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

[Ricardo Robaina]

On Tue, May 12, 2026 at 4:35 PM Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
>
> On May 11, 2026 Ricardo Robaina <rrobaina@xxxxxxxxxx> 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 <sgrubb@xxxxxxxxxx>
> > Signed-off-by: Ricardo Robaina <rrobaina@xxxxxxxxxx>
> > ---
> >  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;
>   }
>

Hi Paul,

Thanks for reviewing this patch. You're definitely right, it seems I
overengineered the fix.
I'm just doing some final testing, and I'll post the V2 shortly.

> >               goto retry;
> > +     }
> >       if (err)
> >               return err;
> >
> > --
> > 2.53.0
>
> --
> paul-moore.com
>