Re: [PATCH] nsfs: fix oops when ns->ops is not provided

[Jakub Kicinski <kuba@kernel.org>]

On Mon, 31 May 2021 23:34:10 +0800 Changbin Du wrote:
> We should not create inode for disabled namespace. A disabled namespace
> sets its ns->ops to NULL. Kernel could panic if we try to create a inode
> for such namespace.
> 
> Here is an example oops in socket ioctl cmd SIOCGSKNS when NET_NS is
> disabled. Kernel panicked 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
> 
> So let's reject such request for disabled namespace.
> 
> Signed-off-by: Changbin Du <changbin.du@gmail.com>
> Cc: <stable@vger.kernel.org>
> Cc: Cong Wang <xiyou.wangcong@gmail.com>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: David Laight <David.Laight@ACULAB.COM>
> ---
>  fs/nsfs.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/fs/nsfs.c b/fs/nsfs.c
> index 800c1d0eb0d0..6c055eb7757b 100644
> --- a/fs/nsfs.c
> +++ b/fs/nsfs.c
> @@ -62,6 +62,10 @@ static int __ns_get_path(struct path *path, struct ns_common *ns)
>  	struct inode *inode;
>  	unsigned long d;
>  
> +	/* In case the namespace is not actually enabled. */
> +	if (!ns->ops)
> +		return -EOPNOTSUPP;
> +
>  	rcu_read_lock();
>  	d = atomic_long_read(&ns->stashed);
>  	if (!d)

I'm not sure why we'd pick runtime checks for something that can be
perfectly easily solved at compilation time. Networking should not
be asking for FDs for objects which don't exist.