Re: [PATCH 1/1] sched/headers: fix thread_info.<first> is overwritten by STACK_END_MAGIC

[Kees Cook]

On Thu, Nov 22, 2018 at 11:54 PM, Wang Dongsheng
<dongsheng.wang@xxxxxxxxxxxxxxxx> wrote:
> When select ARCH_TASK_STRUCT_ON_STACK the first of thread_info variable
> is overwritten by STACK_END_MAGIC. In fact, the ARCH_TASK_STRUCT_ON_STACK
> is not a real task on stack, it's only init_task on init_stack.
>
> Commit 0500871f21b2 ("Construct init thread stack in the linker script
> rather than by union") added this macro and put task_strcut into
> thread_union. This brings us the following possibilities:
>     TASK_ON_STACK    THREAD_INFO_IN_TASK    STACK
>                                             ----- <-- thread_info & stack
>         N                    N             |     |     --- <-- task
>                                            |     |    |   |
>                                             -----      ---
>
>                                             ----- <-- stack
>         N                    Y             |     |     --- <-- task(Including thread_info)
>                                            |     |    |   |
>                                             -----      ---
>
>                                             ----- <-- stack & task & thread_info
>         Y                    N             |     |
>                                            |     |
>                                             -----
>
>                                             ----- <-- stack & task(Including thread_info)
>         Y                    Y             |     |
>                                            |     |
>                                             -----
> The kernel has handled the first two cases correctly.
>
> For the third case:
> TASK_ON_STACK: Y. THREAD_INFO_IN_TASK: N. this case
> should never happen, because the task and thread_info will overlap. So
> when TASK_ON_STACK is selected, THREAD_INFO_IN_TASK must be selected too.
>
> For the fourth case:
> When task on stack, the end of stack should add a sizeof(task_struct) offset.
>
> This patch handled with the third and fourth case.
>
> Fixes: 0500871f21b2 ("Construct init thread stack in the linker ...")
>
> Signed-off-by: Wang Dongsheng <dongsheng.wang@xxxxxxxxxxxxxxxx>
> Signed-off-by: Shunyong Yang <shunyong.yang@xxxxxxxxxxxxxxxx>
> ---
>  arch/Kconfig                     | 1 +
>  include/linux/sched/task_stack.h | 5 ++++-
>  2 files changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index e1e540ffa979..0a2c73e73195 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -251,6 +251,7 @@ config ARCH_HAS_SET_MEMORY
>  # Select if arch init_task must go in the __init_task_data section
>  config ARCH_TASK_STRUCT_ON_STACK
>         bool
> +       depends on THREAD_INFO_IN_TASK || IA64

The "IA64" part shouldn't be needed since IA64 already selects it.

Since it's selected, it also can't have a depends, IIUC.

>
>  # Select if arch has its private alloc_task_struct() function
>  config ARCH_TASK_STRUCT_ALLOCATOR
> diff --git a/include/linux/sched/task_stack.h b/include/linux/sched/task_stack.h
> index 6a841929073f..624c48defb9e 100644
> --- a/include/linux/sched/task_stack.h
> +++ b/include/linux/sched/task_stack.h
> @@ -7,6 +7,7 @@
>   */
>
>  #include <linux/sched.h>
> +#include <linux/sched/task.h>
>  #include <linux/magic.h>
>
>  #ifdef CONFIG_THREAD_INFO_IN_TASK
> @@ -25,7 +26,9 @@ static inline void *task_stack_page(const struct task_struct *task)
>
>  static inline unsigned long *end_of_stack(const struct task_struct *task)
>  {
> -       return task->stack;
> +       if (!IS_ENABLED(CONFIG_ARCH_TASK_STRUCT_ON_STACK) || task != &init_task)
> +               return task->stack;
> +       return (unsigned long *)(task + 1);
>  }

This seems like a strange place for the change. It feels more like
init_task has been defined incorrectly.

-Kees

>
>  #elif !defined(__HAVE_THREAD_FUNCTIONS)
> --
> 2.19.1
>



-- 
Kees Cook