Hello Kees,
On 2018/11/28 6:38, Kees Cook wrote:
> 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.
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
