Re: [PATCH][next] vpda: Fix memory leaks of msg on error return paths

[Stefano Garzarella <sgarzare@xxxxxxxxxx>]

On Fri, Jan 22, 2021 at 02:52:35PM +0000, Colin King wrote:
> From: Colin Ian King <colin.king@xxxxxxxxxxxxx>
>
> There are two error return paths that neglect to free the allocated
> object msg that lead to memory leaks. Fix this by adding an error
> exit path that frees msg.
>
> Addresses-Coverity: ("Resource leak")
> Fixes: 39502d042a70 ("vdpa: Enable user to query vdpa device info")
> Signed-off-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> ---
> drivers/vdpa/vdpa.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/vdpa/vdpa.c b/drivers/vdpa/vdpa.c
> index 9700a0adcca0..eb1f5a514103 100644
> --- a/drivers/vdpa/vdpa.c
> +++ b/drivers/vdpa/vdpa.c
> @@ -540,13 +540,15 @@ static int vdpa_nl_cmd_dev_get_doit(struct sk_buff *skb, struct genl_info *info)
> 	if (!dev) {
> 		mutex_unlock(&vdpa_dev_mutex);
> 		NL_SET_ERR_MSG_MOD(info->extack, "device not found");
> -		return -ENODEV;
> +		err = -ENODEV;
> +		goto err;
> 	}
> 	vdev = container_of(dev, struct vdpa_device, dev);
> 	if (!vdev->mdev) {
> 		mutex_unlock(&vdpa_dev_mutex);
> 		put_device(dev);
> -		return -EINVAL;
> +		err = -EINVAL;
> +		goto err;
> 	}
> 	err = vdpa_dev_fill(vdev, msg, info->snd_portid, info->snd_seq, 0, info->extack);
> 	if (!err)
> @@ -554,6 +556,7 @@ static int vdpa_nl_cmd_dev_get_doit(struct sk_buff *skb, struct genl_info *info)
> 	put_device(dev);
> 	mutex_unlock(&vdpa_dev_mutex);
>
> +err:
> 	if (err)
> 		nlmsg_free(msg);
> 	return err;

The patch looks okay, but reviewing it I figure out that if 
genlmsg_reply() returns an error, it also frees the sk_buff passed, so 
IIUC calling nlmsg_free() when genlmsg_reply() fails should cause a 
double free.

Maybe we should do something like this (not tested):

diff --git a/drivers/vdpa/vdpa.c b/drivers/vdpa/vdpa.c
index 9700a0adcca0..920afcb4aa75 100644
--- a/drivers/vdpa/vdpa.c
+++ b/drivers/vdpa/vdpa.c
@@ -538,24 +538,29 @@ static int vdpa_nl_cmd_dev_get_doit(struct sk_buff *skb, struct genl_info *info)
        mutex_lock(&vdpa_dev_mutex);
        dev = bus_find_device(&vdpa_bus, NULL, devname, vdpa_name_match);
        if (!dev) {
-               mutex_unlock(&vdpa_dev_mutex);
                NL_SET_ERR_MSG_MOD(info->extack, "device not found");
-               return -ENODEV;
+               err= -ENODEV;
+               goto err_msg;
        }
        vdev = container_of(dev, struct vdpa_device, dev);
        if (!vdev->mdev) {
-               mutex_unlock(&vdpa_dev_mutex);
-               put_device(dev);
-               return -EINVAL;
+               err = -EINVAL;
+               goto err_dev;
        }
        err = vdpa_dev_fill(vdev, msg, info->snd_portid, info->snd_seq, 0, info->extack);
-       if (!err)
-               err = genlmsg_reply(msg, info);
+       if (err)
+               goto err_dev;
+
        put_device(dev);
        mutex_unlock(&vdpa_dev_mutex);
 
-       if (err)
-               nlmsg_free(msg);
+       return genlmsg_reply(msg, info);
+
+err_dev:
+       put_device(dev);
+err_msg:
+       mutex_unlock(&vdpa_dev_mutex);
+       nlmsg_free(msg);
        return err;
 }
 

Thanks,
Stefano