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Date: Fri, 20 May 2022 18:56:19 +0530
From: Mukesh Ojha <quic_mojha@...cinc.com>
To: <linux-kernel@...r.kernel.org>
CC: <gregkh@...uxfoundation.org>, <tglx@...utronix.de>,
<sboyd@...nel.org>, <rafael@...nel.org>,
<johannes@...solutions.net>, <keescook@...omium.org>
Subject: Re: [RESEND PATCH v4] devcoredump : Serialize devcd_del work
Gentle reminder for review.
-Mukesh
On 5/13/2022 7:33 PM, Mukesh Ojha wrote:
> Gentle reminder!
>
> Thanks,
> -Mukesh
> On 5/3/2022 11:37 AM, 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>
>> ---
>> 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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