bpf: fix filed access without lock

bpf: fix filed access without lock

The tcp_bpf_recvmsg_parser() function, running in user context,
retrieves seq_copied from tcp_sk without holding the socket lock, and
stores it in a local variable seq. However, the softirq context can
modify tcp_sk->seq_copied concurrently, for example, n tcp_read_sock().

As a result, the seq value is stale when it is assigned back to
tcp_sk->copied_seq at the end of tcp_bpf_recvmsg_parser(), leading to
incorrect behavior.

Due to concurrency, the copied_seq field in tcp_bpf_recvmsg_parser()
might be set to an incorrect value (less than the actual copied_seq) at
the end of function: 'WRITE_ONCE(tcp->copied_seq, seq)'. This causes the
'offset' to be negative in tcp_read_sock()->tcp_recv_skb() when
processing new incoming packets (sk->copied_seq - skb->seq becomes less
than 0), and all subsequent packets will be dropped.

Signed-off-by: Jiayuan Chen <mrpre@163.com>
Link: https://lore.kernel.org/r/20241028065226.35568-1-mrpre@163.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>