On Tue, Nov 30, 2021 at 11:07PM +0100, andrey.konovalov@xxxxxxxxx wrote:
> From: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> ---
> 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
>
