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: stable@vger.kernel.org
Reported-by: Vega <redacted>
Signed-off-by: Zhiling Zou <redacted>

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

Reviewed-by: Ido Schimmel <idosch@nvidia.com>