Re: [PATCH net] net: dev: Detect dev_hold() after netdev_wait_allrefs()

[Eric Dumazet]

On Thu, Jan 27, 2022 at 5:43 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
>
> I've run into a bug where dev_hold() was being called after
> netdev_wait_allrefs(). But at that point, the device is already going
> away, and dev_hold() can't stop that anymore.
>
> To make such problems easier to diagnose in the future:
>
>  - For CONFIG_PCPU_DEV_REFCNT builds: Recheck in free_netdev() whether
>    the net refcount has been elevated. If this is detected, WARN() and
>    leak the object (to prevent worse consequences from a
>    use-after-free).
>  - For builds without CONFIG_PCPU_DEV_REFCNT: Set the refcount to zero.
>    This signals to the generic refcount infrastructure that any attempt
>    to increment the refcount later is a bug.
>
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> ---
>  net/core/dev.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 1baab07820f6..f7916c0d226d 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -9949,8 +9949,18 @@ void netdev_run_todo(void)
>
>                 netdev_wait_allrefs(dev);
>
> +               /* Drop the netdev refcount (which should be 1 at this point)
> +                * to zero. If we're using the generic refcount code, this will
> +                * tell it that any dev_hold() after this point is a bug.
> +                */
> +#ifdef CONFIG_PCPU_DEV_REFCNT
> +               this_cpu_dec(*dev->pcpu_refcnt);
> +               BUG_ON(netdev_refcnt_read(dev) != 0);
> +#else
> +               BUG_ON(!refcount_dec_and_test(&dev->dev_refcnt));
> +#endif
> +
>                 /* paranoia */
> -               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));
> @@ -10293,6 +10303,12 @@ void free_netdev(struct net_device *dev)
>         free_percpu(dev->xdp_bulkq);
>         dev->xdp_bulkq = NULL;
>
> +       /* Recheck in case someone called dev_hold() between
> +        * netdev_wait_allrefs() and here.
> +        */

At this point, dev->pcpu_refcnt per-cpu data has been freed already
(CONFIG_PCPU_DEV_REFCNT=y)

So this should probably crash, or at least UAF ?

> +       if (WARN_ON(netdev_refcnt_read(dev) != 0))
> +               return; /* leak memory, otherwise we might get UAF */
> +
>         /*  Compatibility with error handling in drivers */
>         if (dev->reg_state == NETREG_UNINITIALIZED) {
>                 netdev_freemem(dev);
>
> base-commit: 23a46422c56144939c091c76cf389aa863ce9c18
> --
> 2.35.0.rc0.227.g00780c9af4-goog
>