Patch "bpf, sockmap: Handle fin correctly" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    bpf, sockmap: Handle fin correctly

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     bpf-sockmap-handle-fin-correctly.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 88273fb7af387cd8d6eae1e2db11385d47022a79
Author: John Fastabend <john.fastabend@xxxxxxxxx>
Date:   Mon May 22 19:56:09 2023 -0700

    bpf, sockmap: Handle fin correctly
    
    [ Upstream commit 901546fd8f9ca4b5c481ce00928ab425ce9aacc0 ]
    
    The sockmap code is returning EAGAIN after a FIN packet is received and no
    more data is on the receive queue. Correct behavior is to return 0 to the
    user and the user can then close the socket. The EAGAIN causes many apps
    to retry which masks the problem. Eventually the socket is evicted from
    the sockmap because its released from sockmap sock free handling. The
    issue creates a delay and can cause some errors on application side.
    
    To fix this check on sk_msg_recvmsg side if length is zero and FIN flag
    is set then set return to zero. A selftest will be added to check this
    condition.
    
    Fixes: 04919bed948dc ("tcp: Introduce tcp_read_skb()")
    Signed-off-by: John Fastabend <john.fastabend@xxxxxxxxx>
    Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
    Tested-by: William Findlay <will@xxxxxxxxxxxxx>
    Reviewed-by: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>
    Link: https://lore.kernel.org/bpf/20230523025618.113937-6-john.fastabend@xxxxxxxxx
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
index 89204004eeb5e..5fdef5ddfbbe6 100644
--- a/net/ipv4/tcp_bpf.c
+++ b/net/ipv4/tcp_bpf.c
@@ -174,6 +174,24 @@ static int tcp_msg_wait_data(struct sock *sk, struct sk_psock *psock,
 	return ret;
 }
 
+static bool is_next_msg_fin(struct sk_psock *psock)
+{
+	struct scatterlist *sge;
+	struct sk_msg *msg_rx;
+	int i;
+
+	msg_rx = sk_psock_peek_msg(psock);
+	i = msg_rx->sg.start;
+	sge = sk_msg_elem(msg_rx, i);
+	if (!sge->length) {
+		struct sk_buff *skb = msg_rx->skb;
+
+		if (skb && TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)
+			return true;
+	}
+	return false;
+}
+
 static int tcp_bpf_recvmsg_parser(struct sock *sk,
 				  struct msghdr *msg,
 				  size_t len,
@@ -217,6 +235,19 @@ static int tcp_bpf_recvmsg_parser(struct sock *sk,
 
 msg_bytes_ready:
 	copied = sk_msg_recvmsg(sk, psock, msg, len, flags);
+	/* The typical case for EFAULT is the socket was gracefully
+	 * shutdown with a FIN pkt. So check here the other case is
+	 * some error on copy_page_to_iter which would be unexpected.
+	 * On fin return correct return code to zero.
+	 */
+	if (copied == -EFAULT) {
+		bool is_fin = is_next_msg_fin(psock);
+
+		if (is_fin) {
+			copied = 0;
+			goto out;
+		}
+	}
 	if (!copied) {
 		long timeo;
 		int data;