[PATCH v2] netlink, audit: prevent false ENOBUFS on timeout expiry

When auditd is bottlenecked (e.g., by slow disk I/O), kauditd blocks on
the netlink socket. If the wait timeout fully expires (timeo == 0),
netlink mistakenly interprets the zeroed timeout as a non-blocking
request. It then triggers netlink_overrun that drops the event,
completely bypassing the audit subsystem's internal retry queue, and
falsely returns ENOBUFS to user-space, resulting in the following error:

 auditd[]: Error receiving audit netlink packet (No buffer space available)

Fix this by detecting when a blocking sender's timeout has expired
(timeo == 0 && !nonblock) in netlink_unicast(). In this case, instead
of retrying with timeo=0 (which would incorrectly trigger netlink_overrun
on the next iteration), safely free the skb and return -EAGAIN, allowing
the audit subsystem to gracefully enqueue the pending event into its
internal backlog.

Suggested-by: Steve Grubb <[email protected]>
Suggested-by: Paul Moore <[email protected]>
Signed-off-by: Ricardo Robaina <[email protected]>
---
Changes in v2:
- Use the simple check (timeo == 0 && !nonblock) to detect
  expired timeout, avoiding adding a new NETLINK flag.

 net/netlink/af_netlink.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 2aeb0680807d..fdc3db74b178 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -1351,8 +1351,18 @@ int netlink_unicast(struct sock *ssk, struct sk_buff *skb,
 	}
 
 	err = netlink_attachskb(sk, skb, &timeo, ssk);
-	if (err == 1)
+	if (err == 1) {
+		/* timeo may have been zeroed by schedule_timeout inside
+		 * netlink_attachskb. If the caller is a timed-blocking sender
+		 * (not genuinely nonblocking), don't re-enter with timeo=0 as
+		 * that would misfire netlink_overrun on the next iteration.
+		 */
+		if (timeo == 0 && !nonblock) {
+			kfree_skb(skb);
+			return -EAGAIN;
+		}
 		goto retry;
+	}
 	if (err)
 		return err;
 
-- 
2.53.0