[PATCH v1 net] kcm: Fix error handling for SOCK_DGRAM in kcm_sendmsg().

[PATCH v1 net] kcm: Fix error handling for SOCK_DGRAM in kcm_sendmsg().

[Kuniyuki Iwashima]

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@amazon.com>
---
Resending due to delivery failure for netdev mailing list.
---
 net/kcm/kcmsock.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
index 740539a218b7..dd1d8ffd5f59 100644
--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -930,17 +930,18 @@ 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);
 
-- 
2.30.2

Re: [PATCH v1 net] kcm: Fix error handling for SOCK_DGRAM in kcm_sendmsg().

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Paolo Abeni <pabeni@redhat.com>:

On Mon, 11 Sep 2023 19:27:53 -0700 you wrote:
> 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.
> 
> [...]

Here is the summary with links:
  - [v1,net] kcm: Fix error handling for SOCK_DGRAM in kcm_sendmsg().
    https://git.kernel.org/netdev/net/c/a22730b1b4bf

You are awesome, thank you!
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