RE: [PATCH 4/4] dm: convert table_device.count from atomic_t to refcount_t

[Reshetova, Elena]

> Dne 20.10.2017 v 09:37 Elena Reshetova napsal(a):
> > atomic_t variables are currently used to implement reference
> > counters with the following properties:
> >   - counter is initialized to 1 using atomic_set()
> >   - a resource is freed upon counter reaching zero
> >   - once counter reaches zero, its further
> >     increments aren't allowed
> >   - counter schema uses basic atomic operations
> >     (set, inc, inc_not_zero, dec_and_test, etc.)
> >
> > Such atomic variables should be converted to a newly provided
> > refcount_t type and API that prevents accidental counter overflows
> > and underflows. This is important since overflows and underflows
> > can lead to use-after-free situation and be exploitable.
> >
> > The variable table_device.count is used as pure reference counter.
> > Convert it to refcount_t and fix up the operations.
> >
> > Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > Reviewed-by: David Windsor <dwindsor@xxxxxxxxx>
> > Reviewed-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
> > Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
> > ---
> >   drivers/md/dm.c | 12 +++++++-----
> >   1 file changed, 7 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> > index 4be8532..be12f3f 100644
> > --- a/drivers/md/dm.c
> > +++ b/drivers/md/dm.c
> > @@ -24,6 +24,7 @@
> >   #include <linux/delay.h>
> >   #include <linux/wait.h>
> >   #include <linux/pr.h>
> > +#include <linux/refcount.h>
> >
> >   #define DM_MSG_PREFIX "core"
> >
> > @@ -98,7 +99,7 @@ struct dm_md_mempools {
> >
> >   struct table_device {
> >   	struct list_head list;
> > -	atomic_t count;
> > +	refcount_t count;
> >   	struct dm_dev dm_dev;
> >   };
> >
> > @@ -685,10 +686,11 @@ int dm_get_table_device(struct mapped_device *md,
> dev_t dev, fmode_t mode,
> >
> >   		format_dev_t(td->dm_dev.name, dev);
> >
> > -		atomic_set(&td->count, 0);
> > +		refcount_set(&td->count, 1);
> >   		list_add(&td->list, &md->table_devices);
> > +	} else {
> > +		refcount_inc(&td->count);
> >   	}
> > -	atomic_inc(&td->count);
> >   	mutex_unlock(&md->table_devices_lock);
> >
> 
> 
> NACK
> 
> This patch (2a0b4682e09d76466f7b8f5e347ae2ff02f033af) currently breaks
> accounting of opened devices.
> 
> I.e.   multisegment device  (target with 3 segments is not properly accounted)

Could you please explain what exactly happens (i.e. missing/wrong increment?)
or provide a error dump?
By looking at the code, I don't see where the change in the reference counting
could have caused this. 

Best Regards,
Elena.

> 
> 
> Patch needs reworking and users of 'dm' and 4.15-rc0 kernel should rather
> switch back to 4.14 ATM as it's unclear which other parts can be affected.
> 
> Zdenek
> 
> >   	*result = &td->dm_dev;
> > @@ -701,7 +703,7 @@ void dm_put_table_device(struct mapped_device *md,
> struct dm_dev *d)
> >   	struct table_device *td = container_of(d, struct table_device,
> dm_dev);
> >
> >   	mutex_lock(&md->table_devices_lock);
> > -	if (atomic_dec_and_test(&td->count)) {
> > +	if (refcount_dec_and_test(&td->count)) {
> >   		close_table_device(td, md);
> >   		list_del(&td->list);
> >   		kfree(td);
> > @@ -718,7 +720,7 @@ static void free_table_devices(struct list_head *devices)
> >   		struct table_device *td = list_entry(tmp, struct
> table_device, list);
> >
> >   		DMWARN("dm_destroy: %s still exists with %d
> references",
> > -		       td->dm_dev.name, atomic_read(&td->count));
> > +		       td->dm_dev.name, refcount_read(&td->count));
> >   		kfree(td);
> >   	}
> >   }
> >