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

[Dmitry Vyukov]

On Mon, Oct 5, 2015 at 1:23 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 kasan_disable_current()/kasan_enable_current() to silence
>> these warnings.
>>
>> Reported-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
>> Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
>> ---
>>
>>  Perhaps it would be better to add something like this:
>>       READ_ONCE_NOCHECK()
>>       {
>>               kasan_disable_current();
>>               READ_ONCE();
>>               kasan_enable_current();
>>       }
>>   ?
>>
>>  arch/x86/kernel/process.c | 18 +++++++++++++-----
>>  1 file changed, 13 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
>> index 39e585a..0488eb9 100644
>> --- a/arch/x86/kernel/process.c
>> +++ b/arch/x86/kernel/process.c
>> @@ -13,6 +13,7 @@
>>  #include <linux/random.h>
>>  #include <linux/user-return-notifier.h>
>>  #include <linux/dmi.h>
>> +#include <linux/kasan.h>
>>  #include <linux/utsname.h>
>>  #include <linux/stackprotector.h>
>>  #include <linux/tick.h>
>> @@ -514,7 +515,7 @@ unsigned long arch_randomize_brk(struct mm_struct *mm)
>>   */
>>  unsigned long get_wchan(struct task_struct *p)
>>  {
>> -     unsigned long start, bottom, top, sp, fp, ip;
>> +     unsigned long start, bottom, top, sp, fp, ip, ret = 0;
>>       int count = 0;
>>
>>       if (!p || p == current || p->state == TASK_RUNNING)
>> @@ -550,14 +551,21 @@ unsigned long get_wchan(struct task_struct *p)
>>       if (sp < bottom || sp > top)
>>               return 0;
>>
>> +     kasan_disable_current();
>>       fp = READ_ONCE(*(unsigned long *)sp);
>>       do {
>>               if (fp < bottom || fp > top)
>> -                     return 0;
>> +                     goto out;
>
> a break would do just fine too.
>
>> +
>>               ip = READ_ONCE(*(unsigned long *)(fp + sizeof(unsigned long)));
>> -             if (!in_sched_functions(ip))
>> -                     return ip;
>> +             if (!in_sched_functions(ip)) {
>> +                     ret = ip;
>> +                     goto out;
>
> ditto.
>
>> +             }
>>               fp = READ_ONCE(*(unsigned long *)fp);
>>       } while (count++ < 16 && p->state != TASK_RUNNING);
>> -     return 0;
>> +
>> +out:
>
> and then the label would not be needed.
>
>> +     kasan_enable_current();
>> +     return ret;
>
> But that's all pretty disgusting really.
>
> Cannot we do better, such as annotating the function and then KASAN sorting out
> its false positives, or something like that?


We also plug __attribute__((no_sanitize_address)) on the function.
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