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SubjectRe: [PATCH v4] devcoredump : Serialize devcd_del work
From
This should be v4, corrected in the subject

-Mukesh
On 5/2/2022 9:04 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@quicinc.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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