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Date: Tue, 16 Aug 2022 13:23:28 -0700
From: Kees Cook <keescook@...omium.org>
To: Mukesh Ojha <quic_mojha@...cinc.com>
Cc: Johannes Berg <johannes@...solutions.net>,
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 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. :)
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:
--
Kees Cook
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