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

["Wang, Dongsheng" <dongsheng.wang@hxt-semitech.com>]

Hello Kees,

On 2018/11/28 6:38, Kees Cook wrote:
> On Thu, Nov 22, 2018 at 11:54 PM, Wang Dongsheng
> <dongsheng.wang@hxt-semitech.com> 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@hxt-semitech.com>
>> Signed-off-by: Shunyong Yang <shunyong.yang@hxt-semitech.com>
>> ---
>>  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.

Since the IA64 thread_info including task_struct, it doesn't need to
select THREAD_INFO_IN_TASK.
So we need to allow IA64 select ARCH_TASK_STRUCT_ON_STACK without
THREAD_INFO.

>>  # 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.

The init_task will put into init_stack when ARCH_TASK_STRUCT_ON_STACK is
selected.
include/asm-generic/vmlinux.lds.h:
#define INIT_TASK_DATA(align)                        \
    . = ALIGN(align);                        \
    __start_init_task = .;                        \
    init_thread_union = .;                        \
    init_stack = .;                            \
    KEEP(*(.data..init_task))                    \
    KEEP(*(.data..init_thread_info))                \
    . = __start_init_task + THREAD_SIZE;                \
    __end_init_task = .;

So we need end_of_stack to offset sizeof(task_struct).

Cheers,
Dongsheng