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Message-ID: <b7afe9c1-1112-1605-e0a1-2cfd5c6d88ca@linux.com>
Date: Wed, 23 May 2018 00:00:13 +0300
From: Alexander Popov <alex.popov@...ux.com>
To: Kees Cook <keescook@...omium.org>
Cc: Ingo Molnar <mingo@...nel.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
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LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v12 2/6] x86/entry: Add STACKLEAK erasing the kernel stack
at the end of syscalls
On 22.05.2018 20:20, Kees Cook wrote:
> On Tue, May 22, 2018 at 3:58 AM, Alexander Popov <alex.popov@...ux.com> wrote:
>> Hello Ingo and Kees,
>>
>> On 18.05.2018 09:53, Ingo Molnar wrote:
>>> * Alexander Popov <alex.popov@...ux.com> wrote:
>>>> --- a/arch/x86/kernel/process_32.c
>>>> +++ b/arch/x86/kernel/process_32.c
>>>> @@ -136,6 +136,11 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
>>>> p->thread.sp0 = (unsigned long) (childregs+1);
>>>> memset(p->thread.ptrace_bps, 0, sizeof(p->thread.ptrace_bps));
>>>>
>>>> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
>>>> + p->thread.lowest_stack.val = (unsigned long)end_of_stack(p) +
>>>> + sizeof(unsigned long);
>>>> +#endif
>>>
>>> This should use an inline helper:
>>>
>>> stackleak_task_init(p);
>>
>> Right now I can't define such a helper because 'lowest_stack' is a part of
>> 'thread_struct', but arch/x86/include/asm/processor.h and include/linux/sched.h
>> are independent.
>>
>> I think the best solution is to move 'lowest_stack' variable to 'task_struct'.
>
> It might be possible work around this by using a macro in stackleak.h
> (pardon any whitespace damage):
>
> #ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> # define stackleak_task_init(task) do { \
> task->thread.stack_leak.lowest_stack = \
> (unsigned long)end_of_stack(task) + \
> sizeof(unsigned long); \
> } while (0)
> #else
> # define stackleak_task_init(task) do { } while (0)
> #endif
>
>> Is there any reason to fold this variable into a 'stackleak_info' structure? I
>> see everybody happily uses #ifdef in 'task_struct'.
>
> I'd rather it stay in threadinfo, but I could be convinced otherwise.
Currently 'lowest_stack' is a part of 'thread_struct', which is a "CPU-specific
state of this task". But in fact 'lowest_stack' is not CPU-specific. If I move
it to 'task_struct', Laura will avoid redefining it in 'thread_struct' in
arch/arm64/include/asm/processor.h. And that will also allow me to create a
stackleak_task_init() inline helper in stackleak.h.
Best regards,
Alexander
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