Re: [PATCH v2 2/2] x86/process: Silence KASAN warnings in get_wchan()

[Andrey Ryabinin]

On 10/13/2015 04:57 PM, Dmitry Vyukov wrote:
> On Tue, Oct 13, 2015 at 3:48 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>>
>> * Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote:
>>
>>> get_wchan() is racy by design, it may access volatile stack
>>> of running task, thus it may access redzone in a stack frame
>>> and cause KASAN to warn about this.
>>>
>>> Use READ_ONCE_NOCHECK() to silence these warnings.
>>>
>>> Reported-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
>>> Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
>>> ---
>>>  arch/x86/kernel/process.c | 6 +++---
>>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
>>> index 39e585a..e28db18 100644
>>> --- a/arch/x86/kernel/process.c
>>> +++ b/arch/x86/kernel/process.c
>>> @@ -550,14 +550,14 @@ unsigned long get_wchan(struct task_struct *p)
>>>       if (sp < bottom || sp > top)
>>>               return 0;
>>>
>>> -     fp = READ_ONCE(*(unsigned long *)sp);
>>> +     fp = READ_ONCE_NOCHECK(*(unsigned long *)sp);
>>>       do {
>>>               if (fp < bottom || fp > top)
>>>                       return 0;
>>> -             ip = READ_ONCE(*(unsigned long *)(fp + sizeof(unsigned long)));
>>> +             ip = READ_ONCE_NOCHECK(*(unsigned long *)(fp + sizeof(unsigned long)));
>>>               if (!in_sched_functions(ip))
>>>                       return ip;
>>> -             fp = READ_ONCE(*(unsigned long *)fp);
>>> +             fp = READ_ONCE_NOCHECK(*(unsigned long *)fp);
>>>       } while (count++ < 16 && p->state != TASK_RUNNING);
>>>       return 0;
>>>  }
>>
>> Hm, exactly how is the 'red zone' defined? Is this about the current task mostly,
>> or when doing get_wchan() on other tasks?
> 
> 
> When code is compiled with AddressSanitizer, most variables on stack
> have redzones around them, on entry function "poisons" these redzones
> (any accesses to them will be flagged), on exit function "unpoisons"
> these redzones.
> 

An example bellow (stolen from slides - http://events.linuxfoundation.org/sites/events/files/slides/LinuxCon%20North%20America%202015%20KernelAddressSanitizer.pdf)

The following function:
void foo(void) {
	char a[328];
	...
	a[i] = 0;
}

will be transform by GCC to something like this:

void foo(void) {
       char redzone1[32];
       char a[328];
       char redzone2[24];
       char redzone3[32];

       int *shadow = (&redzone1 >> 3) + shadow_offset;
       shadow[0] = 0xf1f1f1f1; // poison redzone1
       shadow[11] = 0xf4f4f400; // poison redzone2
       shadow[12] = 0xf3f3f3f3; // poison redzone3

       ...
       __asan_store1(&a[i]); //check access to a[i]
       a[i] = 0;

       shadow[0] = shadow[11] = shadow[12] = 0; //unpoison redzones.
}

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