Re: [PATCH 1/2] audit: stop an old auditd being starved out by a new auditd

[Paul Moore <pmoore@redhat.com>]

On Friday, September 18, 2015 03:59:58 AM Richard Guy Briggs wrote:
> Nothing prevents a new auditd starting up and replacing a valid
> audit_pid when an old auditd is still running, effectively starving out
> the old auditd since audit_pid no longer points to the old valid auditd.
> 
> If no message to auditd has been attempted since auditd died unnaturally
> or got killed, audit_pid will still indicate it is alive.  There isn't
> an easy way to detect if an old auditd is still running on the existing
> audit_pid other than attempting to send a message to see if it fails.
> An -ECONNREFUSED almost certainly means it disappeared and can be
> replaced.  Other errors are not so straightforward and may indicate
> transient problems that will resolve themselves and the old auditd will
> recover.  Yet others will likely need manual intervention for which a
> new auditd will not solve the problem.
> 
> Send a new message type (AUDIT_PING) to the old auditd containing a u32
> with the PID of the new auditd.  If the audit ping succeeds (or doesn't
> fail with certainty), fail to register the new auditd and return an
> error (-EEXIST).
> 
> This is expected to make the patch preventing an old auditd orphaning a
> new auditd redundant.
> 
> Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
> ---
>  include/uapi/linux/audit.h |    1 +
>  kernel/audit.c             |   19 +++++++++++++++++--
>  2 files changed, 18 insertions(+), 2 deletions(-)

XXX

> diff --git a/kernel/audit.c b/kernel/audit.c
> index 18cdfe2..3399ab2 100644
> --- a/kernel/audit.c
> +++ b/kernel/audit.c
> @@ -810,6 +810,15 @@ static int audit_set_feature(struct sk_buff *skb)
>  	return 0;
>  }
> 
> +static int audit_ping(pid_t pid, u32 seq, u32 portid)
> +{
> +	struct sk_buff *skb = audit_make_reply(portid, seq, AUDIT_PING, 0, 0,
> +                                       &pid, sizeof(pid));

This is almost surely going to end up using the wrong netlink sequence number 
and portid since you are passing the new requestor's information below.  I 
didn't chase down the netlink_unicast() guts to see if it replaces the portid, 
it might (it probably does), but that still leaves the sequence number.

Also, this is more of a attempted hijack message and not a simple ping, right?  
If we want to create a simple ping message, leave the pid out of it; if we 
want to indicate to an existing auditd that another process is attempting to 
hijack the audit connection then we should probably create a proper audit 
record with a type other than AUDIT_PING.  I tend to think there is more value 
in the hijack message than the ping message, but I can be convinced either 
way.

> +	if (!skb)
> +		return -ENOMEM;
> +	return netlink_unicast(audit_sock, skb, audit_nlk_portid, 0);
> +}

...

> @@ -871,13 +880,19 @@ static int audit_receive_msg(struct sk_buff *skb,
>  		if (s.mask & AUDIT_STATUS_PID) {
>  			int new_pid = s.pid;
> +			pid_t requesting_pid = task_tgid_vnr(current);
> +			u32 portid = NETLINK_CB(skb).portid;
> 
> -			if ((!new_pid) && (task_tgid_vnr(current) != audit_pid))
> +			if ((!new_pid) && (requesting_pid != audit_pid))
>  				return -EACCES;
> +			if (audit_pid && new_pid &&
> +			    audit_ping(requesting_pid, nlmsg_hdr(skb)->..., portid) !=
> +			    -ECONNREFUSED)
> +				return -EEXIST;

See my comments above about audit_ping().

>  			if (audit_enabled != AUDIT_OFF)
>  				audit_log_config_change("audit_pid", new_pid, audit_pid, 1);
>  			audit_pid = new_pid;
> -			audit_nlk_portid = NETLINK_CB(skb).portid;
> +			audit_nlk_portid = portid;
>  			audit_sock = skb->sk;
>  		}
>  		if (s.mask & AUDIT_STATUS_RATE_LIMIT) {

-- 
paul moore
security @ redhat