. */
> +DEFINE_STATIC_KEY_FALSE(kasan_flag_vmalloc);
> +
>  /* Whether to collect alloc/free stack traces. */
>  DEFINE_STATIC_KEY_FALSE(kasan_flag_stacktrace);
>
> @@ -89,6 +99,23 @@ static int __init early_kasan_mode(char *arg)
>  }
>  early_param("kasan.mode", early_kasan_mode);
>
> +/* kasan.vmalloc=off/on */
> +static int __init early_kasan_flag_vmalloc(char *arg)
> +{
> +       if (!arg)
> +               return -EINVAL;
> +
> +       if (!strcmp(arg, "off"))
> +               kasan_arg_vmalloc = KASAN_ARG_VMALLOC_OFF;
> +       else if (!strcmp(arg, "on"))
> +               kasan_arg_vmalloc = KASAN_ARG_VMALLOC_ON;
> +       else
> +               return -EINVAL;
> +
> +       return 0;
> +}
> +early_param("kasan.vmalloc", early_kasan_flag_vmalloc);
> +
>  /* kasan.stacktrace=off/on */
>  static int __init early_kasan_flag_stacktrace(char *arg)
>  {
> @@ -172,6 +199,18 @@ void __init kasan_init_hw_tags(void)
>                 break;
>         }
>
> +       switch (kasan_arg_vmalloc) {
> +       case KASAN_ARG_VMALLOC_DEFAULT:
> +               /* Default to enabling vmalloc tagging. */
> +               fallthrough;
> +       case KASAN_ARG_VMALLOC_ON:
> +               static_branch_enable(&kasan_flag_vmalloc);
> +               break;
> +       case KASAN_ARG_VMALLOC_OFF:
> +               /* Do nothing, kasan_flag_vmalloc keeps its default value. */
> +               break;
> +       }

I think we should be setting the default when defining the static key
(e.g. in this case it should be DEFINE_STATIC_KEY_TRUE), so that:
 - the _DEFAULT case is always empty;
 - the _ON case explicitly enables the static branch
 - the _OFF case explicitly disables the branch
This way we'll only need to change DEFINE_STATIC_KEY_TRUE to
DEFINE_STATIC_KEY_FALSE if we want to change the default, but we don't
have to mess up with the rest of the code.
Right now the switch statement is confusing, because the _OFF case
refers to some "default" value, whereas the _DEFAULT one actively
changes the state.

I see that this code is copied from kasan_flag_stacktrace
implementation, and my comment also applies there (but I don't insist
on fixing that one right now).