linux-kernel - Re: [PATCH 1/3] x86: Create and use a TOP_OF_KERNEL_STACK

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Date:	Tue, 10 Mar 2015 12:47:53 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 1/3] x86: Create and use a TOP_OF_KERNEL_STACK_PADDING macro

On Tue, Mar 10, 2015 at 12:22 PM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
> On 03/10/2015 07:05 PM, Andy Lutomirski wrote:
>> x86_32, unlike x86_64, pads the top of the kernel stack.  Document
>> this padding and give it a name.
>>
>> This should make no change whatsoever to the compiled kernel image.
>> It also doesn't fix any of the current bugs in this area.
>>
>> Signed-off-by: Andy Lutomirski <luto@...capital.net>
>> ---
>>  arch/x86/include/asm/processor.h   |  3 ++-
>>  arch/x86/include/asm/thread_info.h | 30 ++++++++++++++++++++++++++++++
>>  2 files changed, 32 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
>> index 48a61c1c626e..88d9aa745898 100644
>> --- a/arch/x86/include/asm/processor.h
>> +++ b/arch/x86/include/asm/processor.h
>> @@ -849,7 +849,8 @@ extern unsigned long thread_saved_pc(struct task_struct *tsk);
>>  #define task_pt_regs(task)                                             \
>>  ({                                                                     \
>>         struct pt_regs *__regs__;                                       \
>> -       __regs__ = (struct pt_regs *)(KSTK_TOP(task_stack_page(task))-8); \
>> +       __regs__ = (struct pt_regs *)(KSTK_TOP(task_stack_page(task)) - \
>> +                                  TOP_OF_KERNEL_STACK_PADDING);     \
>>         __regs__ - 1;                                                   \
>>  })
>
> The "magic 8" also hides here:
>
>         /*
>          * Push current_thread_info()->sysenter_return to the stack.
>          * A tiny bit of offset fixup is necessary - 4*4 means the 4 words
>          * pushed above; +8 corresponds to copy_thread's esp0 setting.
>          */
>         pushl_cfi ((TI_sysenter_return)-THREAD_SIZE+8+4*4)(%esp)
>                                                    ^^^

I'll just put TOP_OF_KERNEL_STACK_PADDING there, too.

--Andy

>         CFI_REL_OFFSET eip, 0
>
>
> It may make sense to mention TOP_OF_KERNEL_STACK_PADDING here
> (there are no useful comments in copy_thread() anymore):
>
>         movl PT_EFLAGS(%esp), %eax      # mix EFLAGS, SS and CS
>         # Warning: PT_OLDSS(%esp) contains the wrong/random values if we
>         # are returning to the kernel.
>         # See comments in process.c:copy_thread() for details.
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> maybe s/comments in process.c:copy_thread()/TOP_OF_KERNEL_STACK_PADDING/ ?
>         movb PT_OLDSS(%esp), %ah
>         movb PT_CS(%esp), %al
>         andl $(X86_EFLAGS_VM | (SEGMENT_TI_MASK << 8) | SEGMENT_RPL_MASK), %eax
>         cmpl $((SEGMENT_LDT << 8) | USER_RPL), %eax
>         CFI_REMEMBER_STATE
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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