Re: [PATCH v3] staging/android/ion : fix a race condition in the ion driver

[Laura Abbott <labbott@redhat.com>]

On 02/23/2016 08:38 PM, EunTaik Lee wrote:
> There is a use-after-free problem in the ion driver.
> This is caused by a race condition in the ion_ioctl()
> function.
> 
> A handle has ref count of 1 and two tasks on different
> cpus calls ION_IOC_FREE simultaneously.
> 
> cpu 0                                   cpu 1
> -------------------------------------------------------
> ion_handle_get_by_id()
> (ref == 2)
>                              ion_handle_get_by_id()
>                              (ref == 3)
> 
> ion_free()
> (ref == 2)
> 
> ion_handle_put()
> (ref == 1)
> 
>                              ion_free()
>                              (ref == 0 so ion_handle_destroy() is
>                              called
>                              and the handle is freed.)
> 
>                              ion_handle_put() is called and it
>                              decreases the slub's next free pointer
> 
> The problem is detected as an unaligned access in the
> spin lock functions since it uses load exclusive
>   instruction. In some cases it corrupts the slub's
> free pointer which causes a mis-aligned access to the
> next free pointer.(kmalloc returns a pointer like
> ffffc0745b4580aa). And it causes lots of other
> hard-to-debug problems.
> 
> This symptom is caused since the first member in the
> ion_handle structure is the reference count and the
> ion driver decrements the reference after it has been
> freed.
> 
> To fix this problem client->lock mutex is extended
> to protect all the codes that uses the handle.
> 
> Signed-off-by: Eun Taik Lee <eun.taik.lee@samsung.com>

Reviewed-by: Laura Abbott <labbott@redhat.com>

> ---
> changes in v3:
>   1. remove ion_handle_put in ion_free
>   2. remove unnecessary protection in IOC_ION_SHARE/IOC_ION_MAP
> changes in v2 :
>   1. add problem description in the comment
>   2. fix un-matching mutex_lock/unlock pair in ion_share_dma_buf()
> 
>   drivers/staging/android/ion/ion.c | 55 ++++++++++++++++++++++++++++++---------
>   1 file changed, 42 insertions(+), 13 deletions(-)
>   mode change 100644 => 100755 drivers/staging/android/ion/ion.c
> 
> diff --git a/drivers/staging/android/ion/ion.c b/drivers/staging/android/ion/ion.c
> old mode 100644
> new mode 100755
> index e237e9f..1958d58
> --- a/drivers/staging/android/ion/ion.c
> +++ b/drivers/staging/android/ion/ion.c
> @@ -385,13 +385,22 @@ static void ion_handle_get(struct ion_handle *handle)
>   	kref_get(&handle->ref);
>   }
>   
> -static int ion_handle_put(struct ion_handle *handle)
> +static int ion_handle_put_nolock(struct ion_handle *handle)
> +{
> +	int ret;
> +
> +	ret = kref_put(&handle->ref, ion_handle_destroy);
> +
> +	return ret;
> +}
> +
> +int ion_handle_put(struct ion_handle *handle)
>   {
>   	struct ion_client *client = handle->client;
>   	int ret;
>   
>   	mutex_lock(&client->lock);
> -	ret = kref_put(&handle->ref, ion_handle_destroy);
> +	ret = ion_handle_put_nolock(handle);
>   	mutex_unlock(&client->lock);
>   
>   	return ret;
> @@ -415,20 +424,30 @@ static struct ion_handle *ion_handle_lookup(struct ion_client *client,
>   	return ERR_PTR(-EINVAL);
>   }
>   
> -static struct ion_handle *ion_handle_get_by_id(struct ion_client *client,
> +static struct ion_handle *ion_handle_get_by_id_nolock(struct ion_client *client,
>   						int id)
>   {
>   	struct ion_handle *handle;
>   
> -	mutex_lock(&client->lock);
>   	handle = idr_find(&client->idr, id);
>   	if (handle)
>   		ion_handle_get(handle);
> -	mutex_unlock(&client->lock);
>   
>   	return handle ? handle : ERR_PTR(-EINVAL);
>   }
>   
> +struct ion_handle *ion_handle_get_by_id(struct ion_client *client,
> +						int id)
> +{
> +	struct ion_handle *handle;
> +
> +	mutex_lock(&client->lock);
> +	handle = ion_handle_get_by_id_nolock(client, id);
> +	mutex_unlock(&client->lock);
> +
> +	return handle;
> +}
> +
>   static bool ion_handle_validate(struct ion_client *client,
>   				struct ion_handle *handle)
>   {
> @@ -530,22 +549,28 @@ struct ion_handle *ion_alloc(struct ion_client *client, size_t len,
>   }
>   EXPORT_SYMBOL(ion_alloc);
>   
> -void ion_free(struct ion_client *client, struct ion_handle *handle)
> +static void ion_free_nolock(struct ion_client *client, struct ion_handle *handle)
>   {
>   	bool valid_handle;
>   
>   	BUG_ON(client != handle->client);
>   
> -	mutex_lock(&client->lock);
>   	valid_handle = ion_handle_validate(client, handle);
>   
>   	if (!valid_handle) {
>   		WARN(1, "%s: invalid handle passed to free.\n", __func__);
> -		mutex_unlock(&client->lock);
>   		return;
>   	}
> +	ion_handle_put_nolock(handle);
> +}
> +
> +void ion_free(struct ion_client *client, struct ion_handle *handle)
> +{
> +	BUG_ON(client != handle->client);
> +
> +	mutex_lock(&client->lock);
> +	ion_free_nolock(client, handle);
>   	mutex_unlock(&client->lock);
> -	ion_handle_put(handle);
>   }
>   EXPORT_SYMBOL(ion_free);
>   
> @@ -1281,11 +1306,15 @@ static long ion_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
>   	{
>   		struct ion_handle *handle;
>   
> -		handle = ion_handle_get_by_id(client, data.handle.handle);
> -		if (IS_ERR(handle))
> +		mutex_lock(&client->lock);
> +		handle = ion_handle_get_by_id_nolock(client, data.handle.handle);
> +		if (IS_ERR(handle)) {
> +			mutex_unlock(&client->lock);
>   			return PTR_ERR(handle);
> -		ion_free(client, handle);
> -		ion_handle_put(handle);
> +		}
> +		ion_free_nolock(client, handle);
> +		ion_handle_put_nolock(handle);
> +		mutex_unlock(&client->lock);
>   		break;
>   	}
>   	case ION_IOC_SHARE:
>