netdev - Re: [PATCH net-next] net: set initial device refcount to 1

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Date:   Mon, 22 Mar 2021 12:29:44 -0700
From:   Guenter Roeck <groeck@...gle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next] net: set initial device refcount to 1

On Mon, Mar 22, 2021 at 11:21 AM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
>
> When adding CONFIG_PCPU_DEV_REFCNT, I forgot that the
> initial net device refcount was 0.
>
> When CONFIG_PCPU_DEV_REFCNT is not set, this means
> the first dev_hold() triggers an illegal refcount
> operation (addition on 0)
>
> refcount_t: addition on 0; use-after-free.
> WARNING: CPU: 0 PID: 1 at lib/refcount.c:25 refcount_warn_saturate+0x128/0x1a4
>
> Fix is to change initial (and final) refcount to be 1.
>
> Also add a missing kerneldoc piece, as reported by
> Stephen Rothwell.
>
> Fixes: 919067cc845f ("net: add CONFIG_PCPU_DEV_REFCNT")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: Guenter Roeck <groeck@...gle.com>

The problems observed with next-20210322 are gone with this patch applied.

Tested-by: Guenter Roeck <groeck@...gle.com>

Thanks,
Guenter

> ---
>  include/linux/netdevice.h | 1 +
>  net/core/dev.c            | 9 ++++++---
>  2 files changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 8f003955c485b81210ed56f7e1c24080b4bb46eb..b11c2c1890b2a28ba2d02fc4466380703a12efaf 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -1792,6 +1792,7 @@ enum netdev_ml_priv_type {
>   *
>   *     @proto_down_reason:     reason a netdev interface is held down
>   *     @pcpu_refcnt:           Number of references to this device
> + *     @dev_refcnt:            Number of references to this device
>   *     @todo_list:             Delayed register/unregister
>   *     @link_watch_list:       XXX: need comments on this one
>   *
> diff --git a/net/core/dev.c b/net/core/dev.c
> index be941ed754ac71d0839604bcfdd8ab67c339d27f..95c78279d900796c8a3ed0df59b168d5c5e0e309 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -10348,7 +10348,7 @@ static void netdev_wait_allrefs(struct net_device *dev)
>         rebroadcast_time = warning_time = jiffies;
>         refcnt = netdev_refcnt_read(dev);
>
> -       while (refcnt != 0) {
> +       while (refcnt != 1) {
>                 if (time_after(jiffies, rebroadcast_time + 1 * HZ)) {
>                         rtnl_lock();
>
> @@ -10385,7 +10385,7 @@ static void netdev_wait_allrefs(struct net_device *dev)
>
>                 refcnt = netdev_refcnt_read(dev);
>
> -               if (refcnt && time_after(jiffies, warning_time + 10 * HZ)) {
> +               if (refcnt != 1 && time_after(jiffies, warning_time + 10 * HZ)) {
>                         pr_emerg("unregister_netdevice: waiting for %s to become free. Usage count = %d\n",
>                                  dev->name, refcnt);
>                         warning_time = jiffies;
> @@ -10461,7 +10461,7 @@ void netdev_run_todo(void)
>                 netdev_wait_allrefs(dev);
>
>                 /* paranoia */
> -               BUG_ON(netdev_refcnt_read(dev));
> +               BUG_ON(netdev_refcnt_read(dev) != 1);
>                 BUG_ON(!list_empty(&dev->ptype_all));
>                 BUG_ON(!list_empty(&dev->ptype_specific));
>                 WARN_ON(rcu_access_pointer(dev->ip_ptr));
> @@ -10682,6 +10682,9 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
>         dev->pcpu_refcnt = alloc_percpu(int);
>         if (!dev->pcpu_refcnt)
>                 goto free_dev;
> +       dev_hold(dev);
> +#else
> +       refcount_set(&dev->dev_refcnt, 1);
>  #endif
>
>         if (dev_addr_init(dev))
> --
> 2.31.0.291.g576ba9dcdaf-goog
>