Re: [PATCH v2 2/3] kasan: skip HW tagging for all kernel thread stacks

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Mar 24, 2026 at 01:26:28PM +0000, Muhammad Usama Anjum wrote:
> diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
> index 051e429026904..307b8390fc670 100644
> --- a/include/linux/thread_info.h
> +++ b/include/linux/thread_info.h
> @@ -92,7 +92,7 @@ static inline long set_restart_fn(struct restart_block *restart,
>  #define THREAD_ALIGN	THREAD_SIZE
>  #endif
>  
> -#define THREADINFO_GFP		(GFP_KERNEL_ACCOUNT | __GFP_ZERO)
> +#define THREADINFO_GFP		(GFP_KERNEL_ACCOUNT | __GFP_ZERO | __GFP_SKIP_KASAN)
>  
>  /*
>   * flag set/clear/test wrappers
> diff --git a/kernel/fork.c b/kernel/fork.c
> index bb0c2613a5604..4bc7a03662109 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -207,7 +207,7 @@ static DEFINE_PER_CPU(struct vm_struct *, cached_stacks[NR_CACHED_STACKS]);
>   * accounting is performed by the code assigning/releasing stacks to tasks.
>   * We need a zeroed memory without __GFP_ACCOUNT.
>   */
> -#define GFP_VMAP_STACK (GFP_KERNEL | __GFP_ZERO)
> +#define GFP_VMAP_STACK (GFP_KERNEL | __GFP_ZERO | __GFP_SKIP_KASAN)
>  
>  struct vm_stack {
>  	struct rcu_head rcu;
> @@ -345,7 +345,8 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
>  		}
>  
>  		/* Reset stack metadata. */
> -		kasan_unpoison_range(vm_area->addr, THREAD_SIZE);
> +		if (!kasan_hw_tags_enabled())
> +			kasan_unpoison_range(vm_area->addr, THREAD_SIZE);
>  
>  		stack = kasan_reset_tag(vm_area->addr);

I wonder, since with kasan_reset_tag() returns a match-all pointer even
with KASAN_SW_TAGS, is it worth unpoisoning the range (unless it somehow
interferes with vfree() but I couldn't see how).

What the original approach might help with is use-after-realloc in case
we had a tagged pointer in a past life of a page and it still works now.
Oh well, that's I guess for other types of hardening to address like
delayed reallocation.

-- 
Catalin