bpf: reject unhashed sockets in bpf_sk_assign

In the Linux kernel, the following vulnerability has been resolved:

bpf: reject unhashed sockets in bpf_sk_assign

The semantics for bpf_sk_assign are as follows:

sk = some_lookup_func()

bpf_sk_assign(skb, sk)

bpf_sk_release(sk)

That is, the sk is not consumed by bpf_sk_assign. The function

therefore needs to make sure that sk lives long enough to be

consumed from __inet_lookup_skb. The path through the stack for a

TCPv4 packet is roughly:

netif_receive_skb_core: takes RCU read lock

__netif_receive_skb_core:

sch_handle_ingress:

tcf_classify:

bpf_sk_assign()

deliver_ptype_list_skb:

deliver_skb:

ip_packet_type->func == ip_rcv:

ip_rcv_core:

ip_rcv_finish_core:

dst_input:

ip_local_deliver:

ip_local_deliver_finish:

ip_protocol_deliver_rcu:

tcp_v4_rcv:

__inet_lookup_skb:

skb_steal_sock

The existing helper takes advantage of the fact that everything

happens in the same RCU critical section: for sockets with

SOCK_RCU_FREE set bpf_sk_assign never takes a reference.

skb_steal_sock then checks SOCK_RCU_FREE again and does sock_put

if necessary.

This approach assumes that SOCK_RCU_FREE is never set on a sk

between bpf_sk_assign and skb_steal_sock, but this invariant is

violated by unhashed UDP sockets. A new UDP socket is created

in TCP_CLOSE state but without SOCK_RCU_FREE set. That flag is only

added in udp_lib_get_port() which happens when a socket is bound.

When bpf_sk_assign was added it wasn't possible to access unhashed

UDP sockets from BPF, so this wasn't a problem. This changed

in commit 0c48eefae712 ("sock_map: Lift socket state restriction

for datagram sockets"), but the helper wasn't adjusted accordingly.

The following sequence of events will therefore lead to a refcount

leak:

1. Add socket(AF_INET, SOCK_DGRAM) to a sockmap.

2. Pull socket out of sockmap and bpf_sk_assign it. Since

SOCK_RCU_FREE is not set we increment the refcount.

3. bind() or connect() the socket, setting SOCK_RCU_FREE.

4. skb_steal_sock will now set refcounted = false due to

SOCK_RCU_FREE.

5. tcp_v4_rcv() skips sock_put().

Fix the problem by rejecting unhashed sockets in bpf_sk_assign().

This matches the behaviour of __inet_lookup_skb which is ultimately

the goal of bpf_sk_assign().