Re: [PATCH] mm: hwpoison: use do_send_sig_info() instead of force_sig() (Re: PMEM error-handling forces SIGKILL causes kernel panic)

[Jane Chu <jane.chu@oracle.com>]

[-- Attachment #1: Type: text/plain, Size: 4097 bytes --]


On 1/16/2019 8:55 AM, Dan Williams wrote:
> On Wed, Jan 16, 2019 at 1:33 AM Naoya Horiguchi
> <n-horiguchi@ah.jp.nec.com> wrote:
>> [ CCed Andrew and linux-mm ]
>>
>> On Fri, Jan 11, 2019 at 08:14:02AM +0000, Horiguchi Naoya(堀口 直也) wrote:
>>> Hi Dan, Jane,
>>>
>>> Thanks for the report.
>>>
>>> On Wed, Jan 09, 2019 at 03:49:32PM -0800, Dan Williams wrote:
>>>> [ switch to text mail, add lkml and Naoya ]
>>>>
>>>> On Wed, Jan 9, 2019 at 12:19 PM Jane Chu <jane.chu@oracle.com> wrote:
>>> ...
>>>>> 3. The hardware consists the latest revision CPU and Intel NVDIMM, we suspected
>>>>>     the CPU faulty because it generated MCE over PMEM UE in a unlikely high
>>>>>     rate for any reasonable NVDIMM (like a few per 24hours).
>>>>>
>>>>> After swapping the CPU, the problem stopped reproducing.
>>>>>
>>>>> But one could argue that perhaps the faulty CPU exposed a small race window
>>>>> from collect_procs() to unmap_mapping_range() and to kill_procs(), hence
>>>>> caught the kernel  PMEM error handler off guard.
>>>> There's definitely a race, and the implementation is buggy as can be
>>>> seen in __exit_signal:
>>>>
>>>>          sighand = rcu_dereference_check(tsk->sighand,
>>>>                                          lockdep_tasklist_lock_is_held());
>>>>          spin_lock(&sighand->siglock);
>>>>
>>>> ...the memory-failure path needs to hold the proper locks before it
>>>> can assume that de-referencing tsk->sighand is valid.
>>>>
>>>>> Also note, the same workload on the same faulty CPU were run on Linux prior to
>>>>> the 4.19 PMEM error handling and did not encounter kernel crash, probably because
>>>>> the prior HWPOISON handler did not force SIGKILL?
>>>> Before 4.19 this test should result in a machine-check reboot, not
>>>> much better than a kernel crash.
>>>>
>>>>> Should we not to force the SIGKILL, or find a way to close the race window?
>>>> The race should be closed by holding the proper tasklist and rcu read lock(s).
>>> This reasoning and proposal sound right to me. I'm trying to reproduce
>>> this race (for non-pmem case,) but no luck for now. I'll investigate more.
>> I wrote/tested a patch for this issue.
>> I think that switching signal API effectively does proper locking.
>>
>> Thanks,
>> Naoya Horiguchi
>> ---
>>  From 16dbf6105ff4831f73276d79d5df238ab467de76 Mon Sep 17 00:00:00 2001
>> From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>> Date: Wed, 16 Jan 2019 16:59:27 +0900
>> Subject: [PATCH] mm: hwpoison: use do_send_sig_info() instead of force_sig()
>>
>> Currently memory_failure() is racy against process's exiting,
>> which results in kernel crash by null pointer dereference.
>>
>> The root cause is that memory_failure() uses force_sig() to forcibly
>> kill asynchronous (meaning not in the current context) processes.  As
>> discussed in thread https://lkml.org/lkml/2010/6/8/236 years ago for
>> OOM fixes, this is not a right thing to do.  OOM solves this issue by
>> using do_send_sig_info() as done in commit d2d393099de2 ("signal:
>> oom_kill_task: use SEND_SIG_FORCED instead of force_sig()"), so this
>> patch is suggesting to do the same for hwpoison.  do_send_sig_info()
>> properly accesses to siglock with lock_task_sighand(), so is free from
>> the reported race.
>>
>> I confirmed that the reported bug reproduces with inserting some delay
>> in kill_procs(), and it never reproduces with this patch.
>>
>> Note that memory_failure() can send another type of signal using
>> force_sig_mceerr(), and the reported race shouldn't happen on it
>> because force_sig_mceerr() is called only for synchronous processes
>> (i.e. BUS_MCEERR_AR happens only when some process accesses to the
>> corrupted memory.)
>>
>> Reported-by: Jane Chu <jane.chu@oracle.com>
>> Cc: Dan Williams <dan.j.williams@intel.com>
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>> ---
> Looks good to me.
>
> Reviewed-by: Dan Williams <dan.j.williams@intel.com>
>
> ...but it would still be good to get a Tested-by from Jane.

Sure, will let you know how the test goes.

Thanks!
-jane


[-- Attachment #2: Type: text/html, Size: 6214 bytes --]