On Wed 12-10-16 10:57:03, Catalin Marinas wrote:
> Commit 68f24b08ee89 ("sched/core: Free the stack early if
> CONFIG_THREAD_INFO_IN_TASK") may cause the task->stack to be freed
> during kmemleak_scan() execution, leading to either a NULL pointer
> fault (if task->stack is NULL) or kmemleak accessing already freed
> memory. This patch uses the new try_get_task_stack() API to ensure that
> the task stack is not freed during kmemleak stack scanning.

Looks good to me
 
> Fixes: 68f24b08ee89 ("sched/core: Free the stack early if 
> CONFIG_THREAD_INFO_IN_TASK")
> Cc: Andrew Morton <a...@linux-foundation.org>
> Cc: Andy Lutomirski <l...@kernel.org>
> Cc: CAI Qian <caiq...@redhat.com>
> Reported-by: CAI Qian <caiq...@redhat.com>
> Signed-off-by: Catalin Marinas <catalin.mari...@arm.com>

Acked-by: Michal Hocko <mho...@suse.com>

> ---
> 
> This was reported in a subsequent comment here:
> 
> https://bugzilla.kernel.org/show_bug.cgi?id=173901
> 
> However, the original bugzilla entry doesn't look related to task stack
> freeing as it was first reported on 4.8-rc8. Andy, sorry for cc'ing you
> to bugzilla, please feel free to remove your email from the bug above (I
> can't seem to be able to do it).
> 
>  mm/kmemleak.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/kmemleak.c b/mm/kmemleak.c
> index a5e453cf05c4..e5355a5b423f 100644
> --- a/mm/kmemleak.c
> +++ b/mm/kmemleak.c
> @@ -1453,8 +1453,11 @@ static void kmemleak_scan(void)
>  
>               read_lock(&tasklist_lock);
>               do_each_thread(g, p) {
> -                     scan_block(task_stack_page(p), task_stack_page(p) +
> -                                THREAD_SIZE, NULL);
> +                     void *stack = try_get_task_stack(p);
> +                     if (stack) {
> +                             scan_block(stack, stack + THREAD_SIZE, NULL);
> +                             put_task_stack(p);
> +                     }
>               } while_each_thread(g, p);
>               read_unlock(&tasklist_lock);
>       }
> 
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-- 
Michal Hocko
SUSE Labs

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