Re: [PATCH net] ppp: fix race conditions in ppp_fill_forward_path

On Mon, Aug 11, 2025 at 1:44 AM Qingfang Deng <[email protected]> wrote:
>
> ppp_fill_forward_path() has two race conditions:
>
> 1. The ppp->channels list can change between list_empty() and
>    list_first_entry(), as ppp_lock() is not held. If the only channel
>    is deleted in ppp_disconnect_channel(), list_first_entry() may
>    access an empty head or a freed entry, and trigger a panic.
>
> 2. pch->chan can be NULL. When ppp_unregister_channel() is called,
>    pch->chan is set to NULL before pch is removed from ppp->channels.
>
> Fix these by using a lockless RCU approach:
> - Use list_first_or_null_rcu() to safely test and access the first list
>   entry.
> - Convert list modifications on ppp->channels to their RCU variants and
>   add synchronize_rcu() after removal.
> - Check for a NULL pch->chan before dereferencing it.
>
> Fixes: f6efc675c9dd ("net: ppp: resolve forwarding path for bridge pppoe devices")
> Signed-off-by: Qingfang Deng <[email protected]>
> ---
>  drivers/net/ppp/ppp_generic.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
> index 8c98cbd4b06d..fd3ac75a56e3 100644
> --- a/drivers/net/ppp/ppp_generic.c
> +++ b/drivers/net/ppp/ppp_generic.c
> @@ -33,6 +33,7 @@
>  #include <linux/ppp_channel.h>
>  #include <linux/ppp-comp.h>
>  #include <linux/skbuff.h>
> +#include <linux/rculist.h>
>  #include <linux/rtnetlink.h>
>  #include <linux/if_arp.h>
>  #include <linux/ip.h>
> @@ -1598,11 +1599,14 @@ static int ppp_fill_forward_path(struct net_device_path_ctx *ctx,
>         if (ppp->flags & SC_MULTILINK)
>                 return -EOPNOTSUPP;
>
> -       if (list_empty(&ppp->channels))
> +       pch = list_first_or_null_rcu(&ppp->channels, struct channel, clist);

It is unclear if rcu_read_lock() is held at this point.

list_first_or_null_rcu() does not have a builtin __list_check_rcu()



> +       if (!pch)
>                 return -ENODEV;
>
> -       pch = list_first_entry(&ppp->channels, struct channel, clist);
>         chan = pch->chan;

chan = READ_ONCE(pch->chan);

And add a WRITE_ONCE(pch->chan, NULL) in ppp_unregister_channel()

And/or add __rcu to pch->chan

> +       if (!chan)
> +               return -ENODEV;
> +
>         if (!chan->ops->fill_forward_path)
>                 return -EOPNOTSUPP;
>
> @@ -3515,7 +3519,7 @@ ppp_connect_channel(struct channel *pch, int unit)
>         hdrlen = pch->file.hdrlen + 2;  /* for protocol bytes */
>         if (hdrlen > ppp->dev->hard_header_len)
>                 ppp->dev->hard_header_len = hdrlen;
> -       list_add_tail(&pch->clist, &ppp->channels);
> +       list_add_tail_rcu(&pch->clist, &ppp->channels);
>         ++ppp->n_channels;
>         pch->ppp = ppp;
>         refcount_inc(&ppp->file.refcnt);
> @@ -3545,10 +3549,11 @@ ppp_disconnect_channel(struct channel *pch)
>         if (ppp) {
>                 /* remove it from the ppp unit's list */
>                 ppp_lock(ppp);
> -               list_del(&pch->clist);
> +               list_del_rcu(&pch->clist);
>                 if (--ppp->n_channels == 0)
>                         wake_up_interruptible(&ppp->file.rwait);
>                 ppp_unlock(ppp);
> +               synchronize_rcu();

synchronize_net() is preferred.

>                 if (refcount_dec_and_test(&ppp->file.refcnt))
>                         ppp_destroy_interface(ppp);
>                 err = 0;
> --
> 2.43.0
>