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>

diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
index e7658c5d6b79d7367689afbe8638b5638dc588e6..370993c03d31363c0f82a003d9e5b0ca3bbed721 100644 (file)
--- a/net/ipv4/tcp_bpf.c
+++ b/net/ipv4/tcp_bpf.c
@@ -221,11 +221,11 @@ static int tcp_bpf_recvmsg_parser(struct sock *sk,
                                  int flags,
                                  int *addr_len)
 {
-       struct tcp_sock *tcp = tcp_sk(sk);
        int peek = flags & MSG_PEEK;
-       u32 seq = tcp->copied_seq;
        struct sk_psock *psock;
+       struct tcp_sock *tcp;
        int copied = 0;
+       u32 seq;
 
        if (unlikely(flags & MSG_ERRQUEUE))
                return inet_recv_error(sk, msg, len, addr_len);
@@ -238,7 +238,8 @@ static int tcp_bpf_recvmsg_parser(struct sock *sk,
                return tcp_recvmsg(sk, msg, len, flags, addr_len);
 
        lock_sock(sk);
-
+       tcp = tcp_sk(sk);
+       seq = tcp->copied_seq;
        /* We may have received data on the sk_receive_queue pre-accept and
         * then we can not use read_skb in this context because we haven't
         * assigned a sk_socket yet so have no link to the ops. The work-around