kcm: close race conditions on sk_receive_queue

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: kcm: close race conditions on sk_receive_queue sk->sk_receive_queue is protected by skb queue lock, but for KCM sockets its RX path takes mux->rx_lock to protect more than just skb queue. However, kcm_recvmsg() still only grabs the skb queue lock, so race conditions still exist. We can teach kcm_recvmsg() to grab mux->rx_lock too but this would introduce a potential performance regression as struct kcm_mux can be shared by multiple KCM sockets. So we have to enforce skb queue lock in requeue_rx_msgs() and handle skb peek case carefully in kcm_wait_data(). Fortunately, skb_recv_datagram() already handles it nicely and is widely used by other sockets, we can just switch to skb_recv_datagram() after getting rid of the unnecessary sock lock in kcm_recvmsg() and kcm_splice_read(). Side note: SOCK_DONE is not used by KCM sockets, so it is safe to get rid of this check too. I ran the original syzbot reproducer for 30 min without seeing any issue.

References

• https://git.kernel.org/stable/c/22f6b5d47396b4287662668ee3f5c1f766cb4259
• https://git.kernel.org/stable/c/d9ad4de92e184b19bcae4da10dac0275abf83931
• https://git.kernel.org/stable/c/ce57d6474ae999a3b2d442314087473a646a65c7
• https://git.kernel.org/stable/c/4154b6afa2bd639214ff259d912faad984f7413a
• https://git.kernel.org/stable/c/f7b0e95071bb4be4b811af3f0bfc3e200eedeaa3
• https://git.kernel.org/stable/c/bf92e54597d842da127c59833b365d6faeeaf020
• https://git.kernel.org/stable/c/5121197ecc5db58c07da95eb1ff82b98b121a221