[PATCH] Audit: fix audit nlmsg_len from the kernel

[Eric Paris <eparis@redhat.com>]

Currently audit emits it's netlink record setting the nlmsg_len equal to the
size of the data in the netlink message rather than the size of the netlink
header plus the size of the data in the msg.  nlmsg_end() happens to be a
nice function which calculates and sets nlmsg_len for you, so we use that
function to make sure we get it right from now on.

The busted nlmsg_len doesn't hurt anything as long as sizeof(data) >
sizeof(nlmsg_hdr) and we don't pack mulitple netlink messages into one skb.
Since sizeof(nlmsg_hdr)=16 and we send (as it turns out by sheer dumb luck) at
least 16 bytes of audit timestamp into every message we aren't running into
problems.

But lets fix it anyway rather than rely on dumb luck and implementation
details.

Signed-off-by: Eric Paris <eparis@redhat.com>
---

 kernel/audit.c |   13 ++++++++-----
 1 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/kernel/audit.c b/kernel/audit.c
index 9442c35..5c2ccef 100644
--- a/kernel/audit.c
+++ b/kernel/audit.c
@@ -1468,22 +1468,25 @@ void audit_log_end(struct audit_buffer *ab)
 	if (!audit_rate_check()) {
 		audit_log_lost("rate limit exceeded");
 	} else {
-		struct nlmsghdr *nlh = nlmsg_hdr(ab->skb);
-		nlh->nlmsg_len = ab->skb->len - NLMSG_SPACE(0);
+		struct sk_buff *skb = ab->skb;
+		struct nlmsghdr *nlh = nlmsg_hdr(skb);
+
+		/* correctly account for nlh->nlmsg_len */
+		nlmsg_end(skb, nlh);
 
 		if (audit_pid) {
-			skb_queue_tail(&audit_skb_queue, ab->skb);
+			skb_queue_tail(&audit_skb_queue, skb);
 			wake_up_interruptible(&kauditd_wait);
 		} else {
 			if (nlh->nlmsg_type != AUDIT_EOE) {
 				if (printk_ratelimit()) {
 					printk(KERN_NOTICE "type=%d %s\n",
 						nlh->nlmsg_type,
-						ab->skb->data + NLMSG_SPACE(0));
+						skb->data + NLMSG_SPACE(0));
 				} else
 					audit_log_lost("printk limit exceeded\n");
 			}
-			audit_hold_skb(ab->skb);
+			audit_hold_skb(skb);
 		}
 		ab->skb = NULL;
 	}