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Date: Wed, 24 Aug 2022 12:52:09 +0530
From: Mukesh Ojha <quic_mojha@...cinc.com>
To: Kees Cook <keescook@...omium.org>,
Johannes Berg <johannes@...solutions.net>
CC: <gregkh@...uxfoundation.org>, <tglx@...utronix.de>,
<sboyd@...nel.org>, <rafael@...nel.org>,
lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5] devcoredump : Serialize devcd_del work
On 8/17/2022 1:53 AM, Kees Cook wrote:
> On Thu, Aug 11, 2022 at 09:43:48PM +0530, Mukesh Ojha wrote:
>> Hi Johannes/Kees,
>
> Hi!
>
>>
>> Sorry for reminding on it again.
>> Any hope of this one to get into devcoredump ?
>
> I don't know this code well enough to comment on the solution, but it
> seems designed and justified correctly, at least. :)
Thanks Kees for the reply.
Hi @Johannes,
This patch is running in our internal build from a very long time and it
did not show any regression.
Would like to get your ack on it ?
-Mukesh
>
> I'll leave it to Johannes for review.
>
> -Kees
>
>>
>> -Mukesh
>>
>>
>> On 5/27/2022 7:33 PM, Mukesh Ojha wrote:
>>> In following scenario(diagram), when one thread X running dev_coredumpm()
>>> adds devcd device to the framework which sends uevent notification to
>>> userspace and another thread Y reads this uevent and call to
>>> devcd_data_write() which eventually try to delete the queued timer that
>>> is not initialized/queued yet.
>>>
>>> So, debug object reports some warning and in the meantime, timer is
>>> initialized and queued from X path. and from Y path, it gets reinitialized
>>> again and timer->entry.pprev=NULL and try_to_grab_pending() stucks.
>>>
>>> To fix this, introduce mutex and a boolean flag to serialize the behaviour.
>>>
>>> cpu0(X) cpu1(Y)
>>>
>>> dev_coredump() uevent sent to user space
>>> device_add() ======================> user space process Y reads the
>>> uevents writes to devcd fd
>>> which results into writes to
>>>
>>> devcd_data_write()
>>> mod_delayed_work()
>>> try_to_grab_pending()
>>> del_timer()
>>> debug_assert_init()
>>> INIT_DELAYED_WORK()
>>> schedule_delayed_work()
>>> debug_object_fixup()
>>> timer_fixup_assert_init()
>>> timer_setup()
>>> do_init_timer()
>>> /*
>>> Above call reinitializes
>>> the timer to
>>> timer->entry.pprev=NULL
>>> and this will be checked
>>> later in timer_pending() call.
>>> */
>>> timer_pending()
>>> !hlist_unhashed_lockless(&timer->entry)
>>> !h->pprev
>>> /*
>>> del_timer() checks h->pprev and finds
>>> it to be NULL due to which
>>> try_to_grab_pending() stucks.
>>> */
>>>
>>> Link: https://lore.kernel.org/lkml/2e1f81e2-428c-f11f-ce92-eb11048cb271@quicinc.com/
>>> Signed-off-by: Mukesh Ojha <quic_mojha@...cinc.com>
>>> ---
>>> v4->v5:
>>> - Rebased it.
>>>
>>> v3->v4:
>>> - flg variable renamed to delete_work.
>>>
>>> v2->v3:
>>> Addressed comments from gregkh
>>> - Wrapped the commit text and corrected the alignment.
>>> - Described the reason to introduce new variables.
>>> - Restored the blank line.
>>> - rename the del_wk_queued to flg.
>>> Addressed comments from tglx
>>> - Added a comment which explains the race which looks obvious however
>>> would not occur between disabled_store and devcd_del work.
>>>
>>>
>>> v1->v2:
>>> - Added del_wk_queued flag to serialize the race between devcd_data_write()
>>> and disabled_store() => devcd_free().
>>> drivers/base/devcoredump.c | 83 ++++++++++++++++++++++++++++++++++++++++++++--
>>> 1 file changed, 81 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/base/devcoredump.c b/drivers/base/devcoredump.c
>>> index f4d794d..1c06781 100644
>>> --- a/drivers/base/devcoredump.c
>>> +++ b/drivers/base/devcoredump.c
>>> @@ -25,6 +25,47 @@ struct devcd_entry {
>>> struct device devcd_dev;
>>> void *data;
>>> size_t datalen;
>>> + /*
>>> + * Here, mutex is required to serialize the calls to del_wk work between
>>> + * user/kernel space which happens when devcd is added with device_add()
>>> + * and that sends uevent to user space. User space reads the uevents,
>>> + * and calls to devcd_data_write() which try to modify the work which is
>>> + * not even initialized/queued from devcoredump.
>>> + *
>>> + *
>>> + *
>>> + * cpu0(X) cpu1(Y)
>>> + *
>>> + * dev_coredump() uevent sent to user space
>>> + * device_add() ======================> user space process Y reads the
>>> + * uevents writes to devcd fd
>>> + * which results into writes to
>>> + *
>>> + * devcd_data_write()
>>> + * mod_delayed_work()
>>> + * try_to_grab_pending()
>>> + * del_timer()
>>> + * debug_assert_init()
>>> + * INIT_DELAYED_WORK()
>>> + * schedule_delayed_work()
>>> + *
>>> + *
>>> + * Also, mutex alone would not be enough to avoid scheduling of
>>> + * del_wk work after it get flush from a call to devcd_free()
>>> + * mentioned as below.
>>> + *
>>> + * disabled_store()
>>> + * devcd_free()
>>> + * mutex_lock() devcd_data_write()
>>> + * flush_delayed_work()
>>> + * mutex_unlock()
>>> + * mutex_lock()
>>> + * mod_delayed_work()
>>> + * mutex_unlock()
>>> + * So, delete_work flag is required.
>>> + */
>>> + struct mutex mutex;
>>> + bool delete_work;
>>> struct module *owner;
>>> ssize_t (*read)(char *buffer, loff_t offset, size_t count,
>>> void *data, size_t datalen);
>>> @@ -84,7 +125,12 @@ static ssize_t devcd_data_write(struct file *filp, struct kobject *kobj,
>>> struct device *dev = kobj_to_dev(kobj);
>>> struct devcd_entry *devcd = dev_to_devcd(dev);
>>> - mod_delayed_work(system_wq, &devcd->del_wk, 0);
>>> + mutex_lock(&devcd->mutex);
>>> + if (!devcd->delete_work) {
>>> + devcd->delete_work = true;
>>> + mod_delayed_work(system_wq, &devcd->del_wk, 0);
>>> + }
>>> + mutex_unlock(&devcd->mutex);
>>> return count;
>>> }
>>> @@ -112,7 +158,12 @@ static int devcd_free(struct device *dev, void *data)
>>> {
>>> struct devcd_entry *devcd = dev_to_devcd(dev);
>>> + mutex_lock(&devcd->mutex);
>>> + if (!devcd->delete_work)
>>> + devcd->delete_work = true;
>>> +
>>> flush_delayed_work(&devcd->del_wk);
>>> + mutex_unlock(&devcd->mutex);
>>> return 0;
>>> }
>>> @@ -122,6 +173,30 @@ static ssize_t disabled_show(struct class *class, struct class_attribute *attr,
>>> return sysfs_emit(buf, "%d\n", devcd_disabled);
>>> }
>>> +/*
>>> + *
>>> + * disabled_store() worker()
>>> + * class_for_each_device(&devcd_class,
>>> + * NULL, NULL, devcd_free)
>>> + * ...
>>> + * ...
>>> + * while ((dev = class_dev_iter_next(&iter))
>>> + * devcd_del()
>>> + * device_del()
>>> + * put_device() <- last reference
>>> + * error = fn(dev, data) devcd_dev_release()
>>> + * devcd_free(dev, data) kfree(devcd)
>>> + * mutex_lock(&devcd->mutex);
>>> + *
>>> + *
>>> + * In the above diagram, It looks like disabled_store() would be racing with parallely
>>> + * running devcd_del() and result in memory abort while acquiring devcd->mutex which
>>> + * is called after kfree of devcd memory after dropping its last reference with
>>> + * put_device(). However, this will not happens as fn(dev, data) runs
>>> + * with its own reference to device via klist_node so it is not its last reference.
>>> + * so, above situation would not occur.
>>> + */
>>> +
>>> static ssize_t disabled_store(struct class *class, struct class_attribute *attr,
>>> const char *buf, size_t count)
>>> {
>>> @@ -278,13 +353,16 @@ void dev_coredumpm(struct device *dev, struct module *owner,
>>> devcd->read = read;
>>> devcd->free = free;
>>> devcd->failing_dev = get_device(dev);
>>> + devcd->delete_work = false;
>>> + mutex_init(&devcd->mutex);
>>> device_initialize(&devcd->devcd_dev);
>>> dev_set_name(&devcd->devcd_dev, "devcd%d",
>>> atomic_inc_return(&devcd_count));
>>> devcd->devcd_dev.class = &devcd_class;
>>> + mutex_lock(&devcd->mutex);
>>> if (device_add(&devcd->devcd_dev))
>>> goto put_device;
>>> @@ -301,10 +379,11 @@ void dev_coredumpm(struct device *dev, struct module *owner,
>>> INIT_DELAYED_WORK(&devcd->del_wk, devcd_del);
>>> schedule_delayed_work(&devcd->del_wk, DEVCD_TIMEOUT);
>>> -
>>> + mutex_unlock(&devcd->mutex);
>>> return;
>>> put_device:
>>> put_device(&devcd->devcd_dev);
>>> + mutex_unlock(&devcd->mutex);
>>> put_module:
>>> module_put(owner);
>>> free:
>
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