Re: [PATCH v2] net: do not invoke open_related_ns when NET_NS is disabled

[Jakub Kicinski]

On Wed,  9 Jun 2021 23:46:35 +0800 Changbin Du wrote:
> When NET_NS is not enabled, socket ioctl cmd SIOCGSKNS should do nothing
> but acknowledge userspace it is not supported. Otherwise, kernel would
> panic wherever nsfs trys to access ns->ops since the proc_ns_operations
> is not implemented in this case.
> 
> [7.670023] Unable to handle kernel NULL pointer dereference at virtual address 00000010
> [7.670268] pgd = 32b54000
> [7.670544] [00000010] *pgd=00000000
> [7.671861] Internal error: Oops: 5 [#1] SMP ARM
> [7.672315] Modules linked in:
> [7.672918] CPU: 0 PID: 1 Comm: systemd Not tainted 5.13.0-rc3-00375-g6799d4f2da49 #16
> [7.673309] Hardware name: Generic DT based system
> [7.673642] PC is at nsfs_evict+0x24/0x30
> [7.674486] LR is at clear_inode+0x20/0x9c
> 
> The same to tun SIOCGSKNS command.
> 
> Signed-off-by: Changbin Du <changbin.du@xxxxxxxxx>
> Cc: Cong Wang <xiyou.wangcong@xxxxxxxxx>
> Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
> Cc: David Laight <David.Laight@xxxxxxxxxx>
> Cc: Christian Brauner <christian.brauner@xxxxxxxxxx>
> ---
>  drivers/net/tun.c | 4 ++++
>  net/socket.c      | 4 ++++
>  2 files changed, 8 insertions(+)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 84f832806313..8ec5977d2f34 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -3003,9 +3003,13 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
>  	} else if (cmd == TUNSETQUEUE) {
>  		return tun_set_queue(file, &ifr);
>  	} else if (cmd == SIOCGSKNS) {
> +#ifdef CONFIG_NET_NS
>  		if (!ns_capable(net->user_ns, CAP_NET_ADMIN))
>  			return -EPERM;
>  		return open_related_ns(&net->ns, get_net_ns);
> +#else
> +		return -EOPNOTSUPP;
> +#endif

... and why are you not adding that check to get_net_ns like 
I suggested twice and even shared a diff?