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

[Zdenek Kabelac]

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)


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);
>   	}
>   }