Subject: Re: [PATCH nf] netfilter: nf_conntrack_sip: fix use of uninitialized rtp_addr in process_sdp

From: Pablo Neira Ayuso <[email-masked]>
Date: Thu, 26 Mar 2026 21:21:43 +0100

On Mon, Mar 23, 2026 at 04:07:29PM +0800, Weiming Shi wrote:
> process_sdp() declares union nf_inet_addr rtp_addr on the stack and
> passes it to the nf_nat_sip sdp_session hook after walking the SDP
> media descriptions. However rtp_addr is only initialized inside the
> media loop when a recognized media type with a non-zero port is found.
> 
> If the SDP body contains no m= lines, only inactive media sections
> (m=audio 0 ...) or only unrecognized media types, rtp_addr is never
> assigned. Despite that, the function still calls hooks->sdp_session()
> with &rtp_addr, causing nf_nat_sdp_session() to format the stale stack
> value as an IP address and rewrite the SDP session owner and connection
> lines with it.
> 
> With CONFIG_INIT_STACK_ALL_ZERO (default on most distributions) this
> results in the session-level o= and c= addresses being rewritten to
> 0.0.0.0 for inactive SDP sessions. Without stack auto-init the
> rewritten address is whatever happened to be on the stack.
> 
> Fix this by pre-initializing rtp_addr from the session-level connection
> address (caddr) when available, and tracking via a have_rtp_addr flag
> whether any valid address was established. Skip the sdp_session hook
> entirely when no valid address exists.
> 
> Fixes: 4ab9e64e5e3c ("[NETFILTER]: nf_nat_sip: split up SDP mangling")
> Reported-by: Xiang Mei <[email-masked]>
> Signed-off-by: Weiming Shi <[email-masked]>
> ---
>  net/netfilter/nf_conntrack_sip.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/net/netfilter/nf_conntrack_sip.c b/net/netfilter/nf_conntrack_sip.c
> index 4ab5ef71d96db..17af0ff4ea7ab 100644
> --- a/net/netfilter/nf_conntrack_sip.c
> +++ b/net/netfilter/nf_conntrack_sip.c
> @@ -1040,6 +1040,7 @@ static int process_sdp(struct sk_buff *skb, unsigned int protoff,
>  	unsigned int port;
>  	const struct sdp_media_type *t;
>  	int ret = NF_ACCEPT;
> +	bool have_rtp_addr = false;
>  
>  	hooks = rcu_dereference(nf_nat_sip_hooks);
>  
> @@ -1056,8 +1057,11 @@ static int process_sdp(struct sk_buff *skb, unsigned int protoff,
>  	caddr_len = 0;
>  	if (ct_sip_parse_sdp_addr(ct, *dptr, sdpoff, *datalen,
>  				  SDP_HDR_CONNECTION, SDP_HDR_MEDIA,
> -				  &matchoff, &matchlen, &caddr) > 0)
> +				  &matchoff, &matchlen, &caddr) > 0) {
>  		caddr_len = matchlen;
> +		memcpy(&rtp_addr, &caddr, sizeof(rtp_addr));
> +		have_rtp_addr = true;
> +	}
>  
>  	mediaoff = sdpoff;
>  	for (i = 0; i < ARRAY_SIZE(sdp_media_types); ) {
> @@ -1091,9 +1095,11 @@ static int process_sdp(struct sk_buff *skb, unsigned int protoff,
>  					  &matchoff, &matchlen, &maddr) > 0) {
>  			maddr_len = matchlen;
>  			memcpy(&rtp_addr, &maddr, sizeof(rtp_addr));
> -		} else if (caddr_len)
> +			have_rtp_addr = true;
> +		} else if (caddr_len) {
>  			memcpy(&rtp_addr, &caddr, sizeof(rtp_addr));
> -		else {
> +			have_rtp_addr = true;

After this update, this loop sets over rtp_addr, but this was already
set by ct_sip_parse_sdp_addr() a bit above.

This new chunk results in:

                } else if (caddr_len) {
                       memcpy(&rtp_addr, &caddr, sizeof(rtp_addr));
                       have_rtp_addr = true;

which is not needed? Why does caddr need to be copied over and over
again to rtp_addr?

> +		} else {
>  			nf_ct_helper_log(skb, ct, "cannot parse SDP message");
>  			return NF_DROP;
>  		}