Re: [PATCH RFC v2 04/21] kasan: unpoison stack only with CONFIG_KASAN_STACK

[Marco Elver]

On Mon, 16 Nov 2020 at 11:59, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>
> On Thu, Oct 29, 2020 at 8:57 PM 'Andrey Konovalov' via kasan-dev
> <kasan-dev@xxxxxxxxxxxxxxxx> wrote:
> >
> > On Tue, Oct 27, 2020 at 1:44 PM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> > >
> > > On Thu, Oct 22, 2020 at 3:19 PM Andrey Konovalov <andreyknvl@xxxxxxxxxx> wrote:
> > > >
> > > > There's a config option CONFIG_KASAN_STACK that has to be enabled for
> > > > KASAN to use stack instrumentation and perform validity checks for
> > > > stack variables.
> > > >
> > > > There's no need to unpoison stack when CONFIG_KASAN_STACK is not enabled.
> > > > Only call kasan_unpoison_task_stack[_below]() when CONFIG_KASAN_STACK is
> > > > enabled.
> > > >
> > > > Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> > > > Link: https://linux-review.googlesource.com/id/If8a891e9fe01ea543e00b576852685afec0887e3
> > > > ---
> > > >  arch/arm64/kernel/sleep.S        |  2 +-
> > > >  arch/x86/kernel/acpi/wakeup_64.S |  2 +-
> > > >  include/linux/kasan.h            | 10 ++++++----
> > > >  mm/kasan/common.c                |  2 ++
> > > >  4 files changed, 10 insertions(+), 6 deletions(-)
> > > >
> > > > diff --git a/arch/arm64/kernel/sleep.S b/arch/arm64/kernel/sleep.S
> > > > index ba40d57757d6..bdadfa56b40e 100644
> > > > --- a/arch/arm64/kernel/sleep.S
> > > > +++ b/arch/arm64/kernel/sleep.S
> > > > @@ -133,7 +133,7 @@ SYM_FUNC_START(_cpu_resume)
> > > >          */
> > > >         bl      cpu_do_resume
> > > >
> > > > -#ifdef CONFIG_KASAN
> > > > +#if defined(CONFIG_KASAN) && CONFIG_KASAN_STACK
> > > >         mov     x0, sp
> > > >         bl      kasan_unpoison_task_stack_below
> > > >  #endif
> > > > diff --git a/arch/x86/kernel/acpi/wakeup_64.S b/arch/x86/kernel/acpi/wakeup_64.S
> > > > index c8daa92f38dc..5d3a0b8fd379 100644
> > > > --- a/arch/x86/kernel/acpi/wakeup_64.S
> > > > +++ b/arch/x86/kernel/acpi/wakeup_64.S
> > > > @@ -112,7 +112,7 @@ SYM_FUNC_START(do_suspend_lowlevel)
> > > >         movq    pt_regs_r14(%rax), %r14
> > > >         movq    pt_regs_r15(%rax), %r15
> > > >
> > > > -#ifdef CONFIG_KASAN
> > > > +#if defined(CONFIG_KASAN) && CONFIG_KASAN_STACK
> > > >         /*
> > > >          * The suspend path may have poisoned some areas deeper in the stack,
> > > >          * which we now need to unpoison.
> > > > diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> > > > index 3f3f541e5d5f..7be9fb9146ac 100644
> > > > --- a/include/linux/kasan.h
> > > > +++ b/include/linux/kasan.h
> > > > @@ -68,8 +68,6 @@ static inline void kasan_disable_current(void) {}
> > > >
> > > >  void kasan_unpoison_memory(const void *address, size_t size);
> > > >
> > > > -void kasan_unpoison_task_stack(struct task_struct *task);
> > > > -
> > > >  void kasan_alloc_pages(struct page *page, unsigned int order);
> > > >  void kasan_free_pages(struct page *page, unsigned int order);
> > > >
> > > > @@ -114,8 +112,6 @@ void kasan_restore_multi_shot(bool enabled);
> > > >
> > > >  static inline void kasan_unpoison_memory(const void *address, size_t size) {}
> > > >
> > > > -static inline void kasan_unpoison_task_stack(struct task_struct *task) {}
> > > > -
> > > >  static inline void kasan_alloc_pages(struct page *page, unsigned int order) {}
> > > >  static inline void kasan_free_pages(struct page *page, unsigned int order) {}
> > > >
> > > > @@ -167,6 +163,12 @@ static inline size_t kasan_metadata_size(struct kmem_cache *cache) { return 0; }
> > > >
> > > >  #endif /* CONFIG_KASAN */
> > > >
> > > > +#if defined(CONFIG_KASAN) && CONFIG_KASAN_STACK
> > >
> > > && defined(CONFIG_KASAN_STACK) for consistency
> >
> > CONFIG_KASAN_STACK is different from other KASAN configs. It's always
> > defined, and its value is what controls whether stack instrumentation
> > is enabled.
>
> Not sure why we did this instead of the following, but okay.
>
>  config KASAN_STACK
> -       int
> -       default 1 if KASAN_STACK_ENABLE || CC_IS_GCC
> -       default 0
> +       bool
> +       default y if KASAN_STACK_ENABLE || CC_IS_GCC
> +       default n

I wondered the same, but then looking at scripts/Makefile.kasan I
think it's because we directly pass it to the compiler:
    ...
    $(call cc-param,asan-stack=$(CONFIG_KASAN_STACK)) \
    ...