Re: [PATCH] kasan: support panic_on_warn

[Andrey Ryabinin]

On 10/17/2016 11:18 AM, Dmitry Vyukov wrote:
> On Mon, Oct 17, 2016 at 10:13 AM, Andrey Ryabinin
> <aryabinin@xxxxxxxxxxxxx> wrote:
>>
>>
>> On 10/14/2016 08:10 PM, Dmitry Vyukov wrote:
>>> If user sets panic_on_warn, he wants kernel to panic if there is
>>> anything barely wrong with the kernel. KASAN-detected errors
>>> are definitely not less benign than an arbitrary kernel WARNING.
>>>
>>> Panic after KASAN errors if panic_on_warn is set.
>>>
>>> We use this for continuous fuzzing where we want kernel to stop
>>> and reboot on any error.
>>>
>>> Signed-off-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
>>> Cc: kasan-dev@xxxxxxxxxxxxxxxx
>>> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
>>> Cc: Alexander Potapenko <glider@xxxxxxxxxx>
>>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>>> Cc: linux-mm@xxxxxxxxx
>>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>>> ---
>>>  mm/kasan/report.c | 4 ++++
>>>  1 file changed, 4 insertions(+)
>>>
>>> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
>>> index 24c1211..ca0bd48 100644
>>> --- a/mm/kasan/report.c
>>> +++ b/mm/kasan/report.c
>>> @@ -133,6 +133,10 @@ static void kasan_end_report(unsigned long *flags)
>>>       pr_err("==================================================================\n");
>>>       add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
>>>       spin_unlock_irqrestore(&report_lock, *flags);
>>> +     if (panic_on_warn) {
>>> +             panic_on_warn = 0;
>>
>> Why we need to reset panic_on_warn?
>> I assume this was copied from __warn(). AFAIU in __warn() this protects from recursion:
>>  __warn() -> painc() ->__warn() -> panic() -> ...
>> which is possible if WARN_ON() triggered in panic().
>> But KASAN is protected from such recursion via kasan_disable_current().
> 
> But we have recursion into panic via kasan->panic->warning->panic.

We do, like almost every other panic() call in the kernel. But at least it's finite.
So, if finite recursion is a problem for panic() it should be fixed in panic(), rather then on every panic() call site.




> 
>>> +             panic("panic_on_warn set ...\n");
>>> +     }
>>>       kasan_enable_current();
>>>  }