Re: [PATCH 21/31] kasan, fork: don't tag stacks allocated with vmalloc

[Marco Elver <elver@google.com>]

On Tue, Nov 30, 2021 at 11:07PM +0100, andrey.konovalov@linux.dev wrote:
> From: Andrey Konovalov <andreyknvl@google.com>
> 
> Once tag-based KASAN modes start tagging vmalloc() allocations,
> kernel stacks will start getting tagged if CONFIG_VMAP_STACK is enabled.
> 
> Reset the tag of kernel stack pointers after allocation.
> 
> For SW_TAGS KASAN, when CONFIG_KASAN_STACK is enabled, the
> instrumentation can't handle the sp register being tagged.
> 
> For HW_TAGS KASAN, there's no instrumentation-related issues. However,
> the impact of having a tagged SP pointer needs to be properly evaluated,
> so keep it non-tagged for now.

Don't VMAP_STACK stacks have guards? So some out-of-bounds would already
be caught.

What would be the hypothetical benefit of using a tagged stack pointer?
Perhaps wildly out-of-bounds accesses derived from stack pointers?

I agree that unless we understand the impact of using a tagged stack
pointers, it should remain non-tagged for now.

> Note, that the memory for the stack allocation still gets tagged to
> catch vmalloc-into-stack out-of-bounds accesses.

Will the fact it's tagged cause issues for other code? I think kmemleak
already untags all addresses it scans for pointers. Anything else?

> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
>  kernel/fork.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 3244cc56b697..062d1484ef42 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -253,6 +253,7 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
>  	 * so cache the vm_struct.
>  	 */
>  	if (stack) {
> +		stack = kasan_reset_tag(stack);
>  		tsk->stack_vm_area = find_vm_area(stack);
>  		tsk->stack = stack;
>  	}
> -- 
> 2.25.1
>