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

From: 'Simon Horman' <[email protected]>

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---
netlink, audit: prevent false ENOBUFS on timeout expiry

This commit modifies netlink_unicast() to return -EAGAIN when a blocking
sender's timeout expires. This bypasses the netlink_overrun() call to
allow the audit subsystem to gracefully enqueue the pending event into its
internal backlog without falsely returning ENOBUFS to user-space.

> diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
> --- 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;

Could this regression cause silent packet drops in non-audit subsystems?

While this change prevents false ENOBUFS for the audit subsystem, other
subsystems like InfiniBand's IWPM use rdma_nl_unicast_wait(), which sets
nonblock=0 and operates on a kernel socket with a timeout.

If the timeout expires because the receiving user-space daemon is
bottlenecked, this new path will free the skb and return -EAGAIN without
ever invoking netlink_overrun().

Without netlink_overrun(), the receiver's socket error is never set to
ENOBUFS. Would this prevent user-space from being notified of the dropped
message, leading to permanent state desynchronization between the kernel
and user-space?

> +		}
>  		goto retry;
> +	}
>  	if (err)
>  		return err;