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/kernel/audit.c b/kernel/audit.c index e1d489bc2dff..005bfd9dc7a4 100644 --- a/kernel/audit.c +++ b/kernel/audit.c @@ -753,7 +753,7 @@ static int auditd_send_unicast_skb(struct sk_buff *skb) portid = ac->portid; rcu_read_unlock(); - rc = netlink_unicast(sk, skb, portid, 0); + rc = netlink_unicast(sk, skb, portid, NETLINK_UNICAST_TIMED); put_net(net); if (rc < 0) goto err; @@ -811,7 +811,7 @@ static int kauditd_send_queue(struct sock *sk, u32 portid, retry: /* grab an extra skb reference in case of error */ skb_get(skb); - rc = netlink_unicast(sk, skb, portid, 0); + rc = netlink_unicast(sk, skb, portid, NETLINK_UNICAST_TIMED); if (rc < 0) { /* send failed - try a few times unless fatal error */ if (++failed >= retry_limit || @@ -967,7 +967,7 @@ int audit_send_list_thread(void *_dest) audit_ctl_unlock(); while ((skb = __skb_dequeue(&dest->q)) != NULL) - netlink_unicast(sk, skb, dest->portid, 0); + netlink_unicast(sk, skb, dest->portid, NETLINK_UNICAST_TIMED); put_net(dest->net); kfree(dest); @@ -1020,7 +1020,7 @@ static int audit_send_reply_thread(void *arg) /* Ignore failure. It'll only happen if the sender goes away, because our timeout is set to infinite. */ - netlink_unicast(audit_get_sk(reply->net), reply->skb, reply->portid, 0); + netlink_unicast(audit_get_sk(reply->net), reply->skb, reply->portid, NETLINK_UNICAST_TIMED); reply->skb = NULL; audit_free_reply(reply); return 0; 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; + } goto retry; + } if (err) return err; -- 2.53.0
