On Mon, May 31, 2021 at 10:01 PM Jakub Kicinski [off-list ref] wrote:

On Mon, 31 May 2021 23:34:10 +0800 Changbin Du wrote:

quoted

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 <redacted>
Cc: <redacted>
Cc: Cong Wang <redacted>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: David Laight <redacted>
---
 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.

Four reasons:

1) ioctl() is not a hot path, so performance is not a problem here.

2) There are 3 different places (tun has two more) that need the same
fix.

3) init_net always exits, except it does not have an ops when
CONFIG_NET_NS is disabled:

static __net_init int net_ns_net_init(struct net *net)
{
#ifdef CONFIG_NET_NS
        net->ns.ops = &netns_operations;
#endif
        return ns_alloc_inum(&net->ns);
}

4) *I think* other namespaces need this fix too, for instance
init_ipc_ns:

struct ipc_namespace init_ipc_ns = {
        .ns.count = REFCOUNT_INIT(1),
        .user_ns = &init_user_ns,
        .ns.inum = PROC_IPC_INIT_INO,
#ifdef CONFIG_IPC_NS
        .ns.ops = &ipcns_operations,
#endif
};

whose ns->ops is NULL too if disabled.

Thanks.