Re: [PATCH v2] fork: reset pointer tag of vmapped thread stack before vfree

[Lorenzo Stoakes (ARM)]

+cc Ulad for vmalloc stuff.

Please don't send a v2 in-reply-to a v1 or any other email.

Send the patch entirely separately.

I really need to write a bot to say this :)...

On Mon, Sep 14, 2026 at 05:33:00PM +0800, Shaobo Huang wrote:
> thread_stack_free_rcu() frees the vmalloc'd thread stack via
> vfree(vm_area->addr).  In RCU callback context, vfree() routes to
> vfree_atomic(), which calls llist_add((struct llist_node *)addr, ...)
> and writes 8 bytes to the base of the region being freed.
>
> With KASAN_SW_TAGS, vm_area->addr carries a random tag.  If
> kasan_unpoison_task_stack_below() has rewritten the shadow covering
> [base, sp] to KASAN_TAG_KERNEL (0xff) -- which it does on every CPU
> resume for the current task's stack -- the llist_add store checks
> shadow[base] (0xff) against the pointer tag (random) and reports an
> invalid-access, although writing to the base of a stack queued for
> deferred free is legitimate.
>
> Reset the pointer tag to KASAN_TAG_KERNEL before vfree() so that
> kasan_check_range() short-circuits the check, the same way the task
> accesses its own stack at runtime via sp.  The vmalloc lookup is safe:
> __find_vmap_area() resets the tag before comparing against va_start.
>
> Fixes: 9f7d416c3612 ("kprobes: Unpoison stack in jprobe_return() for KASAN")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: zhipuai:glm-5.2

Thanks for adding this!

New convention is to say:

	Assisted-by: LLM

Rather than to list the agent.

See https://docs.kernel.org/process/coding-assistants.html

> Signed-off-by: Shaobo Huang <huangshaobo3@xxxxxxxxxx>

Looks reasonable to me so, with nits addressed:

Acked-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>

But I would like Ulad's input on this from vmalloc side.

> ---
> Changes since v1:

Thanks for adding this!

Nit, but nicer to say 'v2:' here I think.

> - Drop the 12-line comment; keep just the one-line fix.
> - Fix the Fixes: tag to point to the commit that introduced
>   kasan_unpoison_task_stack_below (9f7d416c3612), which added
>   both the function definition and the _cpu_resume call site, not
>   the 2016 vfree_atomic commit.
> - Add Assisted-by tag per Documentation/process/coding-assistants.rst.
> - Use real name (Shaobo Huang) instead of "sparkhuang".
> - Trim the commit message; remove the full KASAN dump.

Please give credit to reviewers for each change, e.g. 'as per XXX' :)

> v1:
>   https://lore.kernel.org/all/20260806123020.90869-1-huangshaobo3@xxxxxxxxxx/
> ---
>  kernel/fork.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 45300f59cf2c..9a66b10749de 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -238,7 +238,7 @@ static void thread_stack_free_rcu(struct rcu_head *rh)
>  	if (try_release_thread_stack_to_cache(vm_stack->stack_vm_area))
>  		return;
>
> -	vfree(vm_area->addr);
> +	vfree(kasan_reset_tag(vm_area->addr));
>  }
>
>  static void thread_stack_delayed_free(struct task_struct *tsk)
> --
> 2.34.1
>

--
Cheers, Lorenzo