[PATCH V6] netfilter: h323: avoid potential attack

[PATCH V6] netfilter: h323: avoid potential attack

[Zhouyi Zhou]

I think hackers chould build a malicious h323 packet to overflow
the pointer p which will panic during the memcpy(addr, p, len)
For example, he may fabricate a very large taddr->ipAddress.ip.

In order to avoid this, I add a valid memory reference check in
 get_h2x5_addr functions.

As suggested by Eric, this module is protected by a lock (nf_h323_lock)
so adding a variable h323_buffer_valid_bytes that would contain
the number of valid bytes would not require to change prototypes of
get_h2x5_addr.

Thanks Sergei for reviewing.

Signed-off-by: Zhouyi Zhou <yizhouzhou@ict.ac.cn>
---
 net/netfilter/nf_conntrack_h323_main.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/net/netfilter/nf_conntrack_h323_main.c b/net/netfilter/nf_conntrack_h323_main.c
index 9511af0..8d24c4b 100644
--- a/net/netfilter/nf_conntrack_h323_main.c
+++ b/net/netfilter/nf_conntrack_h323_main.c
@@ -110,6 +110,21 @@ int (*nat_q931_hook) (struct sk_buff *skb,
 
 static DEFINE_SPINLOCK(nf_h323_lock);
 static char *h323_buffer;
+static int h323_buffer_valid_bytes;
+
+static bool h323_buffer_ref_valid(void *p, int len)
+{
+	if ((unsigned long)len > h323_buffer_valid_bytes)
+		return false;
+
+	if (p + len > (void *)h323_buffer + h323_buffer_valid_bytes)
+		return false;
+
+	if (p < (void *)h323_buffer)
+		return false;
+
+	return true;
+}
 
 static struct nf_conntrack_helper nf_conntrack_helper_h245;
 static struct nf_conntrack_helper nf_conntrack_helper_q931[];
@@ -145,6 +160,7 @@ static int get_tpkt_data(struct sk_buff *skb, unsigned int protoff,
 
 	if (*data == NULL) {	/* first TPKT */
 		/* Get first TPKT pointer */
+		h323_buffer_valid_bytes = tcpdatalen;
 		tpkt = skb_header_pointer(skb, tcpdataoff, tcpdatalen,
 					  h323_buffer);
 		BUG_ON(tpkt == NULL);
@@ -247,6 +263,9 @@ static int get_h245_addr(struct nf_conn *ct, const unsigned char *data,
 		return 0;
 	}
 
+	if (!h323_buffer_ref_valid((void *)p, len + sizeof(__be16)))
+		return 0;
+
 	memcpy(addr, p, len);
 	memset((void *)addr + len, 0, sizeof(*addr) - len);
 	memcpy(port, p + len, sizeof(__be16));
@@ -669,6 +688,9 @@ int get_h225_addr(struct nf_conn *ct, unsigned char *data,
 		return 0;
 	}
 
+	if (!h323_buffer_ref_valid((void *)p, len + sizeof(__be16)))
+		return 0;
+
 	memcpy(addr, p, len);
 	memset((void *)addr + len, 0, sizeof(*addr) - len);
 	memcpy(port, p + len, sizeof(__be16));
@@ -1248,6 +1270,7 @@ static unsigned char *get_udp_data(struct sk_buff *skb, unsigned int protoff,
 	if (dataoff >= skb->len)
 		return NULL;
 	*datalen = skb->len - dataoff;
+	h323_buffer_valid_bytes = *datalen;
 	return skb_header_pointer(skb, dataoff, *datalen, h323_buffer);
 }
 
-- 
1.9.1

Re: [PATCH V6] netfilter: h323: avoid potential attack

[Pablo Neira Ayuso]

On Tue, Feb 02, 2016 at 09:40:04PM +0800, Zhouyi Zhou wrote:
> diff --git a/net/netfilter/nf_conntrack_h323_main.c b/net/netfilter/nf_conntrack_h323_main.c
> index 9511af0..8d24c4b 100644
> --- a/net/netfilter/nf_conntrack_h323_main.c
> +++ b/net/netfilter/nf_conntrack_h323_main.c
> @@ -110,6 +110,21 @@ int (*nat_q931_hook) (struct sk_buff *skb,
>  
>  static DEFINE_SPINLOCK(nf_h323_lock);
>  static char *h323_buffer;
> +static int h323_buffer_valid_bytes;
> +
> +static bool h323_buffer_ref_valid(void *p, int len)

I'd rather see you pass a parameter to this function with the
remaining size in the buffer, so we don't need this global variable
h323_buffer_valid_bytes.

You can probably add an initial patch to add a structure to store the
state information so we reduce the function parameter footprint.

        struct h323_ct_state {
                ...
                int buflen;
        };

So you pass up the h323_ct_state structure to function calls,
something like that.

Thanks.

> +{
> +	if ((unsigned long)len > h323_buffer_valid_bytes)
> +		return false;
> +
> +	if (p + len > (void *)h323_buffer + h323_buffer_valid_bytes)
> +		return false;
> +
> +	if (p < (void *)h323_buffer)
> +		return false;
> +
> +	return true;
> +}
>  
>  static struct nf_conntrack_helper nf_conntrack_helper_h245;
>  static struct nf_conntrack_helper nf_conntrack_helper_q931[];