Re: [PATCH nf 1/2] ipvs: validate cport in received sync records

[netdev-bot+sashiko@kernel.org]

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- [Medium] The new check in ip_vs_proc_conn()
  (net/netfilter/ipvs/ip_vs_sync.c) is incomplete for templates.
- [Low] The new comment and rejection in ip_vs_proc_conn() say ordinary
  connections must have NO_CPORT exactly when cport is 0, and records…

--

ipvs: validate cport in received sync records

This patch adds cport checks to ip_vs_proc_conn() for sync records
received by the backup. Templates with a nonzero cport are dropped.
Ordinary connections are dropped unless IP_VS_CONN_F_NO_CPORT is set
exactly when cport is 0.

> diff --git a/net/netfilter/ipvs/ip_vs_sync.c b/net/netfilter/ipvs/ip_vs_sync.c
> index 5383aeafb0aed..3998e9a0a391c 100644
> --- a/net/netfilter/ipvs/ip_vs_sync.c
> +++ b/net/netfilter/ipvs/ip_vs_sync.c
> @@ -848,6 +848,25 @@ static void ip_vs_proc_conn(struct netns_ipvs *ipvs, struct ip_vs_conn_param *pa
>  	struct ip_vs_dest *dest;
>  	struct ip_vs_conn *cp;
>  
> +	/* Templates require cport 0. Ordinary connections require
> +	 * NO_CPORT exactly when cport is 0.
> +	 */
> +	if (flags & IP_VS_CONN_F_TEMPLATE) {
> +		if (param->cport) {
> +			IP_VS_DBG(2, "BACKUP, template with cport dropped\n");
> +			kfree(param->pe_data);
> +			return;
> +		}

[Severity: Medium]
Should the template branch also reject IP_VS_CONN_F_NO_CPORT?

A template with cport 0 and IP_VS_CONN_F_NO_CPORT set passes this check.
The commit message leaves that case to "ipvs: filter some flags received
in the backup server". That patch is not in this tree, and this one is
tagged for stable.

In this tree, IP_VS_CONN_F_BACKUP_MASK still includes both NO_CPORT and
TEMPLATE, so ip_vs_proc_sync_conn() lets the combination through. The v0
parser applies no mask at all:

net/netfilter/ipvs/ip_vs_sync.c:ip_vs_process_message_v0() {
    ...
		flags = ntohs(s->flags) | IP_VS_CONN_F_SYNC;
		flags &= ~IP_VS_CONN_F_HASHED;
    ...
}

A companion fix that only changes IP_VS_CONN_F_BACKUP_MASK would
therefore not cover v0 records.

If ip_vs_ct_in_get() misses, ip_vs_conn_new() hashes the template and
turns on the cport-zero fallback:

net/netfilter/ipvs/ip_vs_conn.c:ip_vs_conn_new() {
    ...
	if (unlikely(flags & IP_VS_CONN_F_NO_CPORT)) {
		int af_id = ip_vs_af_index(cp->af);

		atomic_inc(&ipvs->no_cport_conns[af_id]);
	}
    ...
}

A later packet from that client, from any source port, misses the first
lookup. ip_vs_conn_in_get() then retries with cport 0. The match in
__ip_vs_conn_in_get() has no IP_VS_CONN_F_TEMPLATE exclusion, so the
retry returns the template:

				    (!p->cport ^
				     (!(cp->flags & IP_VS_CONN_F_NO_CPORT))) &&

The datapath then treats the template as an ordinary connection:

ip_vs_in_hook()
  ip_vs_conn_in_get()        /* returns the template */
  ip_vs_set_state()
  cp->packet_xmit()
    ip_vs_conn_fill_cport()  /* NAT: rewrites cport, clears NO_CPORT */

After fill_cport rehashes the template, ip_vs_ct_in_get() no longer finds
it, which breaks persistence for that client.

set_tcp_state() also changes dest->activeconns for the template, based
only on IP_VS_CONN_F_INACTIVE. For templates, ip_vs_bind_dest() and
ip_vs_unbind_dest() only adjust persistconns, so activeconns on the real
server can drift.

Template state is validated now, so this does not seem to lead to
out-of-bounds reads. Still, isn't this the same class of problem the
commit message describes, a template reaching the protocol state machine
through ip_vs_conn_in_get()?

Would adding "|| (flags & IP_VS_CONN_F_NO_CPORT)" to the template check
make this patch self-contained and cover the v0 path as well?

> +	} else if (!param->cport && !(flags & IP_VS_CONN_F_NO_CPORT)) {
> +		IP_VS_DBG(2, "BACKUP, conn without cport dropped\n");
> +		kfree(param->pe_data);
> +		return;
> +	} else if (param->cport && (flags & IP_VS_CONN_F_NO_CPORT)) {
> +		IP_VS_DBG(2, "BACKUP, conn with cport and NO_CPORT dropped\n");
> +		kfree(param->pe_data);
> +		return;
> +	}

[Severity: Low]
Can the master itself briefly send a record with a nonzero cport and
IP_VS_CONN_F_NO_CPORT still set?

ip_vs_conn_fill_cport() can use hn1 (use2, e.g. NAT/FTP data
connections). In that case the dir 1 pass sets cport while the flag stays
set. cp->lock is then released, and only the dir 0 pass clears the flag:

net/netfilter/ipvs/ip_vs_conn.c:ip_vs_conn_fill_cport() {
    ...
	if (cp->flags & IP_VS_CONN_F_NO_CPORT && (!cp->cport || by_me)) {
		/* If we race with resizing make sure cport is set for dir 1 */
		if (!cp->cport) {
			cp->cport = cport;
			by_me = true;
		}
		if (!dir) {
			atomic_dec(&ipvs->no_cport_conns[af_id]);
			cp->flags &= ~IP_VS_CONN_F_NO_CPORT;
		}
    ...
}

In that window, another CPU can handle a packet of the same connection
and call ip_vs_sync_conn(). That function copies cp->flags and cp->cport
without taking cp->lock:

net/netfilter/ipvs/ip_vs_sync.c:ip_vs_sync_conn() {
    ...
	s->v4.flags = htonl(cp->flags & ~IP_VS_CONN_F_HASHED);
	s->v4.state = htons(cp->state);
	s->v4.protocol = cp->protocol;
	s->v4.cport = cp->cport;
    ...
}

Before this patch, the backup matched such a record to its existing
cport 0 NO_CPORT connection through the cport-zero fallback. It then
updated that connection's INACTIVE/SEQ flags and seq options. Now the
record is dropped.

The next sync record restores the state, so the effect looks small. Is
the comment "NO_CPORT exactly when cport is 0" accurate, given what the
master can send in this window?

-- 
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