Re: [PATCH net] psp: fix NULL genl_sock deref race with concurrent netns teardown

[Daniel Zahka <daniel.zahka@gmail.com>]

On 7/3/26 7:24 AM, Kiran Kella wrote:
> The race occurs between network namespace removal and PSP device
> unregistration.  When a netns is deleted while a PSP device associated
> with that netns is concurrently being removed, psp_dev_unregister()
> triggers psp_nl_notify_dev() to send a device change notification.
> Concurrently, cleanup_net() running in the netns workqueue calls
> genl_pernet_exit(), which sets net->genl_sock to NULL. If
> genl_pernet_exit() wins the race, two sites in psp_nl_multicast_per_ns()
> then dereference the NULL socket and crash:
>
> CPU 0 (netns teardown)       CPU 1 (PSP device unregister)
> ======================       =============================
> cleanup_net [workqueue]
>    genl_pernet_exit()         psp_dev_unregister()
>      net->genl_sock = NULL      psp_nl_notify_dev()
>                                   psp_nl_multicast_per_ns()
>                                     build_ntf()
>                                       -> netlink_has_listeners(NULL)
>                                       /* crash */
>                                     genlmsg_multicast_netns()
>                                       -> nlmsg_multicast_filtered(NULL)
>                                       /* crash */
>
> Both the main_net path (derived from psd->main_netdev) and each
> assoc_net entry in psd->assoc_dev_list are affected.
>
> Fix by replacing the bare dev_net() calls with maybe_get_net().
> maybe_get_net() returns NULL if the namespace is already dying.
> Holding the reference ensures genl_sock remains valid across both the
> build_ntf() and genlmsg_multicast_netns() calls.
>
> Fixes: 00c94ca2b99e ("psp: base PSP device support")
> Fixes: 06c2dce2d0f6 ("psp: add new netlink cmd for dev-assoc and dev-disassoc")
> Signed-off-by: Kiran Kella <kiran.kella@broadcom.com>
> Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
> Reviewed-by: Vikas Gupta <vikas.gupta@broadcom.com>
> Reviewed-by: Bhargava Marreddy <bhargava.marreddy@broadcom.com>


Thanks. I was able to repro myself by inserting a delay after the old:

main_net = dev_net(psd->main_netdev);

and then executing nsim_psp_rereg_write() in parallel with destroying 
the netns that I placed a netdevsim dev into.

Tested-by: Daniel Zahka <daniel.zahka@gmail.com>

Reviewed-by: Daniel Zahka <daniel.zahka@gmail.com>

> ---
>   net/psp/psp_nl.c | 33 +++++++++++++++++++++------------
>   1 file changed, 21 insertions(+), 12 deletions(-)
>
> diff --git a/net/psp/psp_nl.c b/net/psp/psp_nl.c
> index 9610d8c456ff..24ab626a9e8a 100644
> --- a/net/psp/psp_nl.c
> +++ b/net/psp/psp_nl.c
> @@ -62,36 +62,45 @@ psp_nl_multicast_per_ns(struct psp_dev *psd, unsigned int group,
>   	struct net *main_net;
>   	struct sk_buff *ntf;
>   
> -	main_net = dev_net(psd->main_netdev);
> +	main_net = maybe_get_net(dev_net(psd->main_netdev));
> +	if (!main_net)
> +		return;
> +
>   	xa_init(&sent_nets);
>   
>   	list_for_each_entry(entry, &psd->assoc_dev_list, dev_list) {
>   		struct net *assoc_net = dev_net(entry->assoc_dev);
> +		struct net *net;
>   		int ret;
>   
>   		if (net_eq(assoc_net, main_net))
>   			continue;
>   
> -		ret = xa_insert(&sent_nets, (unsigned long)assoc_net, assoc_net,
> -				GFP_KERNEL);
> -		if (ret == -EBUSY)
> +		net = maybe_get_net(assoc_net);
> +		if (!net)
>   			continue;
>   
> -		ntf = build_ntf(psd, assoc_net, ctx);
> -		if (!ntf)
> +		ret = xa_insert(&sent_nets, (unsigned long)assoc_net, assoc_net,
> +				GFP_KERNEL);
> +		if (ret == -EBUSY) {
> +			put_net(net);
>   			continue;
> +		}
>   
> -		genlmsg_multicast_netns(&psp_nl_family, assoc_net, ntf, 0,
> -					group, GFP_KERNEL);
> +		ntf = build_ntf(psd, net, ctx);
> +		if (ntf)
> +			genlmsg_multicast_netns(&psp_nl_family, net, ntf, 0,
> +						group, GFP_KERNEL);
> +		put_net(net);
>   	}

some optional nits if you wanted to respin:

You could eliminate the extra struct net *net, by just doing something 
like if (!maybe_get_net(assoc_net)) directly.

You could get away with a single put_net(net); call site, if you reorder 
the ops that could fail so that maybe_get_net(assoc_net) happens last 
before the build_ntf(psd, net, ctx)

>   	xa_destroy(&sent_nets);
>   
>   	/* Send to main device netns */
>   	ntf = build_ntf(psd, main_net, ctx);
> -	if (!ntf)
> -		return;
> -	genlmsg_multicast_netns(&psp_nl_family, main_net, ntf, 0, group,
> -				GFP_KERNEL);
> +	if (ntf)
> +		genlmsg_multicast_netns(&psp_nl_family, main_net, ntf, 0, group,
> +					GFP_KERNEL);
> +	put_net(main_net);
>   }
>   
>   static struct sk_buff *psp_nl_clone_ntf(struct psp_dev *psd, struct net *net,