Patch "kcm: Fix error handling for SOCK_DGRAM in kcm_sendmsg()." 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

    kcm: Fix error handling for SOCK_DGRAM in kcm_sendmsg().

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:
     kcm-fix-error-handling-for-sock_dgram-in-kcm_sendmsg.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 27f56d46b33a2093dd633bbefd2ad1f89495f98e
Author: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
Date:   Mon Sep 11 19:27:53 2023 -0700

    kcm: Fix error handling for SOCK_DGRAM in kcm_sendmsg().
    
    [ Upstream commit a22730b1b4bf437c6bbfdeff5feddf54be4aeada ]
    
    syzkaller found a memory leak in kcm_sendmsg(), and commit c821a88bd720
    ("kcm: Fix memory leak in error path of kcm_sendmsg()") suppressed it by
    updating kcm_tx_msg(head)->last_skb if partial data is copied so that the
    following sendmsg() will resume from the skb.
    
    However, we cannot know how many bytes were copied when we get the error.
    Thus, we could mess up the MSG_MORE queue.
    
    When kcm_sendmsg() fails for SOCK_DGRAM, we should purge the queue as we
    do so for UDP by udp_flush_pending_frames().
    
    Even without this change, when the error occurred, the following sendmsg()
    resumed from a wrong skb and the queue was messed up.  However, we have
    yet to get such a report, and only syzkaller stumbled on it.  So, this
    can be changed safely.
    
    Note this does not change SOCK_SEQPACKET behaviour.
    
    Fixes: c821a88bd720 ("kcm: Fix memory leak in error path of kcm_sendmsg()")
    Fixes: ab7ac4eb9832 ("kcm: Kernel Connection Multiplexor module")
    Signed-off-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
    Link: https://lore.kernel.org/r/20230912022753.33327-1-kuniyu@xxxxxxxxxx
    Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
index 2d06617e89891..0d1ab4149553c 100644
--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -1064,17 +1064,18 @@ static int kcm_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 out_error:
 	kcm_push(kcm);
 
-	if (copied && sock->type == SOCK_SEQPACKET) {
+	if (sock->type == SOCK_SEQPACKET) {
 		/* Wrote some bytes before encountering an
 		 * error, return partial success.
 		 */
-		goto partial_message;
-	}
-
-	if (head != kcm->seq_skb)
+		if (copied)
+			goto partial_message;
+		if (head != kcm->seq_skb)
+			kfree_skb(head);
+	} else {
 		kfree_skb(head);
-	else if (copied)
-		kcm_tx_msg(head)->last_skb = skb;
+		kcm->seq_skb = NULL;
+	}
 
 	err = sk_stream_error(sk, msg->msg_flags, err);