In the Linux kernel, the following vulnerability has been resolved:
smb: client: fix TCP timers deadlock after rmmod
Commit ef7134c7fc48 ("smb: client: Fix use-after-free of network namespace.")
fixed a netns UAF by manually enabled socket refcounting
(sk->sk_net_refcnt=1 and sock_inuse_add(net, 1)).
The reason the patch worked for that bug was because we now hold
references to the netns (get_net_track() gets a ref internally)
and they're properly released (internally, on __sk_destruct()),
but only because sk->sk_net_refcnt was set.
Problem:
(this happens regardless of CONFIG_NET_NS_REFCNT_TRACKER and regardless
if init_net or other)
Setting sk->sk_net_refcnt=1 manually and after socket creation is not
only out of cifs scope, but also technically wrong -- it's set conditionally
based on user (=1) vs kernel (=0) sockets. And net/ implementations
seem to base their user vs kernel space operations on it.
e.g. upon TCP socket close, the TCP timers are not cleared because
sk->sk_net_refcnt=1:
(cf. commit 151c9c724d05 ("tcp: properly terminate timers for kernel sockets"))
net/ipv4/tcp.c:
void tcp_close(struct sock *sk, long timeout)
{
lock_sock(sk);
__tcp_close(sk, timeout);
release_sock(sk);
if (!sk->sk_net_refcnt)
inet_csk_clear_xmit_timers_sync(sk);
sock_put(sk);
}
Which will throw a lockdep warning and then, as expected, deadlock on
tcp_write_timer().
A way to reproduce this is by running the reproducer from ef7134c7fc48
and then 'rmmod cifs'. A few seconds later, the deadlock/lockdep
warning shows up.
Fix:
We shouldn't mess with socket internals ourselves, so do not set
sk_net_refcnt manually.
Also change __sock_create() to sock_create_kern() for explicitness.
As for non-init_net network namespaces, we deal with it the best way
we can -- hold an extra netns reference for server->ssocket and drop it
when it's released. This ensures that the netns still exists whenever
we need to create/destroy server->ssocket, but is not directly tied to
it.
References
In the Linux kernel, the following vulnerability has been resolved:
smb: client: fix TCP timers deadlock after rmmod
Commit ef7134c7fc48 ("smb: client: Fix use-after-free of network namespace.")
fixed a netns UAF by manually enabled socket refcounting
(sk->sk_net_refcnt=1 and sock_inuse_add(net, 1)).
The reason the patch worked for that bug was because we now hold
references to the netns (get_net_track() gets a ref internally)
and they're properly released (internally, on __sk_destruct()),
but only because sk->sk_net_refcnt was set.
Problem:
(this happens regardless of CONFIG_NET_NS_REFCNT_TRACKER and regardless
if init_net or other)
Setting sk->sk_net_refcnt=1 manually and after socket creation is not
only out of cifs scope, but also technically wrong -- it's set conditionally
based on user (=1) vs kernel (=0) sockets. And net/ implementations
seem to base their user vs kernel space operations on it.
e.g. upon TCP socket close, the TCP timers are not cleared because
sk->sk_net_refcnt=1:
(cf. commit 151c9c724d05 ("tcp: properly terminate timers for kernel sockets"))
net/ipv4/tcp.c:
void tcp_close(struct sock *sk, long timeout)
{
lock_sock(sk);
__tcp_close(sk, timeout);
release_sock(sk);
if (!sk->sk_net_refcnt)
inet_csk_clear_xmit_timers_sync(sk);
sock_put(sk);
}
Which will throw a lockdep warning and then, as expected, deadlock on
tcp_write_timer().
A way to reproduce this is by running the reproducer from ef7134c7fc48
and then 'rmmod cifs'. A few seconds later, the deadlock/lockdep
warning shows up.
Fix:
We shouldn't mess with socket internals ourselves, so do not set
sk_net_refcnt manually.
Also change __sock_create() to sock_create_kern() for explicitness.
As for non-init_net network namespaces, we deal with it the best way
we can -- hold an extra netns reference for server->ssocket and drop it
when it's released. This ensures that the netns still exists whenever
we need to create/destroy server->ssocket, but is not directly tied to
it.
References