Re: [PATCH net v3 1/1] ipmr: only copy pktinfo to cache reports

On Mon, Aug 03, 2026 at 09:17:55AM +0800, Zhiling Zou wrote:
> ipmr_cache_report() builds short IGMP reports for mrouted from a packet
> that may be a synthetic RTM_GETROUTE query. That query skb stores the
> netlink requester portid in NETLINK_CB(), but the report is delivered to
> a raw IPv4 socket, whose receive path interprets skb->cb as IPCB().
> 
> Commit bb7403655b3c ("ipmr: support IP_PKTINFO on cache report IGMP
> msg") added IP_PKTINFO support by calling ipv4_pktinfo_prepare() on the
> original packet and then copying the entire 48-byte skb control block to
> the report skb. For synthetic route-query packets, this copies
> NETLINK_CB() bytes into IPCB() and lets a controlled portid corrupt
> IPCB(skb)->opt. With IP_RECVOPTS or IP_RETOPTS enabled, the raw socket
> receive path can then copy past the short report packet or overflow the
> stack option buffer.
> 
> Keep the IP_PKTINFO support, but copy only the pktinfo fields prepared
> by ipv4_pktinfo_prepare().
> 
> This changes the accidental IP_RECVOPTS/IP_RETOPTS behavior for these
> short cache reports, but that behavior was only a side effect of
> copying the entire control block and was never intended to work.
> 
> Fixes: bb7403655b3c ("ipmr: support IP_PKTINFO on cache report IGMP msg")
> Cc: [email protected]
> Reported-by: Vega <[email protected]>
> Signed-off-by: Zhiling Zou <[email protected]>

There are more bugs in ipmr_cache_report(). I will fix them in a
follow-up.

Reviewed-by: Ido Schimmel <[email protected]>