This is a note to let you know that I've just added the patch titled
devcoredump : Serialize devcd_del work
to the 4.19-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
devcoredump-serialize-devcd_del-work.patch
and it can be found in the queue-4.19 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit ae23f861bc8694e52492bb9298da4d06812bca16
Author: Mukesh Ojha <quic_mojha@xxxxxxxxxxx>
Date: Tue Sep 13 18:20:24 2022 +0530
devcoredump : Serialize devcd_del work
[ Upstream commit 01daccf748323dfc61112f474cf2ba81015446b0 ]
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@xxxxxxxxxxx/
Signed-off-by: Mukesh Ojha <quic_mojha@xxxxxxxxxxx>
Link: https://lore.kernel.org/r/1663073424-13663-1-git-send-email-quic_mojha@xxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Stable-dep-of: af54d778a038 ("devcoredump: Send uevent once devcd is ready")
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/drivers/base/devcoredump.c b/drivers/base/devcoredump.c
index f1a3353f34946..1ba4af7f2f21f 100644
--- a/drivers/base/devcoredump.c
+++ b/drivers/base/devcoredump.c
@@ -29,6 +29,47 @@ 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);
@@ -88,7 +129,12 @@ 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;
}
@@ -116,7 +162,12 @@ 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;
}
@@ -126,6 +177,30 @@ static ssize_t disabled_show(struct class *class, struct class_attribute *attr,
return sprintf(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)
{
@@ -291,13 +366,16 @@ 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;
@@ -311,10 +389,11 @@ 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:
