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

On May 11, 2026 Ricardo Robaina <[email protected]> wrote:
> 
> 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)
> 
> Convert the `nonblock` argument in netlink_unicast() into a `flags`
> bitmask and introduce NETLINK_UNICAST_TIMED. When a caller specifies
> this flag and exhausts its timeout budget, netlink intercepts the
> zeroed timeo state. Instead of defaulting to an overrun condition, it
> safely frees the skb and returns -EAGAIN, allowing the audit subsystem
> to gracefully enqueue the pending event into its internal backlog.
> 
> Suggested-by: Steve Grubb <[email protected]>
> Signed-off-by: Ricardo Robaina <[email protected]>
> ---
>  include/linux/netlink.h  |  5 ++++-
>  kernel/audit.c           |  8 ++++----
>  net/netlink/af_netlink.c | 17 +++++++++++++++--
>  3 files changed, 23 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/netlink.h b/include/linux/netlink.h
> index 882e9c1b6c1d..1888c8ee416a 100644
> --- a/include/linux/netlink.h
> +++ b/include/linux/netlink.h
> @@ -226,7 +226,10 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
>  int netlink_has_listeners(struct sock *sk, unsigned int group);
>  bool netlink_strict_get_check(struct sk_buff *skb);
>  
> -int netlink_unicast(struct sock *ssk, struct sk_buff *skb, __u32 portid, int nonblock);
> +/* Internal flags for netlink_unicast (do not overlap with MSG_* flags) */
> +#define NETLINK_UNICAST_TIMED 0x80000
> +
> +int netlink_unicast(struct sock *ssk, struct sk_buff *skb, __u32 portid, int flags);
>  int netlink_broadcast(struct sock *ssk, struct sk_buff *skb, __u32 portid,
>  		      __u32 group, gfp_t allocation);

...

> diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
> index 2aeb0680807d..acaa96695981 100644
> --- a/net/netlink/af_netlink.c
> +++ b/net/netlink/af_netlink.c
> @@ -1325,14 +1325,17 @@ static int netlink_unicast_kernel(struct sock *sk, struct sk_buff *skb,
>  }
>  
>  int netlink_unicast(struct sock *ssk, struct sk_buff *skb,
> -		    u32 portid, int nonblock)
> +		    u32 portid, int flags)
>  {
>  	struct sock *sk;
>  	int err;
>  	long timeo;
> +	int nonblock;
>  
>  	skb = netlink_trim(skb, gfp_any());
>  
> +	/* Extract blocking mode: strip internal flags, preserve MSG_DONTWAIT */
> +	nonblock = flags & ~NETLINK_UNICAST_TIMED;
>  	timeo = sock_sndtimeo(ssk, nonblock);
>  retry:
>  	sk = netlink_getsockbyportid(ssk, portid);
> @@ -1351,8 +1354,18 @@ 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 && (flags & NETLINK_UNICAST_TIMED)) {
> +			kfree_skb(skb);
> +			return -EAGAIN;
> +		}

Is the new flag needed, couldn't you just check to see if a non-blocking
operation was requested and do the same?

  if (timeo == 0 && !nonblock) {
    kfree_skb(skb);
    return -EAGAIN;
  }

>  		goto retry;
> +	}
>  	if (err)
>  		return err;
>  
> -- 
> 2.53.0

--
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